ClassC.cs

public class ClassC
{
    private string? _state;

    public async Task<int> NotThreadSafe(string state)
    {
        _state = state;

        // Delay this Task so we can end up having ClassB change the state before we return the length
        if (state.Length > 0)
        {
            await Task.Delay(10);
        }

        return _state.Length;
    }
}