Fix #810

src/Microsoft.AspNetCore.Routing/Internal/LinkGenerationDecisionTree.cs

@@ -15,6 +15,11 @@ namespace Microsoft.AspNetCore.Routing.Internal
     [DebuggerDisplay("{DebuggerDisplayString,nq}")]
     public class LinkGenerationDecisionTree
     {
+        // Fallback value for cases where the ambient values weren't provided.
+        //
+        // This is safe because we don't mutate the route values in here.
+        private static readonly RouteValueDictionary EmptyAmbientValues = new RouteValueDictionary();
+
         private readonly DecisionTreeNode<OutboundMatch> _root;
 
         public LinkGenerationDecisionTree(IReadOnlyList<OutboundMatch> entries)
@@ -30,7 +35,7 @@ namespace Microsoft.AspNetCore.Routing.Internal
             if (_root.Matches.Count > 0 || _root.Criteria.Count > 0)
             {
                 var results = new List<OutboundMatchResult>();
-                Walk(results, values, ambientValues, _root, isFallbackPath: false);
+                Walk(results, values, ambientValues ?? EmptyAmbientValues, _root, isFallbackPath: false);
                 results.Sort(OutboundMatchResultComparer.Instance);
                 return results;
             }

test/Microsoft.AspNetCore.Routing.Tests/Internal/LinkGenerationDecisionTreeTest.cs

@@ -14,6 +14,26 @@ namespace Microsoft.AspNetCore.Routing.Internal.Routing
 {
     public class LinkGenerationDecisionTreeTest
     {
+        [Fact]
+        public void GetMatches_AllowsNullAmbientValues()
+        {
+            // Arrange
+            var entries = new List<OutboundMatch>();
+
+            var entry = CreateMatch(new { });
+            entries.Add(entry);
+
+            var tree = new LinkGenerationDecisionTree(entries);
+
+            var context = CreateContext(new { });
+
+            // Act
+            var matches = tree.GetMatches(context.Values, ambientValues: null);
+
+            // Assert
+            Assert.Same(entry, Assert.Single(matches).Match);
+        }
+
         [Fact]
         public void SelectSingleEntry_NoCriteria()
         {