private SqlDataReader QueryConnectionString (string query)
    {
        // New SQL Connection
        SqlConnection cnn;

        // New Connection String
        string connetionString = ConfigurationManager.ConnectionStrings["ConnectionStringName"].ConnectionString;

        // Instantiate the SQL Connection with Connection String
        cnn = new SqlConnection(connetionString);

        // Create a SqlCommand with the input Query
        SqlCommand command = new SqlCommand(query, cnn);

        cnn.Open();

        // Create a SqlDataReader and tie it to the query
        SqlDataReader reader = command.ExecuteReader();


        cnn.Close();

        return reader;
    }

SqlDataReader reader = QueryConnectionString("SELECT * FROM tTable");

lblOutput.Text = reader.GetString(0);

System.InvalidOperationException: 'Invalid attempt to call CheckDataIsReady when reader is closed.'