Handle ternary expressions inside interpolated attributes. Fixes #446

src/Microsoft.AspNetCore.Blazor.Razor.Extensions/BlazorRuntimeNodeWriter.cs

@@ -594,7 +594,21 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                                 writer.Write(" + ");
                             }
 
+                            // If it's a C# expression, we have to wrap it in parens, otherwise
+                            // things like ternary expressions don't compose with concatenation
+                            var isCSharp = concatenatedValue is IntermediateToken intermediateToken
+                                && intermediateToken.Kind == TokenKind.CSharp;
+                            if (isCSharp)
+                            {
+                                writer.Write("(");
+                            }
+
                             WriteAttributeValue(writer, concatenatedValue);
+
+                            if (isCSharp)
+                            {
+                                writer.Write(")");
+                            }
                         }
                         break;
                     }

test/Microsoft.AspNetCore.Blazor.Build.Test/RenderingRazorIntegrationTest.cs

@@ -180,6 +180,20 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
                 frame => AssertFrame.Attribute(frame, "attr", "Hello, WORLD    with number 246!", 1));
         }
 
+        [Fact]
+        public void SupportsAttributesWithInterpolatedTernaryExpressionValues()
+        {
+            // Arrange/Act
+            var component = CompileToComponent(
+                "@{ var myValue = \"world\"; }"
+                + "<elem attr=\"Hello, @(true ? myValue : \"nothing\")!\" />");
+
+            // Assert
+            Assert.Collection(GetRenderTree(component),
+                frame => AssertFrame.Element(frame, "elem", 2, 0),
+                frame => AssertFrame.Attribute(frame, "attr", "Hello, world!", 1));
+        }
+
         [Fact]
         public void SupportsHyphenedAttributesWithCSharpExpressionValues()
         {