Refactored the header value to coalesce to the multi-value CSV format as defined in RFC 2616, section 4.2. The built-in APIs appear to always write the headers using the long form without any alternate configuration

Author: Chris Martinez

src/Common/ReportApiVersionsAttribute.cs

@@ -22,5 +22,6 @@ public sealed partial class ReportApiVersionsAttribute : ActionFilterAttribute
     {
         private const string ApiSupportedVersions = "api-supported-versions";
         private const string ApiDeprecatedVersions = "api-deprecated-versions";
+        private const string ValueSeparator = ", ";
     }
 }

src/Microsoft.AspNet.WebApi.Versioning/ReportApiVersionsAttribute.cs

@@ -1,13 +1,13 @@
-using System.Diagnostics.Contracts;
-namespace Microsoft.Web.Http
+namespace Microsoft.Web.Http
 {
     using System.Collections.Generic;
     using System.Diagnostics.CodeAnalysis;
+    using System.Diagnostics.Contracts;
     using System.Linq;
-    using System.Net.Http;
     using System.Net.Http.Headers;
     using System.Web.Http;
     using System.Web.Http.Filters;
+    using static System.String;
 
     /// <content>
     /// Provides the implementation for ASP.NET Web API.
@@ -40,40 +40,20 @@ public override void OnActionExecuted( HttpActionExecutedContext actionExecutedC
 
             var headers = response.Headers;
 
-            if ( model.SupportedApiVersions.Count == 0 )
-            {
-                TryAddHeaderWithDefaultSupportedApiVersion( headers, actionExecutedContext.Request );
-                return;
-            }
-
             AddApiVersionHeader( headers, ApiSupportedVersions, model.SupportedApiVersions );
             AddApiVersionHeader( headers, ApiDeprecatedVersions, model.DeprecatedApiVersions );
         }
 
-        private static void TryAddHeaderWithDefaultSupportedApiVersion( HttpHeaders headers, HttpRequestMessage request )
-        {
-            Contract.Requires( headers != null );
-            Contract.Requires( request != null );
-
-            if ( headers.Contains( ApiSupportedVersions ) )
-            {
-                return;
-            }
-
-            var supportedVersion = request.GetRequestedApiVersion() ?? request.GetConfiguration().GetDefaultApiVersion();
-            headers.Add( ApiSupportedVersions, supportedVersion.ToString() );
-        }
-
         private static void AddApiVersionHeader( HttpHeaders headers, string headerName, IReadOnlyList<ApiVersion> versions )
         {
             Contract.Requires( headers != null );
-            Contract.Requires( !string.IsNullOrEmpty( headerName ) );
+            Contract.Requires( !IsNullOrEmpty( headerName ) );
             Contract.Requires( versions != null );
 
             if ( versions.Count > 0 && !headers.Contains( headerName ) )
             {
-                headers.Add( headerName, versions.Select( v => v.ToString() ).ToArray() );
+                headers.Add( headerName, Join( ValueSeparator, versions.Select( v => v.ToString() ) ) );
             }
         }
     }
-}
+}

src/Microsoft.AspNetCore.Mvc.Versioning/ReportApiVersionsAttribute.cs

@@ -9,7 +9,7 @@
     using System.Diagnostics.Contracts;
     using System.Linq;
     using Versioning;
-    using static ApiVersion;
+    using static System.String;
 
     /// <content>
     /// Provides additional implementation specific to ASP.NET Core.
@@ -48,13 +48,13 @@ public override void OnActionExecuted( ActionExecutedContext context )
         private static void AddApiVersionHeader( IHeaderDictionary headers, string headerName, IReadOnlyList<ApiVersion> versions )
         {
             Contract.Requires( headers != null );
-            Contract.Requires( !string.IsNullOrEmpty( headerName ) );
+            Contract.Requires( !IsNullOrEmpty( headerName ) );
             Contract.Requires( versions != null );
 
             if ( versions.Count > 0 && !headers.ContainsKey( headerName ) )
             {
-                headers.Add( headerName, versions.Select( v => v.ToString() ).ToArray() );
+                headers.Add( headerName, Join( ValueSeparator, versions.Select( v => v.ToString() ).ToArray() ) );
             }
         }
     }
-}
+}

test/Microsoft.AspNet.WebApi.Versioning.Tests/ReportApiVersionsAttributeTest.cs

@@ -42,7 +42,7 @@ public void on_action_executed_should_add_version_headers()
             attribute.OnActionExecuted( context );
 
             // assert
-            context.Response.Headers.GetValues( "api-supported-versions" ).Should().BeEquivalentTo( "1.0", "2.0" );
+            context.Response.Headers.GetValues( "api-supported-versions" ).Single().Should().Be( "1.0, 2.0" );
             context.Response.Headers.GetValues( "api-deprecated-versions" ).Single().Should().Be( "0.5" );
         }
 

test/Microsoft.AspNetCore.Mvc.Versioning.Tests/ReportApiVersionsAttributeTest.cs

@@ -41,7 +41,7 @@ public void on_action_executed_should_add_version_headers()
             attribute.OnActionExecuted( context );
 
             // assert
-            context.HttpContext.Response.Headers["api-supported-versions"].Should().BeEquivalentTo( "1.0", "2.0" );
+            context.HttpContext.Response.Headers["api-supported-versions"].Single().Should().Be( "1.0, 2.0" );
             context.HttpContext.Response.Headers["api-deprecated-versions"].Single().Should().Be( "0.5" );
         }