Winform Richtextbox控件本身并不直接支持撤銷和重做功能，但可以通過添加自定義的撤銷和重做功能來實現該功能。以下是一種實現方式：

1. 創建一個名為UndoRedoStack的類，其中包含兩個棧用于保存操作歷史記錄。

public class UndoRedoStack
{
    private Stack<string> undoStack = new Stack<string>();
    private Stack<string> redoStack = new Stack<string>();

    public void AddOperation(string operation)
    {
        undoStack.Push(operation);
        redoStack.Clear();
    }

    public string Undo()
    {
        if (undoStack.Count == 0)
            return null;

        string operation = undoStack.Pop();
        redoStack.Push(operation);
        return operation;
    }

    public string Redo()
    {
        if (redoStack.Count == 0)
            return null;

        string operation = redoStack.Pop();
        undoStack.Push(operation);
        return operation;
    }
}

2. 在窗體中創建一個UndoRedoStack實例，并在Richtextbox的TextChanged事件中保存操作歷史記錄。

UndoRedoStack undoRedoStack = new UndoRedoStack();

private void richTextBox1_TextChanged(object sender, EventArgs e)
{
    undoRedoStack.AddOperation(richTextBox1.Text);
}

3. 在撤銷和重做按鈕的Click事件中調用Undo和Redo方法。

private void undoButton_Click(object sender, EventArgs e)
{
    string operation = undoRedoStack.Undo();
    if (operation != null)
    {
        richTextBox1.Text = operation;
    }
}

private void redoButton_Click(object sender, EventArgs e)
{
    string operation = undoRedoStack.Redo();
    if (operation != null)
    {
        richTextBox1.Text = operation;
    }
}

通過以上步驟，就可以在Winform Richtextbox控件中實現撤銷和重做功能。當用戶進行文本編輯操作時，可以通過點擊撤銷按鈕來撤銷上一步操作，通過點擊重做按鈕來重做上一步撤銷的操作。