Need help on my datareader in visual studio 2015

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My code was working fine until when I added parameters. Since then I get this error:

An unhandled exception of type 'System.NullReferenceException' occurred in NewMonitoringSystem.exe

Additional information: Object reference not set to an instance of an object.

I need help on how to fix this error. Thank you very much. Here's my code: Error on LINE 10 of this code

Private Sub btnLogin_Click(sender As Object, e As EventArgs) Handles btnLogin.Click
    frmMainMenu.Height = 720
    frmMainMenu.Width = 950
    Dim numID As Integer = 2
    cmd.Connection = cn
    cn.Open()
    cmd.CommandText = "SELECT ID, username, password FROM tblUsers WHERE username = @username and password = @password"
    cmd.Parameters.Add(New SqlClient.SqlParameter("@username", SqlDbType.VarChar, 20)).Value = txtUsername.Text
    cmd.Parameters.Add(New SqlClient.SqlParameter("@password", SqlDbType.VarChar, 20)).Value = txtPassword.Text
    If dr.HasRows Then
        dr.Read()
        If dr("ID") = numID Then
            Me.Hide()
            frmMainMenu.lblAccessLevel.Text = "Admin"
            frmMainMenu.Show()
        Else
            Me.Hide()
            frmMainMenu.lblAccessLevel.Text = "User"
            frmMainMenu.TSMOffertory.Enabled = False
            frmMainMenu.TSMOffertory.Visible = False
            frmMainMenu.Show()
        End If
    Else
        MsgBox("Invalid Credentials", MsgBoxStyle.Exclamation, "Invalid LogIn")
    End If
    cn.Close()
End Sub

What I have tried:

Haven't tried anything because I don't really know what to do.

Posted 19-Sep-16 2:02am

Paolo Tansengco

Updated 19-Sep-16 2:17am

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[no name] 19-Sep-16 8:14am

What do you mean you don't know what to do? The error means exactly that same as everyone else that has gotten it and you fix it the exact same what that everyone else in the world has fixed it. Learn how to do some research and learn how to use your debugger.

2 solutions

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OriginalGriff · Accepted Answer · 2016-09-19T02:12:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Paolo Tansengco · Accepted Answer · 2016-09-19T02:17:00

Solution 2

VB

dr = cmd.ExecuteReader

This fixed the code.

Posted 19-Sep-16 2:17am