Must declare the scalar variable "@UserName"

Question

I keep getting an error that I don't understand.

Must declare the scalar variable "@varname"

My aim is to create a login page that uses 2 textboxes and a button, where it checks if the user exits based on the information stored in a SQL database.

This is where I think the problem is coming from:

private bool DBConnection(string userName, string password)
{
    SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
    
    //string cmdString = ("SELECT UserName, Password FROM Users WHERE UserName ='" + userName +
    //                    "'AND Password ='" + password + "'");         //REMOVED AS THIS IS PRONE TO SQL INJECTIONS
    
    string cmdString = ("SELECT * FROM Users WHERE UserName = @uname AND Password = @pw");
    
    SqlCommand cmd = new SqlCommand(cmdString, conn);
    
    cmd.Parameters.Add("uname", SqlDbType.VarChar).Value = userName;
    cmd.Parameters.Add("pw", SqlDbType.VarChar).Value = password;

    DataSet loginCredentials = new DataSet();
    SqlDataAdapter dataAdapter;

    try
    {
        if (conn.State.Equals(ConnectionState.Closed))
        {
            conn.Open();

            dataAdapter = new SqlDataAdapter(cmdString, conn);
            dataAdapter.Fill(loginCredentials);

            conn.Close();

            if (loginCredentials != null)
            {
                if (loginCredentials.Tables[0].Rows.Count > 0)
                {
                    return true;
                }
                else
                {
                    lblMessage.Text = "Incorrect Username or Password";
                    lblMessage.Visible = true;
                }
            }   
        }
    }
    catch (Exception err)
    {
        lblMessage.Text = err.Message.ToString() + " Error connecting to the Database // " + cmd.Parameters.Count;
        lblMessage.Visible = true;
        return false;
    }

    return false;
}

specifically where dataAdapter.Fill(loginCredentials); is being executed.

The commented-out statement works successfully in logging in a user with the correct username and password, but as far as I know is not secure, as it's vulnerable to SQL injections and this is why I'm trying to parameterize the SQL statement.

error screenshot below:

Answer 1

You should pass the sqlcommand to the dataAdapter because in your case the sqlcommand (cmd) has more information than mere commandtext and connectionstring. Your code may look like the following:

private bool DBConnection(string userName, string password)
{
 SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);

//string cmdString = ("SELECT UserName, Password FROM Users WHERE UserName ='" + userName +
//                    "'AND Password ='" + password + "'");         //REMOVED AS THIS IS PRONE TO SQL INJECTIONS

string cmdString = ("SELECT * FROM Users WHERE UserName = @uname AND Password = @pw");

SqlCommand cmd = new SqlCommand(cmdString, conn);

cmd.Parameters.Add("uname", SqlDbType.VarChar).Value = userName;
cmd.Parameters.Add("pw", SqlDbType.VarChar).Value = password;

DataSet loginCredentials = new DataSet();
SqlDataAdapter dataAdapter;

try
{
    if (conn.State.Equals(ConnectionState.Closed))
    {
        conn.Open();

        dataAdapter = new SqlDataAdapter(cmd);
        dataAdapter.Fill(loginCredentials);

        conn.Close();

        if (loginCredentials != null)
        {
            if (loginCredentials.Tables[0].Rows.Count > 0)
            {
                return true;
            }
            else
            {
                lblMessage.Text = "Incorrect Username or Password";
                lblMessage.Visible = true;
            }
        }   
    }
}
catch (Exception err)
{
    lblMessage.Text = err.Message.ToString() + " Error connecting to the Database // " + cmd.Parameters.Count;
    lblMessage.Visible = true;
    return false;
}

return false;
}

Answer 2

cmd.Parameters.Add("@uname", SqlDbType.VarChar).Value = userName;

Note the @ in front of uname.