#77 Catch startup exceptions and show them in the browser.

src/Microsoft.AspNet.Hosting/Internal/HostingEngine.cs

@@ -14,6 +14,7 @@ using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Http.Features;
 using Microsoft.AspNet.Http.Features.Internal;
 using Microsoft.AspNet.Server.Features;
+using Microsoft.Dnx.Runtime;
 using Microsoft.Framework.Configuration;
 using Microsoft.Framework.DependencyInjection;
 using Microsoft.Framework.Logging;
@@ -24,11 +25,13 @@ namespace Microsoft.AspNet.Hosting.Internal
     {
         // This is defined by IIS's HttpPlatformHandler.
         private static readonly string ServerPort = "HTTP_PLATFORM_PORT";
+        private static readonly string DetailedErrors = "Hosting:DetailedErrors";
 
         private readonly IServiceCollection _applicationServiceCollection;
         private readonly IStartupLoader _startupLoader;
         private readonly ApplicationLifetime _applicationLifetime;
         private readonly IConfiguration _config;
+        private readonly bool _captureStartupErrors;
 
         private IServiceProvider _applicationServices;
 
@@ -45,7 +48,8 @@ namespace Microsoft.AspNet.Hosting.Internal
         public HostingEngine(
             IServiceCollection appServices,
             IStartupLoader startupLoader,
-            IConfiguration config)
+            IConfiguration config,
+            bool captureStartupErrors)
         {
             if (appServices == null)
             {
@@ -65,6 +69,7 @@ namespace Microsoft.AspNet.Hosting.Internal
             _config = config;
             _applicationServiceCollection = appServices;
             _startupLoader = startupLoader;
+            _captureStartupErrors = captureStartupErrors;
             _applicationLifetime = new ApplicationLifetime();
         }
 
@@ -79,8 +84,6 @@ namespace Microsoft.AspNet.Hosting.Internal
 
         public virtual IApplication Start()
         {
-            EnsureApplicationServices();
-
             var application = BuildApplication();
 
             var logger = _applicationServices.GetRequiredService<ILogger<HostingEngine>>();
@@ -174,6 +177,67 @@ namespace Microsoft.AspNet.Hosting.Internal
         }
 
         private RequestDelegate BuildApplication()
+        {
+            try
+            {
+                EnsureApplicationServices();
+                EnsureServer();
+
+                var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>();
+                var builder = builderFactory.CreateBuilder(_serverInstance);
+                builder.ApplicationServices = _applicationServices;
+
+                var startupFilters = _applicationServices.GetService<IEnumerable<IStartupFilter>>();
+                var configure = Startup.ConfigureDelegate;
+                foreach (var filter in startupFilters)
+                {
+                    configure = filter.Configure(configure);
+                }
+
+                configure(builder);
+
+                return builder.Build();
+            }
+            catch (Exception ex)
+            {
+                if (!_captureStartupErrors)
+                {
+                    throw;
+                }
+
+                // EnsureApplicationServices may have failed due to a missing or throwing Startup class.
+                if (_applicationServices == null)
+                {
+                    _applicationServices = _applicationServiceCollection.BuildServiceProvider();
+                }
+
+                EnsureServer();
+
+                // Write errors to standard out so they can be retrieved when not in development mode.
+                Console.Out.WriteLine("Application startup exception: " + ex.ToString());
+                var logger = _applicationServices.GetRequiredService<ILogger<HostingEngine>>();
+                logger.LogError("Application startup exception", ex);
+
+                // Generate an HTML error page.
+                var runtimeEnv = _applicationServices.GetRequiredService<IRuntimeEnvironment>();
+                var hostingEnv = _applicationServices.GetRequiredService<IHostingEnvironment>();
+                var showDetailedErrors = hostingEnv.IsDevelopment()
+                    || string.Equals("true", _config[DetailedErrors], StringComparison.OrdinalIgnoreCase)
+                    || string.Equals("1", _config[DetailedErrors], StringComparison.OrdinalIgnoreCase);
+                var errorBytes = StartupExceptionPage.GenerateErrorHtml(showDetailedErrors, runtimeEnv, ex);
+
+                return context =>
+                {
+                    context.Response.StatusCode = 500;
+                    context.Response.Headers["Cache-Control"] = "private, max-age=0";
+                    context.Response.ContentType = "text/html; charset=utf-8";
+                    context.Response.ContentLength = errorBytes.Length;
+                    return context.Response.Body.WriteAsync(errorBytes, 0, errorBytes.Length);
+                };
+            }
+        }
+
+        private void EnsureServer()
         {
             if (ServerFactory == null)
             {
@@ -186,37 +250,25 @@ namespace Microsoft.AspNet.Hosting.Internal
                 ServerFactory = _applicationServices.GetRequiredService<IServerLoader>().LoadServerFactory(ServerFactoryLocation);
             }
 
-            _serverInstance = ServerFactory.Initialize(_config);
-            var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>();
-            var builder = builderFactory.CreateBuilder(_serverInstance);
-            builder.ApplicationServices = _applicationServices;
-
-            var addresses = builder.ServerFeatures?.Get<IServerAddressesFeature>()?.Addresses;
-            if (addresses != null && !addresses.IsReadOnly)
+            if (_serverInstance == null)
             {
-                var port = _config[ServerPort];
-                if (!string.IsNullOrEmpty(port))
+                _serverInstance = ServerFactory.Initialize(_config);
+                var addresses = _serverInstance?.Get<IServerAddressesFeature>()?.Addresses;
+                if (addresses != null && !addresses.IsReadOnly)
                 {
-                    addresses.Add("http://localhost:" + port);
-                }
+                    var port = _config[ServerPort];
+                    if (!string.IsNullOrEmpty(port))
+                    {
+                        addresses.Add("http://localhost:" + port);
+                    }
 
-                // Provide a default address if there aren't any configured.
-                if (addresses.Count == 0)
-                {
-                    addresses.Add("http://localhost:5000");
+                    // Provide a default address if there aren't any configured.
+                    if (addresses.Count == 0)
+                    {
+                        addresses.Add("http://localhost:5000");
+                    }
                 }
             }
-
-            var startupFilters = _applicationServices.GetService<IEnumerable<IStartupFilter>>();
-            var configure = Startup.ConfigureDelegate;
-            foreach (var filter in startupFilters)
-            {
-                configure = filter.Configure(configure);
-            }
-
-            configure(builder);
-
-            return builder.Build();
         }
 
         private string GetRequestIdentifier(HttpContext httpContext)

src/Microsoft.AspNet.Hosting/Program.cs

@@ -36,7 +36,7 @@ namespace Microsoft.AspNet.Hosting
             builder.AddCommandLine(args);
             var config = builder.Build();
 
-            var host = new WebHostBuilder(_serviceProvider, config).Build();
+            var host = new WebHostBuilder(_serviceProvider, config, captureStartupErrors: true).Build();
             using (var app = host.Start())
             {
                 var hostingEnv = app.Services.GetRequiredService<IHostingEnvironment>();

src/Microsoft.AspNet.Hosting/Startup/StartupExceptionPage.cs

@@ -0,0 +1,520 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.ExceptionServices;
+using System.Text;
+using Microsoft.Dnx.Compilation;
+using Microsoft.Dnx.Runtime;
+using Microsoft.Framework.WebEncoders;
+
+namespace Microsoft.AspNet.Hosting.Startup
+{
+    internal static class StartupExceptionPage
+    {
+        private const int MaxCompilationErrorsToShow = 20;
+
+        private static readonly string _errorPageFormatString = GetResourceString("GenericError.html", escapeBraces: true);
+        private static readonly string _errorMessageFormatString = GetResourceString("GenericError_Message.html");
+        private static readonly string _errorExceptionFormatString = GetResourceString("GenericError_Exception.html");
+        private static readonly string _errorFooterFormatString = GetResourceString("GenericError_Footer.html");
+        private static readonly string _compilationExceptionFormatString = GetResourceString("Compilation_Exception.html");
+
+        public static byte[] GenerateErrorHtml(bool showDetails, IRuntimeEnvironment runtimeEnvironment, params object[] errorDetails)
+        {
+            if (!showDetails)
+            {
+                errorDetails = new[] { "An error occurred while starting the application." };
+            }
+
+            // Build the message for each error
+            var wasSourceCodeWrittenOntoPage = false;
+            var builder = new StringBuilder();
+            var rawExceptionDetails = new StringBuilder();
+
+            foreach (object error in errorDetails ?? new object[0])
+            {
+                var ex = error as Exception;
+                if (ex == null && error is ExceptionDispatchInfo)
+                {
+                    ex = ((ExceptionDispatchInfo)error).SourceException;
+                }
+
+                if (ex != null)
+                {
+                    var flattenedExceptions = FlattenAndReverseExceptionTree(ex);
+
+                    var compilationException = flattenedExceptions.OfType<ICompilationException>()
+                                                                  .FirstOrDefault();
+                    if (compilationException != null)
+                    {
+                        WriteException(compilationException, builder, ref wasSourceCodeWrittenOntoPage);
+
+                        var compilationErrorMessages = compilationException.CompilationFailures
+                            .SelectMany(f => f.Messages.Select(m => m.FormattedMessage))
+                            .Take(MaxCompilationErrorsToShow);
+
+                        WriteRawExceptionDetails("Show raw compilation error details", compilationErrorMessages, rawExceptionDetails);
+                    }
+                    else
+                    {
+                        foreach (var innerEx in flattenedExceptions)
+                        {
+                            WriteException(innerEx, builder, ref wasSourceCodeWrittenOntoPage);
+                        }
+
+                        WriteRawExceptionDetails("Show raw exception details", new[] { ex.ToString() }, rawExceptionDetails);
+                    }
+                }
+                else
+                {
+                    var message = Convert.ToString(error, CultureInfo.InvariantCulture);
+                    WriteMessage(message, builder);
+                }
+            }
+
+            // Generate the footer
+            var footer = showDetails ? GenerateFooterEncoded(runtimeEnvironment) : null;
+
+            // And generate the full markup
+            return Encoding.UTF8.GetBytes(string.Format(CultureInfo.InvariantCulture, _errorPageFormatString, builder, rawExceptionDetails, footer));
+        }
+
+        private static string BuildCodeSnippetDiv(StackFrame frame)
+        {
+            var filename = frame.GetFileName();
+            if (!string.IsNullOrEmpty(filename))
+            {
+                int failingLineNumber = frame.GetFileLineNumber();
+                if (failingLineNumber >= 1)
+                {
+                    var lines = GetFailingCallSiteInFile(filename, failingLineNumber);
+                    if (lines != null)
+                    {
+                        return @"<div class=""codeSnippet"">"
+                            + @"<div class=""filename""><code>" + HtmlEncodeAndReplaceLineBreaks(filename) + "</code></div>" + Environment.NewLine
+                            + string.Join(Environment.NewLine, lines) + "</div>" + Environment.NewLine;
+                    }
+                }
+            }
+
+            // fallback
+            return null;
+        }
+
+        private static string BuildLineForStackFrame(StackFrame frame)
+        {
+            var builder = new StringBuilder("<pre>");
+            var method = frame.GetMethod();
+
+            // Special case: no method available
+            if (method == null)
+            {
+                return null;
+            }
+
+            // First, write the type name
+            var type = method.DeclaringType;
+            if (type != null)
+            {
+                // Special-case ExceptionDispatchInfo.Throw()
+                if (type == typeof(ExceptionDispatchInfo) && method.Name == "Throw")
+                {
+                    return @"<pre><span class=""faded"">--- exception rethrown ---</span></pre>";
+                }
+
+                string prefix, friendlyName;
+                SplitTypeIntoPrefixAndFriendlyName(type, out prefix, out friendlyName);
+                builder.AppendFormat(CultureInfo.InvariantCulture, @"<span class=""faded"">at {0}</span>", HtmlEncodeAndReplaceLineBreaks(prefix));
+                builder.Append(HtmlEncodeAndReplaceLineBreaks(friendlyName));
+            }
+
+            // Next, write the method signature
+            builder.Append(HtmlEncodeAndReplaceLineBreaks("." + method.Name));
+
+            // Is this method generic?
+            if (method.IsGenericMethod)
+            {
+                builder.Append(HtmlEncodeAndReplaceLineBreaks(BuildMethodGenericParametersUnescaped(method)));
+            }
+
+            // Build method parameters
+            builder.AppendFormat(CultureInfo.InvariantCulture, @"<span class=""faded"">{0}</span>", HtmlEncodeAndReplaceLineBreaks(BuildMethodParametersUnescaped(method)));
+
+            // Do we have source information for this frame?
+            if (frame.GetILOffset() != -1)
+            {
+                var filename = frame.GetFileName();
+                if (!string.IsNullOrEmpty(filename))
+                {
+                    builder.AppendFormat(CultureInfo.InvariantCulture, " in {0}:line {1:D}", HtmlEncodeAndReplaceLineBreaks(filename), frame.GetFileLineNumber());
+                }
+            }
+
+            // Finish
+            builder.Append("</pre>");
+            return builder.ToString();
+        }
+
+        private static string BuildMethodGenericParametersUnescaped(MethodBase method)
+        {
+            Debug.Assert(method.IsGenericMethod);
+            return "<" + string.Join(", ", method.GetGenericArguments().Select(PrettyPrintTypeName)) + ">";
+        }
+
+        private static string BuildMethodParametersUnescaped(MethodBase method)
+        {
+            return "(" + string.Join(", ", method.GetParameters().Select(p => {
+                Type parameterType = p.ParameterType;
+                return ((parameterType != null) ? PrettyPrintTypeName(parameterType) : "?") + " " + p.Name;
+            })) + ")";
+        }
+
+        private static void BuildCodeSnippetDiv(CompilationFailure failure,
+                                               StringBuilder builder,
+                                               ref int totalErrorsShown)
+        {
+            const int NumContextLines = 3;
+            var fileName = failure.SourceFilePath;
+            if (totalErrorsShown < MaxCompilationErrorsToShow &&
+                !string.IsNullOrEmpty(fileName))
+            {
+                builder.Append(@"<div class=""codeSnippet"">")
+                       .AppendFormat(@"<div class=""filename""><code>{0}</code></div>", HtmlEncodeAndReplaceLineBreaks(fileName))
+                       .AppendLine();
+
+                IEnumerable<string> fileContent;
+                if (string.IsNullOrEmpty(failure.SourceFileContent))
+                {
+                    fileContent = File.ReadLines(fileName);
+                }
+                else
+                {
+                    fileContent = failure.SourceFileContent.Split(new[] { Environment.NewLine }, StringSplitOptions.None);
+                }
+                foreach (var message in failure.Messages)
+                {
+                    if (totalErrorsShown++ > MaxCompilationErrorsToShow)
+                    {
+                        break;
+                    }
+
+                    if (totalErrorsShown > 1)
+                    {
+                        builder.AppendLine("<br />");
+                    }
+
+                    builder.Append(@"<div class=""error-message"">")
+                           .Append(HtmlEncodeAndReplaceLineBreaks(message.Message))
+                           .Append("</div>");
+
+                    // StartLine and EndLine are 1-based
+                    var startLine = message.StartLine - 1;
+                    var endLine = message.EndLine - 1;
+                    var preContextIndex = Math.Max(startLine - NumContextLines, 0);
+                    var index = preContextIndex + 1;
+                    foreach (var line in fileContent.Skip(preContextIndex).Take(startLine - preContextIndex))
+                    {
+                        builder.Append(@"<div class=""line faded"">")
+                               .AppendFormat(@"<span class=""line-number"">{0}</span><code>", index++)
+                               .Append(HtmlEncodeAndReplaceLineBreaks(line))
+                               .AppendLine("</code></div>");
+                    }
+
+                    var numErrorLines = 1 + Math.Max(0, endLine - startLine);
+                    foreach (var line in fileContent.Skip(startLine).Take(numErrorLines))
+                    {
+                        builder.Append(@"<div class=""line error"">")
+                               .AppendFormat(@"<span class=""line-number"">{0}</span><code>", index++)
+                               .Append(HtmlEncodeAndReplaceLineBreaks(line))
+                               .AppendLine("</code></div>");
+                    }
+                    foreach (var line in fileContent.Skip(message.EndLine).Take(NumContextLines))
+                    {
+                        builder.Append(@"<div class=""line faded"">")
+                               .AppendFormat(@"<span class=""line-number"">{0}</span><code>", index++)
+                               .Append(HtmlEncodeAndReplaceLineBreaks(line))
+                               .AppendLine("</code></div>");
+                    }
+                }
+
+                builder.AppendLine("</div>"); // Close codeSnippet div
+            }
+        }
+
+        private static string GetResourceString(string name, bool escapeBraces = false)
+        {
+            // '{' and '}' are special in CSS, so we use "[[[0]]]" instead for {0} (and so on).
+            var assembly = typeof(StartupExceptionPage).GetTypeInfo().Assembly;
+            var resourceName = assembly.GetName().Name + ".compiler.resources." + name;
+            var manifestStream = assembly.GetManifestResourceStream(resourceName);
+            var formatString = new StreamReader(manifestStream, Encoding.UTF8, detectEncodingFromByteOrderMarks: false).ReadToEnd();
+            if (escapeBraces)
+            {
+                formatString = formatString.Replace("{", "{{").Replace("}", "}}").Replace("[[[", "{").Replace("]]]", "}");
+            }
+
+            return formatString;
+        }
+
+        private static List<string> GetFailingCallSiteInFile(string filename, int failedLineNumber)
+        {
+            // We figure out the [first, last] range of lines to read from the file.
+            var firstLineNumber = failedLineNumber - 2;
+            firstLineNumber = Math.Max(1, firstLineNumber);
+            var lastLineNumber = failedLineNumber + 2;
+            lastLineNumber = Math.Max(lastLineNumber, failedLineNumber);
+
+            // Figure out how many characters lastLineNumber will take to print.
+            var lastLineNumberCharLength = lastLineNumber.ToString("D", CultureInfo.InvariantCulture).Length;
+
+            var errorSubContents = new List<string>();
+            var didReadFailingLine = false;
+
+            try
+            {
+                var thisLineNum = 0;
+                foreach (var line in File.ReadLines(filename))
+                {
+                    thisLineNum++;
+
+                    // Are we within the correct range?
+                    if (thisLineNum < firstLineNumber)
+                    {
+                        continue;
+                    }
+                    if (thisLineNum > lastLineNumber)
+                    {
+                        break;
+                    }
+
+                    var encodedLine = HtmlEncodeAndReplaceLineBreaks("Line "
+                        + thisLineNum.ToString("D", CultureInfo.InvariantCulture).PadLeft(lastLineNumberCharLength)
+                        + ":  "
+                        + line);
+
+                    if (thisLineNum == failedLineNumber)
+                    {
+                        didReadFailingLine = true;
+                        errorSubContents.Add(@"<div class=""line error""><code>" + encodedLine + "</code></div>");
+                    }
+                    else
+                    {
+                        errorSubContents.Add(@"<div class=""line""><code>" + encodedLine + "</code></div>");
+                    }
+                }
+            }
+            catch
+            {
+                // If there's an error for any reason, don't show source.
+                return null;
+            }
+
+            return (didReadFailingLine) ? errorSubContents : null;
+        }
+
+        private static string PrettyPrintTypeName(Type t)
+        {
+            try
+            {
+                RuntimeHelpers.EnsureSufficientExecutionStack();
+
+                var name = t.Name;
+
+                // Degenerate case
+                if (string.IsNullOrEmpty(name))
+                {
+                    name = "?";
+                }
+
+                // Handle generic types
+                if (t.GetTypeInfo().IsGenericType)
+                {
+                    // strip off the CLR generic type marker if it exists
+                    var indexOfGenericTypeMarker = name.LastIndexOf('`');
+                    if (indexOfGenericTypeMarker >= 0)
+                    {
+                        name = name.Substring(0, indexOfGenericTypeMarker);
+                        name += "<" + string.Join(", ", t.GetGenericArguments().Select(PrettyPrintTypeName)) + ">";
+                    }
+                }
+
+                // Handle nested types
+                if (!t.IsGenericParameter)
+                {
+                    var containerType = t.DeclaringType;
+                    if (containerType != null)
+                    {
+                        name = PrettyPrintTypeName(containerType) + "." + name;
+                    }
+                }
+
+                return name;
+            }
+            catch
+            {
+                // If anything at all goes wrong, fall back to the full type name so that we don't crash the server.
+                return t.FullName;
+            }
+        }
+
+        private static void SplitTypeIntoPrefixAndFriendlyName(Type type, out string prefix, out string friendlyName)
+        {
+            prefix = type.Namespace;
+            friendlyName = PrettyPrintTypeName(type);
+
+            if (!string.IsNullOrEmpty(friendlyName) && !string.IsNullOrEmpty(prefix))
+            {
+                prefix += ".";
+            }
+        }
+
+        private static string GenerateFooterEncoded(IRuntimeEnvironment environment)
+        {
+            var runtimeType = HtmlEncodeAndReplaceLineBreaks(environment.RuntimeType);
+#if DNXCORE50
+            var systemRuntimeAssembly = typeof(System.ComponentModel.DefaultValueAttribute).GetTypeInfo().Assembly;
+            var assemblyVersion = new AssemblyName(systemRuntimeAssembly.FullName).Version.ToString();
+            var clrVersion = HtmlEncodeAndReplaceLineBreaks(assemblyVersion);
+#else
+            var clrVersion = HtmlEncodeAndReplaceLineBreaks(Environment.Version.ToString());
+#endif
+            var runtimeArch = HtmlEncodeAndReplaceLineBreaks(environment.RuntimeArchitecture);
+            var dnxVersion = HtmlEncodeAndReplaceLineBreaks(environment.RuntimeVersion);
+            var currentAssembly = typeof(StartupExceptionPage).GetTypeInfo().Assembly;
+            var currentAssemblyVersion = currentAssembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>().InformationalVersion;
+            currentAssemblyVersion = HtmlEncodeAndReplaceLineBreaks(currentAssemblyVersion);
+
+            var os = HtmlEncodeAndReplaceLineBreaks(environment.OperatingSystem);
+            var osVersion = HtmlEncodeAndReplaceLineBreaks(environment.OperatingSystemVersion);
+
+            return string.Format(CultureInfo.InvariantCulture, _errorFooterFormatString, runtimeType, clrVersion,
+                runtimeArch, dnxVersion, currentAssemblyVersion, os, osVersion);
+        }
+
+        private static string HtmlEncodeAndReplaceLineBreaks(string input)
+        {
+            if (string.IsNullOrEmpty(input))
+            {
+                return string.Empty;
+            }
+
+            // Split on line breaks before passing it through the encoder.
+            // We use the static default encoder since we can't depend on DI in the error handling logic.
+            return string.Join("<br />" + Environment.NewLine,
+                input.Split(new[] { "\r\n" }, StringSplitOptions.None)
+                .SelectMany(s => s.Split(new[] { '\r', '\n' }, StringSplitOptions.None))
+                .Select(HtmlEncoder.Default.HtmlEncode));
+        }
+
+        private static void WriteException(Exception ex, StringBuilder builder, ref bool wasFailingCallSiteSourceWritten)
+        {
+            string inlineSourceDiv = null;
+
+            // First, build the stack trace
+            var firstStackFrame = true;
+            var stackTraceBuilder = new StringBuilder();
+            var needFileInfo = true;
+            foreach (var frame in new StackTrace(ex, needFileInfo).GetFrames() ?? Enumerable.Empty<StackFrame>())
+            {
+                if (!firstStackFrame)
+                {
+                    stackTraceBuilder.Append("<br />");
+                }
+                firstStackFrame = false;
+                var thisFrameLine = BuildLineForStackFrame(frame);
+                stackTraceBuilder.AppendLine(thisFrameLine);
+
+                // Try to include the source code in the error page if we can.
+                if (!wasFailingCallSiteSourceWritten && inlineSourceDiv == null)
+                {
+                    inlineSourceDiv = BuildCodeSnippetDiv(frame);
+                    if (inlineSourceDiv != null)
+                    {
+                        wasFailingCallSiteSourceWritten = true;
+                    }
+                }
+            }
+
+            // Finally, build the rest of the <div>
+            builder.AppendFormat(CultureInfo.InvariantCulture, _errorExceptionFormatString,
+                HtmlEncodeAndReplaceLineBreaks(ex.GetType().FullName),
+                HtmlEncodeAndReplaceLineBreaks(ex.Message),
+                inlineSourceDiv,
+                stackTraceBuilder);
+        }
+
+        private static void WriteRawExceptionDetails(string linkText, IEnumerable<string> lines, StringBuilder rawExceptionDetails)
+        {
+            rawExceptionDetails
+                .AppendLine("<div class=\"rawExceptionBlock\">")
+                .AppendFormat($"    <div><a href=\"#\" onclick=\"javascript: showRawException(); return false;\">{linkText}</a></div>")
+                .AppendLine()
+                .AppendLine("    <div id=\"rawException\">")
+                .Append("        <pre>");
+
+            foreach (var line in lines)
+            {
+                rawExceptionDetails.AppendLine(line);
+            }
+
+            rawExceptionDetails
+                .AppendLine("</pre>")
+                .AppendLine("    </div>")
+                .AppendLine("</div>");
+        }
+
+        private static void WriteException(ICompilationException compilationException,
+                                           StringBuilder builder,
+                                           ref bool wasSourceCodeWrittenOntoPage)
+        {
+            var totalErrorsShown = 0;
+            var inlineSourceDiv = new StringBuilder();
+            var firstStackFrame = true;
+            foreach (var failure in compilationException.CompilationFailures)
+            {
+                if (firstStackFrame)
+                {
+                    firstStackFrame = false;
+                }
+                else
+                {
+                    inlineSourceDiv.AppendLine("<br/>");
+                }
+
+                BuildCodeSnippetDiv(failure, inlineSourceDiv, ref totalErrorsShown);
+            }
+
+            wasSourceCodeWrittenOntoPage = totalErrorsShown > 0;
+
+            builder.AppendFormat(CultureInfo.InvariantCulture,
+                                 _compilationExceptionFormatString,
+                                 inlineSourceDiv);
+        }
+
+        private static void WriteMessage(string message, StringBuilder builder)
+        {
+            // Build the <div>
+            builder.AppendFormat(CultureInfo.InvariantCulture, _errorMessageFormatString,
+                HtmlEncodeAndReplaceLineBreaks(message));
+        }
+
+        private static IEnumerable<Exception> FlattenAndReverseExceptionTree(Exception ex)
+        {
+            var list = new List<Exception>();
+            for (; ex != null; ex = ex.InnerException)
+            {
+                list.Add(ex);
+            }
+            list.Reverse();
+            return list;
+        }
+    }
+}

src/Microsoft.AspNet.Hosting/WebHostBuilder.cs

@@ -39,6 +39,7 @@ namespace Microsoft.AspNet.Hosting
         private StartupMethods _startup;
         private Type _startupType;
         private string _startupAssemblyName;
+        private readonly bool _captureStartupErrors;
 
         // Only one of these should be set
         private string _serverFactoryLocation;
@@ -50,6 +51,11 @@ namespace Microsoft.AspNet.Hosting
         }
 
         public WebHostBuilder(IServiceProvider services, IConfiguration config)
+            : this(services, config: config, captureStartupErrors: false)
+        {
+        }
+
+        public WebHostBuilder(IServiceProvider services, IConfiguration config, bool captureStartupErrors)
         {
             if (services == null)
             {
@@ -65,6 +71,7 @@ namespace Microsoft.AspNet.Hosting
             _loggerFactory = new LoggerFactory();
             _services = services;
             _config = config;
+            _captureStartupErrors = captureStartupErrors;
         }
 
         private IServiceCollection BuildHostingServices()
@@ -117,8 +124,7 @@ namespace Microsoft.AspNet.Hosting
             var startupLoader = hostingContainer.GetRequiredService<IStartupLoader>();
 
             _hostingEnvironment.Initialize(appEnvironment.ApplicationBasePath, _config?[EnvironmentKey] ?? _config?[OldEnvironmentKey]);
-
-            var engine = new HostingEngine(hostingServices, startupLoader, _config);
+            var engine = new HostingEngine(hostingServices, startupLoader, _config, _captureStartupErrors);
 
             // Only one of these should be set, but they are used in priority
             engine.ServerFactory = _serverFactory;

src/Microsoft.AspNet.Hosting/compiler/resources/Compilation_Exception.html

@@ -0,0 +1,6 @@
+<div class="message">
+    <span class="light exception">One or more compilation errors occured:</span><br />
+    <div class="stacktrace">
+        {0}
+    </div>
+</div>

src/Microsoft.AspNet.Hosting/compiler/resources/GenericError.html

@@ -0,0 +1,151 @@
+<!DOCTYPE html>
+
+<html>
+<head>
+    <meta charset="utf-8" />
+    <title>500 Internal Server Error</title>
+    <style type="text/css">
+        body {
+            background-color: white;
+            color: #111111;
+            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
+            margin: 2em 4em;
+        }
+
+        footer a {
+            color: darkblue;
+            text-decoration: none;
+            font-weight: bolder;
+        }
+
+        #header {
+            margin-bottom: 2.5em;
+        }
+
+        .stacktrace pre {
+            display: inline;
+        }
+
+        .faded {
+            color: #999999;
+            font-weight: normal;
+        }
+
+        div.message {
+            margin-top: 2.5em;
+            padding: 0.3em 1em;
+            border-left: 0.25em solid red;
+        }
+
+        .light {
+            font-size: 1.3em;
+            font-weight: lighter;
+        }
+
+        .heavy {
+            font-size: 1.5em;
+        }
+
+        .exception {
+            color: red;
+        }
+
+        .stacktrace {
+            padding-top: 0.3em;
+            padding-left: 2em;
+            display: block;
+            font-weight: bold;
+        }
+
+        .codeSnippet {
+            margin-left: 2em;
+            margin-top: 1em;
+            margin-bottom: 1em;
+            display: inline-block;
+            border-top: 0.2em solid #cccccc;
+            border-bottom: 0.2em solid #cccccc;
+            color: black;
+        }
+
+        .codeSnippet div:nth-of-type(2n) {
+            background-color: #f0f0f0;
+        }
+
+        .codeSnippet div:nth-of-type(2n + 1) {
+            background-color: #f6f6f6;
+        }
+
+        .codeSnippet .error-message {
+            color: red;
+            font-weight: normal;
+        }
+
+        .codeSnippet div.filename {
+            font-weight: bold;
+            background-color: white;
+            margin: 0.6em;
+        }
+
+        .codeSnippet div.line {
+            padding: 0.2em;
+            line-height: 1em;
+        }
+
+        .codeSnippet div.line .line-number {
+            color: #999999;
+            text-align: right;
+            margin-right: 0.5em;
+        }
+
+        .codeSnippet div.error {
+            color: red;
+            font-weight: bolder;
+            background-color: #ffeda7;
+        }
+
+        .codeSnippet code {
+            white-space: pre;
+        }
+
+        .rawExceptionBlock {
+            margin-top: 1em;
+            margin-left: 1em;
+        }
+
+        #rawException {
+            display: none;
+        }
+
+        footer {
+            margin-top: 2em;
+            font-size: smaller;
+            font-weight: lighter;
+        }
+    </style>
+    <script type="text/javascript">
+        function showRawException() {
+            var div = document.getElementById('rawException');
+            div.style.display = 'inline-block';
+            div.scrollIntoView(true);
+        }
+    </script>
+</head>
+<body>
+
+    <div id="header">
+        <div style="font-size: 6em; display: inline-block;">
+            :(
+        </div>
+        <div style="display: inline-block; padding-left: 3em;">
+            <span style="font-size: 2em;">Oops.</span><br />
+            <span style="font-size: 1.65em; font-weight: lighter;">500 Internal Server Error</span>
+        </div>
+    </div>
+
+    [[[0]]]
+
+    [[[1]]]
+
+    [[[2]]]
+</body>
+</html>

src/Microsoft.AspNet.Hosting/compiler/resources/GenericError_Exception.html

@@ -0,0 +1,8 @@
+<div class="message">
+    <span class="light exception">{0}</span><br />
+    <span class="heavy">{1}</span><br />
+    {2}
+    <div class="stacktrace">
+        {3}
+    </div>
+</div>

src/Microsoft.AspNet.Hosting/compiler/resources/GenericError_Footer.html

@@ -0,0 +1,3 @@
+<footer>
+    .NET Framework {0} version {1}&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;DNX {2} version {3}&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;Microsoft.AspNet.Hosting version {4}&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;{5} {6}&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;<a href="http://go.microsoft.com/fwlink/?LinkId=517394">Need help?</a>
+</footer>

src/Microsoft.AspNet.Hosting/compiler/resources/GenericError_Message.html

@@ -0,0 +1,3 @@
+<div class="message">
+    <span class="heavy">{0}</span><br />
+</div>

src/Microsoft.AspNet.Hosting/project.json

@@ -18,6 +18,7 @@
         "Microsoft.Framework.Configuration.Json": "1.0.0-*",
         "Microsoft.Framework.DependencyInjection": "1.0.0-*",
         "Microsoft.Framework.Logging": "1.0.0-*",
+        "Microsoft.Dnx.Compilation.Abstractions": "1.0.0-*",
         "Microsoft.Dnx.Runtime.Abstractions": "1.0.0-*",
         "Newtonsoft.Json": "6.0.6",
         "System.Diagnostics.Tracing.Telemetry": "4.0.0-beta-*"
@@ -30,7 +31,8 @@
         },
         "dnxcore50": {
             "dependencies": {
-                "System.Console": "4.0.0-beta-*"
+                "System.Console": "4.0.0-beta-*",
+                "System.Diagnostics.StackTrace": "4.0.1-beta-*"
             }
         }
     }

test/Microsoft.AspNet.Hosting.Tests/Fakes/RuntimeEnvironment.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.Dnx.Runtime;
+
+namespace Microsoft.AspNet.Hosting.Fakes
+{
+    public class RuntimeEnvironment : IRuntimeEnvironment
+    {
+        public string OperatingSystem { get; } = "TestOs";
+
+        public string OperatingSystemVersion { get; } = "TestOsVersion";
+
+        public string RuntimeArchitecture { get; } = "TestArch";
+
+        public string RuntimeType { get; } = "TestRuntime";
+
+        public string RuntimeVersion { get; } = "TestRuntimeVersion";
+    }
+}

test/Microsoft.AspNet.Hosting.Tests/Fakes/StartupConfigureServicesThrows.cs

@@ -0,0 +1,22 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNet.Builder;
+using Microsoft.Framework.DependencyInjection;
+
+namespace Microsoft.AspNet.Hosting.Fakes
+{
+    public class StartupConfigureServicesThrows
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+            throw new Exception("Exception from ConfigureServices");
+        }
+
+        public void Configure(IApplicationBuilder builder)
+        {
+
+        }
+    }
+}

test/Microsoft.AspNet.Hosting.Tests/Fakes/StartupConfigureThrows.cs

@@ -0,0 +1,21 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNet.Builder;
+using Microsoft.Framework.DependencyInjection;
+
+namespace Microsoft.AspNet.Hosting.Fakes
+{
+    public class StartupConfigureThrows
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+        }
+
+        public void Configure(IApplicationBuilder builder)
+        {
+            throw new Exception("Exception from Configure");
+        }
+    }
+}

test/Microsoft.AspNet.Hosting.Tests/Fakes/StartupCtorThrows.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNet.Builder;
+
+namespace Microsoft.AspNet.Hosting.Fakes
+{
+    public class StartupCtorThrows
+    {
+        public StartupCtorThrows()
+        {
+            throw new Exception("Exception from constructor");
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+        }
+    }
+}

test/Microsoft.AspNet.Hosting.Tests/Fakes/StartupStaticCtorThrows.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNet.Builder;
+
+namespace Microsoft.AspNet.Hosting.Fakes
+{
+    public class StartupStaticCtorThrows
+    {
+        static StartupStaticCtorThrows()
+        {
+            throw new Exception("Exception from static constructor");
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+        }
+    }
+}

test/Microsoft.AspNet.Hosting.Tests/WebHostBuilderTests.cs

@@ -1,8 +1,17 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Hosting.Fakes;
 using Microsoft.AspNet.Hosting.Internal;
+using Microsoft.AspNet.Hosting.Server;
+using Microsoft.AspNet.Http.Features;
+using Microsoft.AspNet.Http.Internal;
 using Microsoft.Dnx.Runtime.Infrastructure;
+using Microsoft.Framework.Configuration;
 using Xunit;
 
 namespace Microsoft.AspNet.Hosting
@@ -29,6 +38,110 @@ namespace Microsoft.AspNet.Hosting
             Assert.Equal("MyStartupAssembly", engine.StartupAssemblyName);
         }
 
-        private WebHostBuilder CreateWebHostBuilder() => new WebHostBuilder(CallContextServiceLocator.Locator.ServiceProvider);
+        [Fact]
+        public async Task StartupMissing_Fallback()
+        {
+            var builder = CreateWebHostBuilder();
+            var serverFactory = new TestServerFactory();
+            var engine = (HostingEngine)builder.UseServer(serverFactory).UseStartup("MissingStartupAssembly").Build();
+            using (engine.Start())
+            {
+                await AssertResponseContains(serverFactory.Application, "MissingStartupAssembly");
+            }
+        }
+
+        [Fact]
+        public async Task StartupStaticCtorThrows_Fallback()
+        {
+            var builder = CreateWebHostBuilder();
+            var serverFactory = new TestServerFactory();
+            var engine = (HostingEngine)builder.UseServer(serverFactory).UseStartup<StartupStaticCtorThrows>().Build();
+            using (engine.Start())
+            {
+                await AssertResponseContains(serverFactory.Application, "Exception from static constructor");
+            }
+        }
+
+        [Fact]
+        public async Task StartupCtorThrows_Fallback()
+        {
+            var builder = CreateWebHostBuilder();
+            var serverFactory = new TestServerFactory();
+            var engine = (HostingEngine)builder.UseServer(serverFactory).UseStartup<StartupCtorThrows>().Build();
+            using (engine.Start())
+            {
+                await AssertResponseContains(serverFactory.Application, "Exception from constructor");
+            }
+        }
+
+        [Fact]
+        public async Task StartupConfigureServicesThrows_Fallback()
+        {
+            var builder = CreateWebHostBuilder();
+            var serverFactory = new TestServerFactory();
+            var engine = (HostingEngine)builder.UseServer(serverFactory).UseStartup<StartupConfigureServicesThrows>().Build();
+            using (engine.Start())
+            {
+                await AssertResponseContains(serverFactory.Application, "Exception from ConfigureServices");
+            }
+        }
+
+        [Fact]
+        public async Task StartupConfigureThrows_Fallback()
+        {
+            var builder = CreateWebHostBuilder();
+            var serverFactory = new TestServerFactory();
+            var engine = (HostingEngine)builder.UseServer(serverFactory).UseStartup<StartupConfigureServicesThrows>().Build();
+            using (engine.Start())
+            {
+                await AssertResponseContains(serverFactory.Application, "Exception from Configure");
+            }
+        }
+
+        private WebHostBuilder CreateWebHostBuilder()
+        {
+            var vals = new Dictionary<string, string>
+            {
+                { "server", "Microsoft.AspNet.Hosting.Tests" },
+                { "Hosting:DetailedErrors", "true" },
+            };
+            var builder = new ConfigurationBuilder()
+                .AddInMemoryCollection(vals);
+            var config = builder.Build();
+            return new WebHostBuilder(CallContextServiceLocator.Locator.ServiceProvider, config, captureStartupErrors: true);
+        }
+
+        private async Task AssertResponseContains(Func<IFeatureCollection, Task> app, string expectedText)
+        {
+            var httpContext = new DefaultHttpContext();
+            httpContext.Response.Body = new MemoryStream();
+            await app(httpContext.Features);
+            httpContext.Response.Body.Seek(0, SeekOrigin.Begin);
+            var bodyText = new StreamReader(httpContext.Response.Body).ReadToEnd();
+            Assert.Contains(expectedText, bodyText);
+        }
+
+        private class TestServerFactory : IServerFactory
+        {
+            public Func<IFeatureCollection, Task> Application { get; set; }
+
+            public IFeatureCollection Initialize(IConfiguration configuration)
+            {
+                return new FeatureCollection();
+            }
+
+            public IDisposable Start(IFeatureCollection serverFeatures, Func<IFeatureCollection, Task> application)
+            {
+                Application = application;
+                return new Disposable();
+            }
+
+            private class Disposable : IDisposable
+            {
+                public void Dispose()
+                {
+                }
+            }
+        }
     }
 }