asp.net - 变量“std”在被赋值之前被使用。运行时可能会导致空引用异常

Question

为什么我会出现这种错误。我一直在练习教程，但我仍然明白这一点。应该如何清除这个错误。我已经开始练习.net。所以请指导我

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim std As DataSet1TableAdapters.studentTableAdapter

    If txtname.Text = "" Then
        lblMessage.Text = "Please enter name"
    ElseIf txtclass.Text = "" Then
        lblMessage.Text = "Please enter class "
    ElseIf txtsemseter.Text = "" Then
        lblMessage.Text = "Please enter semester"
    Else
        std.Insert(Name:=txtname.Text, Class:=txtclass.Text, Semester:=txtsemseter.Text)
    End If
End Sub

教程链接：http ://www.youtube.com/watch?v=wDFo-nGdUQY

Answer 1

做如下

Dim std As New DataSet1TableAdapters.studentTableAdapter()

当你打电话时，std.Insert你需要有对象DataSet1TableAdapters.studentTableAdapter

Answer 2

您需要更新变量，如下所示：

Dim std As New DataSet1TableAdapters.studentTableAdapter()

编辑：我试图在手机上做到这一点，不容易:)