Tolerate leading "~/" or "/" (#499)

Addresses #441

src/Microsoft.AspNetCore.Dispatcher/Patterns/RoutePatternBuilder.cs

@@ -58,8 +58,7 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
                 var segment = PathSegments[i];
                 for (var j = 0; j < segment.Parts.Count; j++)
                 {
-                    var parameter = segment.Parts[j] as RoutePatternParameter;
-                    if (parameter != null)
+                    if (segment.Parts[j] is RoutePatternParameter parameter)
                     {
                         parameters.Add(parameter);
                     }

src/Microsoft.AspNetCore.Dispatcher/Patterns/RoutePatternParser.cs

@@ -33,13 +33,9 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
                 throw new ArgumentNullException(nameof(pattern));
             }
 
-            if (IsInvalidPattern(pattern))
-            {
-                throw new RoutePatternException(pattern, Resources.TemplateRoute_InvalidRouteTemplate);
-            }
+            var trimmedPattern = TrimPrefix(pattern);
 
-            var context = new TemplateParserContext(pattern);
-            var builder = RoutePatternBuilder.Create(pattern);
+            var context = new TemplateParserContext(trimmedPattern);
             var segments = new List<RoutePatternPathSegment>();
 
             while (context.MoveNext())
@@ -69,6 +65,7 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
 
             if (IsAllValid(context, segments))
             {
+                var builder = RoutePatternBuilder.Create(pattern);
                 for (var i = 0; i < segments.Count; i++)
                 {
                     builder.PathSegments.Add(segments[i]);
@@ -257,7 +254,7 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
         private static bool ParseLiteral(TemplateParserContext context, List<RoutePatternPart> parts)
         {
             context.Mark();
-            
+
             while (true)
             {
                 if (context.Current == Separator)
@@ -469,10 +466,21 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
             return true;
         }
 
-        private static bool IsInvalidPattern(string routeTemplate)
+        private static string TrimPrefix(string routePattern)
         {
-            return routeTemplate.StartsWith("~", StringComparison.Ordinal) ||
-                   routeTemplate.StartsWith("/", StringComparison.Ordinal);
+            if (routePattern.StartsWith("~/", StringComparison.Ordinal))
+            {
+                return routePattern.Substring(2);
+            }
+            else if (routePattern.StartsWith("/", StringComparison.Ordinal))
+            {
+                return routePattern.Substring(1);
+            }
+            else if (routePattern.StartsWith("~", StringComparison.Ordinal))
+            {
+                throw new RoutePatternException(routePattern, Resources.TemplateRoute_InvalidRouteTemplate);
+            }
+            return routePattern;
         }
 
         [DebuggerDisplay("{DebuggerToString()}")]
@@ -555,8 +563,8 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
                 }
                 else if (_mark.HasValue)
                 {
-                    return _template.Substring(0, _mark.Value) + 
-                        "|" + 
+                    return _template.Substring(0, _mark.Value) +
+                        "|" +
                         _template.Substring(_mark.Value, _index - _mark.Value) +
                         "|" +
                         _template.Substring(_index);

src/Microsoft.AspNetCore.Dispatcher/Properties/Resources.Designer.cs

@@ -221,7 +221,7 @@ namespace Microsoft.AspNetCore.Dispatcher
             => string.Format(CultureInfo.CurrentCulture, GetString("TemplateRoute_InvalidParameterName"), p0);
 
         /// <summary>
-        /// The route template cannot start with a '/' or '~' character.
+        /// The route template cannot start with a '~' character.
         /// </summary>
         internal static string TemplateRoute_InvalidRouteTemplate
         {
@@ -229,7 +229,7 @@ namespace Microsoft.AspNetCore.Dispatcher
         }
 
         /// <summary>
-        /// The route template cannot start with a '/' or '~' character.
+        /// The route template cannot start with a '~' character.
         /// </summary>
         internal static string FormatTemplateRoute_InvalidRouteTemplate()
             => GetString("TemplateRoute_InvalidRouteTemplate");

src/Microsoft.AspNetCore.Dispatcher/Resources.resx

@@ -164,7 +164,7 @@
     <value>The route parameter name '{0}' is invalid. Route parameter names must be non-empty and cannot contain these characters: '{{', '}}', '/'. The '?' character marks a parameter as optional, and can occur only at the end of the parameter. The '*' character marks a parameter as catch-all, and can occur only at the start of the parameter.</value>
   </data>
   <data name="TemplateRoute_InvalidRouteTemplate" xml:space="preserve">
-    <value>The route template cannot start with a '/' or '~' character.</value>
+    <value>The route template cannot start with a '~' character unless followed by a '/'.</value>
   </data>
   <data name="TemplateRoute_MismatchedParameter" xml:space="preserve">
     <value>There is an incomplete parameter in the route template. Check that each '{' character has a matching '}' character.</value>

src/Microsoft.AspNetCore.Dispatcher/RoutePatternBinder.cs

@@ -74,8 +74,7 @@ namespace Microsoft.AspNetCore.Dispatcher
                 // If it's a parameter subsegment, examine the current value to see if it matches the new value
                 var parameterName = parameter.Name;
 
-                object newParameterValue;
-                var hasNewParameterValue = values.TryGetValue(parameterName, out newParameterValue);
+                var hasNewParameterValue = values.TryGetValue(parameterName, out var newParameterValue);
 
                 object currentParameterValue = null;
                 var hasCurrentParameterValue = ambientValues != null &&
@@ -179,8 +178,7 @@ namespace Microsoft.AspNetCore.Dispatcher
                     continue;
                 }
 
-                object value;
-                if (values.TryGetValue(filter.Key, out value))
+                if (values.TryGetValue(filter.Key, out var value))
                 {
                     if (!RoutePartsEqual(value, filter.Value))
                     {

src/Microsoft.AspNetCore.Routing/Template/TemplateParser.cs

@@ -17,7 +17,7 @@ namespace Microsoft.AspNetCore.Routing.Template
 
             try
             {
-                var inner = Microsoft.AspNetCore.Dispatcher.Patterns.RoutePattern.Parse(routeTemplate);
+                var inner = RoutePattern.Parse(routeTemplate);
                 return new RouteTemplate(inner);
             }
             catch (ArgumentException ex) when (ex.InnerException is RoutePatternException)

test/Microsoft.AspNetCore.Dispatcher.Test/Patterns/RoutePatternParserTest.cs

@@ -622,12 +622,16 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
                 "a literal string.");
         }
 
-        [Fact]
-        public void InvalidTemplate_CannotStartWithSlash()
+        [Theory]
+        [InlineData("/foo")]
+        [InlineData("~/foo")]
+        public void ValidTemplate_CanStartWithSlashOrTildeSlash(string routePattern)
         {
-            ExceptionAssert.Throws<RoutePatternException>(
-                () => RoutePatternParser.Parse("/foo"),
-                "The route template cannot start with a '/' or '~' character.");
+            // Arrange & Act
+            var pattern = RoutePatternParser.Parse(routePattern);
+
+            // Assert
+            Assert.Equal(routePattern, pattern.RawText);
         }
 
         [Fact]
@@ -635,7 +639,7 @@ namespace Microsoft.AspNetCore.Dispatcher.Patterns
         {
             ExceptionAssert.Throws<RoutePatternException>(
                 () => RoutePatternParser.Parse("~foo"),
-                "The route template cannot start with a '/' or '~' character.");
+                "The route template cannot start with a '~' character unless followed by a '/'.");
         }
 
         [Fact]

test/Microsoft.AspNetCore.Dispatcher.Test/RoutePatternBinderTest.cs

@@ -5,7 +5,6 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text.Encodings.Web;
-using Microsoft.AspNetCore.Dispatcher.Patterns;
 using Microsoft.Extensions.ObjectPool;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Xunit;
@@ -651,6 +650,28 @@ namespace Microsoft.AspNetCore.Dispatcher
                 UrlEncoder.Default);
         }
 
+        [Fact]
+        public void GetUrlWithLeadingTildeSlash()
+        {
+            RunTest(
+                "~/foo",
+                null,
+                null,
+                new DispatcherValueCollection(new { }),
+                "/UrlEncode[[foo]]");
+        }
+
+        [Fact]
+        public void GetUrlWithLeadingSlash()
+        {
+            RunTest(
+                "/foo",
+                null,
+                null,
+                new DispatcherValueCollection(new { }),
+                "/UrlEncode[[foo]]");
+        }
+
         [Fact]
         public void GetUrlWithCatchAllWithValue()
         {
@@ -1192,8 +1213,7 @@ namespace Microsoft.AspNetCore.Dispatcher
 
                     foreach (var kvp in expectedParts.Parameters)
                     {
-                        string value;
-                        Assert.True(actualParts.Parameters.TryGetValue(kvp.Key, out value));
+                        Assert.True(actualParts.Parameters.TryGetValue(kvp.Key, out var value));
                         Assert.Equal(kvp.Value, value);
                     }
                 }

test/Microsoft.AspNetCore.Routing.Tests/Template/TemplateParserTests.cs

@@ -765,13 +765,16 @@ namespace Microsoft.AspNetCore.Routing.Template.Tests
                 "Parameter name: routeTemplate");
         }
 
-        [Fact]
-        public void InvalidTemplate_CannotStartWithSlash()
+        [Theory]
+        [InlineData("/foo")]
+        [InlineData("~/foo")]
+        public void ValidTemplate_CanStartWithSlashOrTildeSlash(string routeTemplate)
         {
-            ExceptionAssert.Throws<ArgumentException>(
-                () => TemplateParser.Parse("/foo"),
-                "The route template cannot start with a '/' or '~' character." + Environment.NewLine +
-                "Parameter name: routeTemplate");
+            // Arrange & Act
+            var pattern = TemplateParser.Parse(routeTemplate);
+
+            // Assert
+            Assert.Equal(routeTemplate, pattern.TemplateText);
         }
 
         [Fact]
@@ -779,7 +782,7 @@ namespace Microsoft.AspNetCore.Routing.Template.Tests
         {
             ExceptionAssert.Throws<ArgumentException>(
                 () => TemplateParser.Parse("~foo"),
-                "The route template cannot start with a '/' or '~' character." + Environment.NewLine +
+                "The route template cannot start with a '~' character unless followed by a '/'." + Environment.NewLine +
                 "Parameter name: routeTemplate");
         }