对象引用不设置为对象的实例

Question

错误出现在此图片中：

我该如何解决这个问题？

我的代码：

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Dim con As New MySqlConnection("host=localhost; username=root; database= login")
    Dim cmd As MySqlCommand
    Dim dr As MySqlDataReader

    con.Open()
    cmd.Connection = con 'this is where I have an error

    cmd.CommandText = "select userID, password from login where userID= ' " & userIDtxt.Text & " ' and password = ' " & pwTxt.Text & " '"
    dr = cmd.ExecuteReader

    If dr.HasRows Then
        MsgBox("Login successful!")
        Me.Hide()
        MainForm.Show()
    Else
        MsgBox("Username or password does not match")
        userIDtxt.Text = ""
        pwTxt.Text = " "
    End If

End Sub
End Class

Answer 1

你的sql连接字符串缺少真正的凭据，这个示例用vb和c＃连接到mysql：

https://dev.mysql.com/doc/connector-net/en/connector-net-programming-connecting-open.html

Answer 2

您尚未初始化cmd变量的新实例。

变化：

Dim cmd As MySqlCommand

为：

Dim cmd As New MySqlCommand