Compiler Error CS8177

Async methods cannot have by-reference locals

To manage asynchronous state, async methods use a state machine, capturing variable state in closures implemented in compiler-generated classes and properties. A local variable reference (on the stack) can't be captured within the instance of a class in the heap, so the compiler issues an error.

Example

The following sample generates CS8177 before C# 13:

// CS8177.cs (20,26)

using System.Threading.Tasks;

class E
{
    public class Enumerator
    {
        public ref int Current => throw new System.NotImplementedException();
        public bool MoveNext() => throw new System.NotImplementedException();
    }

    public Enumerator GetEnumerator() => new Enumerator();
}

class C
{
    public async static Task Test()
    {
        await Task.CompletedTask;

        foreach (ref int x in new E())
        {
            System.Console.Write(x);
        }
    }
}

To correct this error

Remove the ref modifier. Or, you can upgrade to C# 13, which ships with .NET 9.

class C
{
    public async static Task Test()
    {
        await Task.CompletedTask;

        foreach (int x in new E())
        {
            System.Console.Write(x);
        }
    }
}