text1-text2的值小于400且不等于0,则结果为400，值为0时结果为0，大于400按实际值为结果。这个要如何写函数！

参考:

Public Function aaa(text1, text2) As Double

    On Error GoTo er
    Dim a As Double, b As Double
    If IsNumeric(text1) = True Then
        a = CDbl(text1)
    Else
        MsgBox "第一个参数错误!"
        Exit Function
    End If
    If IsNumeric(text2) = True Then
        b = CDbl(text2)
    Else
        MsgBox "第二个参数错误!"
        Exit Function
    End If
    Select Case a - b
    Case 0
        aaa = 0
    Case Is <= 400
        aaa = 400
    Case Is > 400
        aaa = a - b
    End Select
    Exit Function
er:
    MsgBox Err.Description
End Function