Question

Possible Duplicate:
ExecuteScalar returns null or DBNull (development or production server)

I have a stored procedure that checks to see if a pre-existing file id is associated to an item. If the select statement returns values, it should be true and assign “true” to the bool. however when the select statement returns a null because it doesn’t exist, my code behind still makes the .Execute return “true”

This is my stored proc:

ALTER PROCEDURE [dbo].[Events_TaskIDExists] 
@EventID int
AS
BEGIN
    select TaskID from Events where EventID = @EventID
END

and here is my code behind:

public void hasTaskAssociatedToNote()
{
    String[] Notes = hidSelectedEventIDs.Value.Split(',');
    bool exists = false;
    foreach (var note in Notes)
    {
        var connection = new SqlConnection(ConfigurationManager.ConnectionStrings["OSCIDConnectionString"].ToString());
        var command = new SqlCommand("Events_TaskIDExists", connection);
        command.Parameters.Add(new SqlParameter("@EventID", SqlDbType.Int));
        command.Parameters["@EventID"].Value = Convert.ToInt32(note.Trim());
        command.CommandType = CommandType.StoredProcedure;
        try
        {
            connection.Open();
            exists = command.ExecuteScalar() != null;//causes true when it returns null......
            var temp = command.ExecuteScalar();//this was just to check something else
            if (exists)
            {
                exhibitWarning.Visible = true;
                Warning1.Text = "There is an existing Task associated 0.";
            }
        }
        catch (SqlException sql)
        {
            lblStatus.Text = "Couldn't connect to the Database - Error";
            lblStatus.ForeColor = System.Drawing.Color.Red;
        }
        catch (Exception ex)
        {
            lblStatus.Text = "An error occured";
            lblStatus.ForeColor = System.Drawing.Color.Red;
        }
        finally
        {
            if (connection.State == ConnectionState.Open)
                connection.Close();
        }
    }
}

Answer 1

Your exists variable should be set as:

object result = command.ExecuteScalar();
exists = result != DBNull.Value && result != null;

A null result from SqlCommand.ExecuteScalar() returns a DBNull.Value, not null. Only an empty result set will return a null.

Since you’re selecting TaskID based on EventID, my guess is that you don’t have your database constrained to require a TaskID for every Event, and thus you have null TaskID fields. In other words, you have the Event record containing @EventID, but not an associated Task record (based on TaskID). This condition will return a DBNull.Value rather than a null.

“Return Value Type: System.Object The first column of the first
row in the result set, or a null reference (Nothing in Visual Basic)
if the result set is empty. Returns a maximum of 2033 characters.” –
MSDN –
SqlCommand.ExecuteScalar()