Vb.net 如果不符合条件，则在文本框中保留旧值

我有一个函数，其中一个特定值不允许超过另一个值。但是，如果是这样的话，我希望保留在文本框中的旧值，在进行更改之前就在那里。我该怎么做

Private Sub TextBox20_TextChanged(sender As Object, e As EventArgs) Handles TextBox20.TextChanged

    'PREVENTS THE USER FROM APPLYING A THICKNESS GREATER THAN THE HIGHT AND LENGTH OF THE EXTRUSION SPECIFIDE...S
    If TextBox20.Text >= ((TextBox17.Text / 2) + 1) Or TextBox20.Text >= ((TextBox18.Text / 2) + 1) Then
        MessageBox.Show("CAUTION!" & vbCrLf & vbCrLf & "The material thickness cannot exceed the" & vbCrLf & "total height or width of the extrussion " & vbCrLf & "Either reduce the material thickness or increase the total " & vbCrLf & "height and or width of the extrusion", "Important Note", MessageBoxButtons.OK, MessageBoxIcon.Exclamation, MessageBoxDefaultButton.Button1)
    End If
    'PREVENTS THE USER FROM APPLYING A THICKNESS GREATER THAN THE HIGHT AND LENGTH OF THE EXTRUSION SPECIFIDE...E

End Sub

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您可以使用验证事件并设置e.Cancel=true以防止发生文本更改-

Private Sub TextBox20_Validating(sender As Object, e As System.ComponentModel.CancelEventArgs) Handles TextBox20.Validating
        If {your logic} Then
            'messagebox
            e.Cancel = true
            Return
        End If
    End Sub

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您可以使用验证事件并设置e.Cancel=true以防止发生文本更改-

Private Sub TextBox20_Validating(sender As Object, e As System.ComponentModel.CancelEventArgs) Handles TextBox20.Validating
        If {your logic} Then
            'messagebox
            e.Cancel = true
            Return
        End If
    End Sub