Implement a 'required' constraint

This is useful for a variety of interesting scenarios in link generation
where a default value doesn't appear in the route template as a parameter.

This can be used to implement the desired behavior for areas - where the
'area' key is sticky.

samples/RoutingSample.Web/RoutingSample.Web.kproj

@@ -20,6 +20,9 @@
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x86'">
     <DevelopmentServerPort>28778</DevelopmentServerPort>
   </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
+    <DevelopmentServerPort>22209</DevelopmentServerPort>
+  </PropertyGroup>
   <ItemGroup>
     <Content Include="project.json" />
     <Content Include="web.config" />

src/Microsoft.AspNet.Routing/Constraints/RequiredRouteConstraint.cs

@@ -0,0 +1,39 @@
+// Copyright (c) Microsoft Open Technologies, Inc. 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.Globalization;
+using Microsoft.AspNet.Http;
+
+namespace Microsoft.AspNet.Routing.Constraints
+{
+    /// <summary>
+    /// Constraints a route parameter that must have a value.
+    /// </summary>
+    /// <remarks>
+    /// This constraint is primarily used to enforce that a non-parameter value is present during 
+    /// URL generation.
+    /// </remarks>
+    public class RequiredRouteConstraint : IRouteConstraint
+    {
+        /// <inheritdoc />
+        public bool Match(
+            [NotNull]HttpContext httpContext, 
+            [NotNull]IRouter route, 
+            [NotNull]string routeKey, 
+            [NotNull]IDictionary<string, object> values, 
+            RouteDirection routeDirection)
+        {
+            object value;
+            if (values.TryGetValue(routeKey, out value) && value != null)
+            {
+                // In routing the empty string is equivalent to null, which is equivalent to an unset value.
+                var valueString = Convert.ToString(value, CultureInfo.InvariantCulture);
+                return !string.IsNullOrEmpty(valueString);
+            }
+
+            return false;
+        }
+    }
+}

src/Microsoft.AspNet.Routing/Microsoft.AspNet.Routing.kproj

@@ -41,6 +41,7 @@
     <Compile Include="Constraints\BoolRouteConstraint.cs" />
     <Compile Include="Constraints\IntRouteConstraint.cs" />
     <Compile Include="Constraints\RegexRouteConstraint.cs" />
+    <Compile Include="Constraints\RequiredRouteConstraint.cs" />
     <Compile Include="DefaultInlineConstraintResolver.cs" />
     <Compile Include="IInlineConstraintResolver.cs" />
     <Compile Include="INamedRouter.cs" />

test/Microsoft.AspNet.Routing.Tests/Constraints/RequiredRouteConstraintTests.cs

@@ -0,0 +1,98 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+#if NET45
+
+using System;
+using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Routing.Constraints;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNet.Routing.Tests
+{
+    public class RequiredRouteConstraintTests
+    {
+        [Theory]
+        [InlineData(RouteDirection.IncomingRequest)]
+        [InlineData(RouteDirection.UrlGeneration)]
+        public void RequiredRouteConstraint_NoValue(RouteDirection direction)
+        {
+            // Arrange
+            var constraint = new RequiredRouteConstraint();
+
+            // Act
+            var result = constraint.Match(
+                Mock.Of<HttpContext>(), 
+                Mock.Of<IRouter>(), 
+                "area", 
+                new RouteValueDictionary(new { controller = "Home", action = "Index" }),
+                direction);
+
+            // Assert
+            Assert.False(result);
+        }
+
+        [Theory]
+        [InlineData(RouteDirection.IncomingRequest)]
+        [InlineData(RouteDirection.UrlGeneration)]
+        public void RequiredRouteConstraint_Null(RouteDirection direction)
+        {
+            // Arrange
+            var constraint = new RequiredRouteConstraint();
+
+            // Act
+            var result = constraint.Match(
+                Mock.Of<HttpContext>(),
+                Mock.Of<IRouter>(),
+                "area",
+                new RouteValueDictionary(new { controller = "Home", action = "Index", area = (string)null }),
+                direction);
+
+            // Assert
+            Assert.False(result);
+        }
+
+        [Theory]
+        [InlineData(RouteDirection.IncomingRequest)]
+        [InlineData(RouteDirection.UrlGeneration)]
+        public void RequiredRouteConstraint_EmptyString(RouteDirection direction)
+        {
+            // Arrange
+            var constraint = new RequiredRouteConstraint();
+
+            // Act
+            var result = constraint.Match(
+                Mock.Of<HttpContext>(),
+                Mock.Of<IRouter>(),
+                "area",
+                new RouteValueDictionary(new { controller = "Home", action = "Index", area = string.Empty}),
+                direction);
+
+            // Assert
+            Assert.False(result);
+        }
+
+        [Theory]
+        [InlineData(RouteDirection.IncomingRequest)]
+        [InlineData(RouteDirection.UrlGeneration)]
+        public void RequiredRouteConstraint_WithValue(RouteDirection direction)
+        {
+            // Arrange
+            var constraint = new RequiredRouteConstraint();
+
+            // Act
+            var result = constraint.Match(
+                Mock.Of<HttpContext>(),
+                Mock.Of<IRouter>(),
+                "area",
+                new RouteValueDictionary(new { controller = "Home", action = "Index", area = "Store" }),
+                direction);
+
+            // Assert
+            Assert.True(result);
+        }
+    }
+}
+
+#endif

test/Microsoft.AspNet.Routing.Tests/Microsoft.AspNet.Routing.Tests.kproj

@@ -40,6 +40,7 @@
     <Compile Include="Constraints\AlphaRouteConstraintTests.cs" />
     <Compile Include="Constraints\RegexConstraintTests.cs" />
     <Compile Include="Constraints\IntRouteConstraintsTests.cs" />
+    <Compile Include="Constraints\RequiredRouteConstraintTests.cs" />
     <Compile Include="DefaultInlineConstraintResolverTest.cs" />
     <Compile Include="RouteCollectionTest.cs" />
     <Compile Include="InlineRouteParameterParserTests.cs" />