MutateState.cs

var continuationWrapper = func.Target;

var continuation = continuationWrapper.GetType().GetField("_continuation", BindingFlags.Instance | BindingFlags.NonPublic)
    .GetValue(continuationWrapper) as Action;

if (continuation != null)
{
    var asyncStateMachineBox = continuation.Target;

    var stateMachineField = asyncStateMachineBox?.GetType().GetField("StateMachine");

    if (stateMachineField != null)
    {
        var stateMachine = stateMachineField.GetValue(asyncStateMachineBox);
    }
}