VB.NET如何区分空字符和0？？？

[xurilenovo]

我用VS2010编写一个成绩判断程序，代码如下

Public Class frm_cjzhdj

    Private Sub cmd_pd_Click(sender As System.Object, e As System.EventArgs) Handles cmd_pd.Click
        If Val(txt_cj.Text) > 100 Or Val(txt_cj.Text) < 0 Then
            txt_dj.Text = "你所输入的数据超出范围"
        ElseIf Val(txt_cj.Text) >= 90 Then
            txt_dj.Text = "优"
        ElseIf Val(txt_cj.Text) >= 80 Then
            txt_dj.Text = "良"
        ElseIf Val(txt_cj.Text) >= 60 Then
            txt_dj.Text = "中"
        ElseIf Val(txt_cj.Text) < 60 Then
            txt_dj.Text = "差"
        ElseIf Trim(txt_cj.TextLength) = 0 Then
            MsgBox("请输入数据")
        End If
    End Sub

    Private Sub cmd_qc_Click(sender As System.Object, e As System.EventArgs) Handles cmd_qc.Click
        txt_cj.Text = ""
        txt_dj.Text = ""
    End Sub
End Class

就是如何让小于60的成绩显示为“差”
然而不输入就会弹出msgbox，可是我这样写，始终不会弹出，仅仅显示“差”
我还试过ElseIf Trim(txt_cj.Text) ="" Then

[Kation]

txt_cj.Text=""
or
txt_cj.Text.Length=0

[ningweidong]

if .text = ""

if .text = string.empty

[fanr_forever]

在60的地方把空字符排除掉
ElseIf Val(txt_cj.Text) < 60 andalso txt_cj.Text.trim<>""Then

[gxingmin]

    Private Sub cmd_pd_Click(sender As System.Object, e As System.EventArgs) Handles cmd_pd.Click
        If Trim(txt_cj.TextLength) = 0 Then  '先判断是否为空
            MsgBox("请输入数据")
        ELSEIf Val(txt_cj.Text) > 100 Or Val(txt_cj.Text) < 0 Then
            txt_dj.Text = "你所输入的数据超出范围"
        ElseIf Val(txt_cj.Text) >= 90 Then
            txt_dj.Text = "优"
        ElseIf Val(txt_cj.Text) >= 80 Then
            txt_dj.Text = "良"
        ElseIf Val(txt_cj.Text) >= 60 Then
            txt_dj.Text = "中"
        ElseIf Val(txt_cj.Text) < 60 Then
            txt_dj.Text = "差"
        End If
    End Sub

[xurilenovo]

[Quote=引用 3 楼 fanr_forever 的回复:]
在60的地方把空字符排除掉
ElseIf Val(txt_cj.Text) < 60 andalso txt_cj.Text.trim<>""Then
[/Quote]


3Q搞定了，为什么要这么写呢？
下面的空白判断不行？