If you are trying to do a sum in a text box, then start by not using a DataReader - they are sequential and only allow you to process through the results in order: you can't "rewind" and go back.
Instead, consider using a DataAdapter to file a DataTable (if you are accessing information from a DB for example) or a DataTable directly, and then use that to calculate your totals.
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlDataAdapter da = new SqlDataAdapter("SELECT MyColumn1, MyColumn2 FROM myTable WHERE mySearchColumn = @SEARCH", con))
{
da.SelectCommand.Parameters.AddWithValue("@SEARCH", myTextBox.Text);
DataTable dt = new DataTable();
da.Fill(dt);
myDataGridView.DataSource = dt;
}
}
If you keep numeric values in your DB, they will arrive in your DataTable as numbers as well.
Then the calculation is simple:
int total = 0;
foreach (DataRow row in dt.Rows)
{
total += (int)row[0];
}
myTextBox.Text = total.ToString();