[Fixes #8250] Improve the error message when TEntryPoint is not in an entry point assembly

Validates that the type used as a generic argument in WebApplicaitonFactory is contained within the entry point to assembly and throws InvalidOperationException otherwise

src/Microsoft.AspNetCore.Mvc.Testing/Properties/Resources.Designer.cs

@@ -38,6 +38,20 @@ namespace Microsoft.AspNetCore.Mvc.Testing
         internal static string FormatMissingDepsFile(object p0, object p1)
             => string.Format(CultureInfo.CurrentCulture, GetString("MissingDepsFile"), p0, p1);
 
+        /// <summary>
+        /// The provided Type '{0}' does not belong to an assembly with an entry point. A common cause for this error is providing a Type from a class library.
+        /// </summary>
+        internal static string InvalidAssemblyEntryPoint
+        {
+            get => GetString("InvalidAssemblyEntryPoint");
+        }
+
+        /// <summary>
+        /// The provided Type '{0}' does not belong to an assembly with an entry point. A common cause for this error is providing a Type from a class library.
+        /// </summary>
+        internal static string FormatInvalidAssemblyEntryPoint(string p0)
+            => string.Format(CultureInfo.CurrentCulture, GetString("InvalidAssemblyEntryPoint"), p0);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Mvc.Testing/Resources.resx

@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="InvalidAssemblyEntryPoint" xml:space="preserve">
+    <value>The provided Type '{0}' does not belong to an assembly with an entry point. A common cause for this error is providing a Type from a class library.</value>
+  </data>
   <data name="MissingCreateWebHostBuilderMethod" xml:space="preserve">
     <value>No method 'public static {0} CreateWebHostBuilder(string[] args)' found on '{1}'. Alternatively, {2} can be extended and 'protected virtual {0} {3}()' can be overridden to provide your own {0} instance.</value>
   </data>

src/Microsoft.AspNetCore.Mvc.Testing/WebApplicationFactory.cs

@@ -251,6 +251,11 @@ namespace Microsoft.AspNetCore.Mvc.Testing
 
         private void EnsureDepsFile()
         {
+            if (typeof(TEntryPoint).Assembly.EntryPoint == null)
+            {
+                throw new InvalidOperationException(Resources.FormatInvalidAssemblyEntryPoint(typeof(TEntryPoint).Name));
+            }
+
             var depsFileName = $"{typeof(TEntryPoint).Assembly.GetName().Name}.deps.json";
             var depsFile = new FileInfo(Path.Combine(AppContext.BaseDirectory, depsFileName));
             if (!depsFile.Exists)

test/Microsoft.AspNetCore.Mvc.FunctionalTests/TestingInfrastructureTests.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Formatting;
@@ -11,6 +12,7 @@ using Microsoft.AspNetCore.Mvc.Testing;
 using Microsoft.AspNetCore.Mvc.Testing.Handlers;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.DependencyInjection;
+using RazorPagesClassLibrary;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.FunctionalTests
@@ -39,6 +41,15 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("Test", response);
         }
 
+        [Fact]
+        public void TestingInfrastructure_CreateClientThrowsInvalidOperationForNonEntryPoint()
+        {
+            var factory = new WebApplicationFactory<ClassLibraryStartup>();
+            var ex = Assert.Throws<InvalidOperationException>(() => factory.CreateClient());
+            Assert.Equal($"The provided Type '{typeof(RazorPagesClassLibrary.ClassLibraryStartup).Name}' does not belong to an assembly with an entry point. A common cause for this error is providing a Type from a class library.",
+               ex.Message);
+        }
+
         [Fact]
         public async Task TestingInfrastructure_RedirectHandlerWorksWithPreserveMethod()
         {

test/WebSites/RazorPagesClassLibrary/ClassLibraryStartup.cs

@@ -0,0 +1,21 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace RazorPagesClassLibrary
+{
+    /// <summary>
+    /// Empty Startup for testing a Startup file within a class library
+    /// </summary>
+    public class ClassLibraryStartup
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+
+        }
+    }
+}