如何修改点击按钮的文本？PowerPoint宏开发问题咨询

我需要制作一个宏，实现点击按钮后修改按钮上的文本。

普通形状按钮实现方案 #

参考微软论坛的oShape声明，我编写了以下代码，实现文本在"a"和"b"之间切换：

Sub changeLetter(oShape As Shape)
    If ActivePresentation.Slides(oShape.Parent.SlideID).Shapes(oShape.Name).TextFrame.TextRange.Text = "a" Then
        ActivePresentation.Slides(oShape.Parent.SlideID).Shapes(oShape.Name).TextFrame.TextRange.Text = "b"
    ElseIf ActivePresentation.Slides(oShape.Parent.SlideID).Shapes(oShape.Name).TextFrame.TextRange.Text = "b" Then
        ActivePresentation.Slides(oShape.Parent.SlideID).Shapes(oShape.Name).TextFrame.TextRange.Text = "a"
    End If
End Sub

命令按钮实现及问题解决 #

后续改用命令按钮，编写了如下代码：

Private Sub letter1_Click()
    If letter1.Caption = "a" Then
        letter1.Caption = "b"
    ElseIf letter1.Caption = "b" Then
        letter1.Caption = "a"
    End If
End Sub

使用过程中持续出现**"Invalid outside procedure"**错误，目前该问题已解决。

内容的提问来源于stack exchange，提问作者ColurMine