NullReferenceException未处理？

Question

当我触发IF语句时发生错误或者如果我将文本框留空，应用程序不应该关闭并修复文本框问题，但事实并非如此。

并指向Dim i As Integer = cmd.ExecuteNonQuery()

Imports MySql.Data.MySqlClient
Public Class Form3

Public sConnection As New MySqlConnection
Private Sub Form3_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    If sConnection.State = ConnectionState.Closed Then
        sConnection.ConnectionString = "server=localhost;user id=root;database=db"
        sConnection.Open()
    End If

    LoadPeople()

End Sub
Private Sub btnsave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnsave.Click
    Dim Query As String
    If txtfname.Text = "" Or txtlname.Text = "" Or txtmname.Text = "" Or txtparty.Text = "" Or txtyr.Text = "" Or cmbpos.Text = "" Then
        MessageBox.Show("Please complete the required fields..", "Error", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
    Else
        Query = "INSERT INTO candidate(cfname,cmname,clname,cpos,cyr,cparty) VALUES('" & txtfname.Text & "','" & txtmname.Text & "','" & txtlname.Text & "','" & cmbpos.Text & "','" & txtyr.Text & "','" & txtparty.Text & "')"
    End If

    Dim cmd As MySqlCommand = New MySqlCommand(Query, sConnection)

    Dim i As Integer = cmd.ExecuteNonQuery()
    If (i > 0) Then
        MsgBox("Record Inserted")
    Else
        MsgBox("Record is not Inserted")
    End If

    sConnection.Close()

    txtfname.Text = ""
    txtlname.Text = ""
    txtmname.Text = ""
    txtparty.Text = ""
    txtyr.Text = ""
    cmbpos.Invalidate()
    txtfname.Focus()

    LoadPeople()

End Sub

Answer 1

我会使用try catch ....

Try
    cmd.ExecuteNonQuery()
    MsgBox("Record Inserted")
Catch ex as Exception
    MsgBox("Error:" & ex.message)
Finally
    'Optional but I would use it to close of DB Connections
End Try

您也容易受到SQL注入攻击....使用参数化SQL或其他方法来限制风险。