Object reference not set to an instance of an object error when checking value from DB table

Question

I am checking username from DB table when a user enters username in registration page an textbox OnTextChanged event fired which checks if username is already exists or not. Here is code :

public void HandleTextbox1OnTextChanged(Object sender, EventArgs e)
{
    string _connString = ConfigurationManager.AppSettings["connString"];
    string username = txtUserName.Text;
    int result = 0;
    using (SqlConnection conn = new SqlConnection(_connString))
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand("select * from  ref_CourseRegistration_Users where nm_UserName=@nm_UserName", conn);
        cmd.Parameters.AddWithValue("@nm_UserName", username);
        cmd.ExecuteNonQuery();
        result = (int)cmd.ExecuteScalar();
        if (result > 0)
        {
            lblMessage.Text = "username already exists, please enter different User Name!";
            lblMessage.ForeColor = Color.Red;
        }
        else
        {

        }
    }
}

it works fine when username exists but when username doesn't exists it shows an error "Object reference not set to an instance of an object." if i am missing anything?

Always point out the line where the exception is occurring.

Jonathon Reinhart
– Jonathon Reinhart

2014-04-28 07:22:51 +00:00
Commented Apr 28, 2014 at 7:22 — Jonathon Reinhart
– Jonathon Reinhart, Commented Apr 28, 2014 at 7:22

nvoigt · Accepted Answer · 2014-04-28 07:24:18Z

3

You expect the number of users with the same name from your query, so you should select it:

Remove the line cmd.ExecuteNonQuery(); from your code, you already use cmd.ExecuteScalar(); and that's fine.

Your select should return a number, so you should select a number:

select COUNT(*) from  ref_CourseRegistration_Users where nm_UserName=@nm_UserName

If you don't do this, you will get null from your query and that results in a null reference exception on your end because you don't handle it.

answered Apr 28, 2014 at 7:24

nvoigt

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