How to display SQL query selection into a gridview?

See more:

ASP.NET

SQL

C#3.5

I have a web form in ASP.NET using c#. By using this query:

SELECT INST_ID, LongName FROM TableCOCINSE
EXCEPT
SELECT INST_ID, LongName from TableAFTE where AFTEYR = 2016

I am trying to display the results into a GridView. This is what I have as my code:

SqlConnection con3 = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["PassoneConnectionString"].ConnectionString);
            con3.Open();

            SqlCommand com = new SqlCommand("(SELECT INST_ID, LongName FROM TableCOCINSE) EXCEPT (SELECT INST_ID, LongName FROM TableAFTE)", con3);

            DataTable dt = new DataTable();

            SqlDataReader sqlDataReader = com.ExecuteReader();

            dt.Load(sqlDataReader);
            sqlDataReader.Close();

            GridView dataGridView4 = new GridView();
            dataGridView4.DataSource = dt;

What am I missing?

What I have tried:

I have tried almost everything. The SQL Query works in SQL Server Management Studio.