asp.net - 在为变量 'std'分配值之前，先使用它。空引用异常可能在运行时导致

Question

为什么我会出现这种错误。我一直在练习有关教程的内容，但仍然可以理解。应该如何清除此错误。我已经开始练习.net。所以请引导我

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()

当您调用 std.Insert时，您需要拥有 DataSet1TableAdapters.studentTableAdapter的对象