textbox1失去焦点，检查输入的值是否为数字。
textbox2中按下Enter键，检查输入的值是否为数字。
textbox3获得焦点，计算textbox1和textbox2的和。

Public Class Form1


    Private Sub TextBox1_LostFocus(sender As Object, e As EventArgs) Handles TextBox1.LostFocus
        If Not IsNumeric(TextBox1.Text) Then
            TextBox1.Text = ""
            TextBox1.Focus()
        End If

    End Sub




    Private Sub TextBox2_KeyUp(sender As Object, e As KeyEventArgs) Handles TextBox2.KeyUp
        If e.KeyValue = 13 Then
            If Not IsNumeric(TextBox2.Text) Then
                TextBox2.Text = ""
            End If
        End If
    End Sub

    Private Sub TextBox3_GotFocus(sender As Object, e As EventArgs) Handles TextBox3.GotFocus
        TextBox3.Text = Val(TextBox1.Text) + Val(TextBox2.Text)
    End Sub
End Class