运行时出现错误 [英] error comes out at time of run

问题描述

Public Class grid1
    Inherits System.Web.UI.Page



    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        Dim db As empClassesDataContext
        Dim empall = From p In db.emp Select p//Warning	1	Variable 'db' is used before it has been assigned a value. A null reference exception could result at runtime 



        GridView1.DataSource = empall

        GridView1.DataBind()

    End Sub


    Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim db As New empClassesDataContext

        Dim myemp As New emp

        myemp.emp_id = 1006

        myemp.firstname = "joy"

        myemp.lastname = "dutta"

        myemp.age = 26

        myemp.address = "mumbai - 400 008"

        db.emp.InsertOnSubmit(myemp)

        db.SubmitChanges()

        Label1.Text = "inserted successfully"



    End Sub

    Protected Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click

        Dim db As New empClassesDataContext

        Dim empupdt = (From p In db.emp Where p.age = 26 Select p)

        For Each emp In empupdt

            emp.age = 27
        Next

        db.SubmitChanges()

        Label1.Text = "update successfully"
    End Sub

    Protected Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

        Dim db As New empClassesDataContext

        Dim empdel = From p In db.emp Where p.emp_id = 1006 Select p

        If empdel.Count > 0 Then

            db.emp.DeleteOnSubmit(empdel.First)

            db.SubmitChanges()

        End If
        Label1.Text = "Deleted successfully."
    End Sub
End Class

推荐答案

您正在创建db值，但从未在其中添加任何内容或对其进行初始化.这条线

You are creating the db value but you are never putting anything into it or initializing it. This line

Dim db As empClassesDataContext

应该是

should be

Dim db As new empClassesDataContext

我相信，就像您的其余代码一样.

I believe, just like it is in the rest of your code.

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