roslyn [Visual Studio] [C# 14] Private extension methods incorrectly marked as unused members

This issue has been moved from a ticket on Developer Community.

[severity:It bothers me. A fix would be nice] When using the new C# 14 extension method syntax, Visual Studio 18.0.2 marks private extension methods as "unused members" but only if it is not called using the extension method signature. Surely this is the side-effect of some compiler lowering to old-style extension syntax, but the IDE suggestion is just wrong and would lead to a compiler error.

Example:

public static class NewStyleExtensions
{
    extension(string input)
    {
        public T[] SplitToArray&lt;T&gt;(char separator) where T : IConvertible
            =&gt; [.. input.Split(separator).Select(ConvertTo&lt;T&gt;)];

        private T ConvertTo&lt;T&gt;() where T : IConvertible   // &lt;--- This method gets wrongfully marked as "unused member"
            =&gt; (T)Convert.ChangeType(input, typeof(T));
    }
}

Note that changing the call from the public method affects this behavior:

// Alternative 1
// When called with the true extension method signature, the private method is now correctly marked as "used".
public T[] SplitToArray&lt;T&gt;(char separator) where T : IConvertible
    =&gt; [.. input.Split(separator).Select(x =&gt; x.ConvertTo&lt;T&gt;())];

// Alternative 2
// When called with the target as an input parameter, the private method is incorrectly marked as "unused" and a refactor is suggested to remove the lambda expression (to original example above).
public T[] SplitToArray&lt;T&gt;(char separator) where T : IConvertible
    =&gt; [.. input.Split(separator).Select(x =&gt; ConvertTo&lt;T&gt;(x))];

Original Comments

Benjamin Copass on 12/11/2025, 01:27 AM:

Another example / use case:

internal static class PrivateExtensionPropertyTest
{
    extension(object o)
    {
        private bool Foo => o is null; // faded out in Visual Studio (VS thinks it's an unused member)
    }
public static void Test()
{
object o = new object();
bool b = o.Foo; // Foo is actually used
}
}

Feedback Bot on 12/11/2025, 06:50 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

Dec 11 '25 14:12 vsfeedback

Dec 11 '25 15:12 deepakrathore33

Related to https://github.com/dotnet/roslyn/issues/80645

Dec 11 '25 15:12 deepakrathore33

Moved to Area-IDE as this seems driven by IDE0051 Also relates to https://github.com/dotnet/roslyn/issues/81213

I'm not sure if this is handled by the same IDE logic, but extension members and their implementation methods are also not tracked together in FAR and other refactorings: https://github.com/dotnet/roslyn/issues/81507

Dec 12 '25 07:12 jcouv

Closing as duplicate of https://github.com/dotnet/roslyn/issues/80645 I'm working on a fix for 18.3

Dec 17 '25 04:12 jcouv