Executereader: commandtext property has not been initialized

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protected void btnBuyNow_Click(object sender, EventArgs e)
  {
      string id = Session["UserID"].ToString();

      con.Open();
      SqlCommand cmd = con.CreateCommand();
      cmd.CommandType = CommandType.Text;
      using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
      {
          DataTable dt = new DataTable();
          sda.Fill(dt);
          if (dt.Rows.Count > 0)
          {
              Int32 myQty = Convert.ToInt32(dt.Rows[0]["Qty"].ToString());
              cmd.CommandText = "UPDATE tblProducts SET PQuantity=PQuantity-" + myQty + " WHERE id=" + id;
              cmd.ExecuteNonQuery();
              Response.Redirect("OrderConfirmation.aspx");

          }
      } con.Close();

  }

What I have tried:

After I click the button, it turns out to get this error.

Posted 8-May-21 6:54am

Member 15100384

Updated 8-May-21 8:16am

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Wendelius · Answer 1 · 2021-05-08T07:49:00

Solution 1

You're calling the Fill method for the data adapter before you have set the SQL statement. Try setting the CommandText before Fill

Another observation: Never concatenate values directly to a statement, instead always use parameters.

Posted 8-May-21 7:49am

Wendelius

Updated 8-May-21 8:02am

v2

OriginalGriff · Answer 2 · 2021-05-08T08:16:00

To exaplian what Wendelius has said: never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?