Escape braces in ReferenceExpression.AppendFormatted() (#4869)

src/Aspire.Hosting/ApplicationModel/ReferenceExpression.cs

@@ -112,7 +112,9 @@ public readonly void AppendLiteral(string value)
         /// <param name="value">The formatted string to be appended to the interpolated string.</param>
         public readonly void AppendFormatted(string? value)
         {
-            _builder.Append(value);
+            // The value that comes in is a literal string that is not meant to be interpreted.
+            // But the _builder later gets treated as a format string, so we just need to escape the braces.
+            _builder.Append(value?.Replace("{", "{{").Replace("}", "}}"));
         }
 
         /// <summary>

tests/Aspire.Hosting.Tests/ReferenceExpressionTests.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace Aspire.Hosting.Tests;
+public class ReferenceExpressionTests
+{
+    [Theory]
+    [InlineData("world", "Hello world", "Hello world")]
+    [InlineData("{", "Hello {{", "Hello {")]
+    [InlineData("}", "Hello }}", "Hello }")]
+    [InlineData("{1}", "Hello {{1}}", "Hello {1}")]
+    [InlineData("{x}", "Hello {{x}}", "Hello {x}")]
+    [InlineData("{{x}}", "Hello {{{{x}}}}", "Hello {{x}}")]
+    public void TestReferenceExpressionCreateInputStringTreatedAsLiteral(string input, string expectedFormat, string expectedExpression)
+    {
+        var refExpression = ReferenceExpression.Create($"Hello {input}");
+        Assert.Equal(expectedFormat, refExpression.Format);
+
+        // Generally, the input string should end up unchanged in the expression, since it's a literal
+        var expr = refExpression.ValueExpression;
+        Assert.Equal(expectedExpression, expr);
+    }
+}