加法器的实现:

Public Class Form1

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        If Not IsNumeric(TextBox1.Text) Then
            TextBox1.Text = ""
        End If
    End Sub

    Private Sub TextBox2_TextChanged(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles TextBox2.TextChanged
        If Not IsNumeric(TextBox2.Text) Then
            TextBox2.Text = ""
        End If

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button1.Click
        TextBox3.Text = Val(TextBox1.Text) + Val(TextBox2.Text)
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button2.Click
        TextBox2.Text = ""
        TextBox1.Text = ""
        TextBox3.Text = ""
        TextBox1.Focus()
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button3.Click
        End
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub
End Class