Login - check database if user exists... (c#)

I have managed to do the following...

string connectionString = "datasource=localhost;username=xxx;password=xxx;database=xxx";
MySqlConnection mySqlConnection = new MySqlConnection(connectionString);

string selectString =
"SELECT username, password " +
"FROM forum_members " +
"WHERE username = '" + frmUsername.Text + "' AND password = '" + frmPassword.Text + "'";

MySqlCommand mySqlCommand = new MySqlCommand(selectString, mySqlConnection);
mySqlConnection.Open();
String strResult = String.Empty;
strResult = (String)mySqlCommand.ExecuteScalar();
mySqlConnection.Close();

if (strResult.Length == 0)
{
Label1.Text = "INCORRECT USER/PASS!"
//could redirect to register page
} else {
Label1.Text = "YOU ARE LOGGED IN!";
//set loggin in sessions variables
}