---

This section details on the original issue you should resolve

<issue_title>Introduce Constant IDE feature doesn't work with extension blocks</issue_title>
<issue_description>Version Used:
VS 2026 11206.111

Steps to Reproduce:

Consider a simple extension property with a quoted string:

public static class CEx
{
    extension(C c)
    {
        public string P => c["P"]; // can't use nameof(P) since it doesn't exist
    }
}

// to compile
public class C { public string this[string k] => ""; }

Trying to introduce a constant over "P" mangles the code to:

public static class CEx
{
    extension(C c)
    {
        private const string V = "P"; // can't have constants in an extension block

        public string P => c[extension(C).V]; // this isn't even valid C#
    }
}

Diagnostic Id:

N/A, just compilation errors.

Expected Behavior:

I would expect the constant to be introduced at the class level, outside of the extension. Also, of course, valid C# code to use the constant.</issue_description>

<agent_instructions>@copilot CSharpIntroduceVariableService.IntroduceFieldAsync should get the Ancestor TypeDeclarationSyntax that is not an ExtensionBlockDeclarationSyntax. See if that is sufficient to solve this. If not, figure out what other problems you encounter in this scenario and address them. Tests go in IntroduceVariableTests</agent_instructions>

Comments on the Issue (you are @copilot in this section)