Error: Object reference not set to an instance of an object

Question

I'm not really good at programming and I don't know what's happening in my codes, it works well before but now I get this error:

NullReferenceException was unhadled by user code.

Object Reference not set to an instance of an object.

Here's my code:

 void GetProfileInfo()
    {
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = "SELECT FirstName, LastName, Address, ContactNo, EmailAddress FROM Users " +
            "WHERE UserID=@UserID";
        cmd.Parameters.Add("@UserID", SqlDbType.Int).Value = Session["userid"].ToString();
        SqlDataReader data = cmd.ExecuteReader();
        while (data.Read())
        {
            txtFN.Text = data["FirstName"].ToString();
            txtLN.Text = data["LastName"].ToString();
            txtAddress.Text = data["Address"].ToString();
            txtContact.Text = data["ContactNo"].ToString();
            txtEmail.Text = data["EmailAddress"].ToString();
        }
        con.Close();
    }

this line: cmd.Parameters.Add("@UserID", SqlDbType.Int).Value = Session["userid"].ToString(); — user2514307
– user2514307, Commented Jun 23, 2013 at 20:09
Then Session["userid"] return null and you try to convert to a string. — Steve
– Steve, Commented Jun 23, 2013 at 20:10
I'm sorry guys, i'm not really good at this. I want to learn though, what should I do :) — user2514307
– user2514307, Commented Jun 23, 2013 at 20:13

Nick · Accepted Answer · 2013-06-23 20:17:51Z

First you have to check all occurrences where Session["userid"] is initialized, then in order to avoid exception error you have to check this variable value whether is null or wrap the code with try catch finally blocks

Ex:

try
{
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = "SELECT FirstName, LastName, Address, ContactNo, EmailAddress FROM Users " +
            "WHERE UserID=@UserID";
        cmd.Parameters.Add("@UserID", SqlDbType.Int).Value = Session["userid"].ToString();
        SqlDataReader data = cmd.ExecuteReader();
        while (data.Read())
        {
            txtFN.Text = data["FirstName"].ToString();
            txtLN.Text = data["LastName"].ToString();
            txtAddress.Text = data["Address"].ToString();
            txtContact.Text = data["ContactNo"].ToString();
            txtEmail.Text = data["EmailAddress"].ToString();
        }
}
catch(Exception e)
{
    string message = e.Message;
    MessageBox.Show(message);
}
finally
{
        con.Close();
}

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