public CompletionSource(ITextView textView)
        {
            _textView = textView;

            if (!Workspace.TryGetWorkspace(textView.TextBuffer.AsTextContainer(), out var workspace))
            {
                throw new InvalidOperationException();
            }

            _completionService = workspace.Services.GetLanguageServices(textView.TextBuffer).GetService <CompletionService>();
        }