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[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<T>(char separator) where T : IConvertible
            => [.. input.Split(separator).Select(ConvertTo<T>)];

        private T ConvertTo<T>() where T : IConvertible   // <--- This method gets wrongfully marked as "unused member"
            => (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<T>(char separator) where T : IConvertible
    => [.. input.Split(separator).Select(x => x.ConvertTo<T>())];

// 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<T>(char separator) where T : IConvertible
    => [.. input.Split(separator).Select(x => ConvertTo<T>(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:

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