SQL Server存储过程（如果存在）

Question

Ideally, I'm trying to get a stored procedure to return 1 if exists, or 0 if not.

This is the stored procedure:

CREATE PROCEDURE [dbo].[spCheckForExistingTimecard]
   @userId int,
   @paYPeriodId int,
   @exists bit = 0 OUTPUT
AS
BEGIN
   IF EXISTS (SELECT COUNT (t.TimeCardId) 
              FROM TimeCard AS t
              WHERE t.PayPeriodId = @payPeriodId
                AND t.UserId = @userId )
      RETURN 1
   ELSE
      RETURN 0

Here's the code calling the stored procedure:

 public static int CheckForExistingTimecard(int userId, int payPeriodId)
 {
        using (SqlConnection connection = new SqlConnection(dbMaintenanceConnectionString))
        {
            connection.Open();

            using (SqlCommand sqlCommand = new SqlCommand("spCheckForExistingTimecard", connection))
            {
                sqlCommand.CommandType = CommandType.StoredProcedure;
                sqlCommand.Parameters.AddWithValue("@userId", userId);
                sqlCommand.Parameters.AddWithValue("@payPeriodId", payPeriodId);
                return (int)sqlCommand.ExecuteScalar();
            }
        }
    }

Problem is that I am getting an error

Object reference not set to an instance of an object

on the return line of the calling code.

Any help would be greatly appreciated

Answer 1

As documeneted in officeil site

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.

ExecuteScalar returns null if no records were returned by the query

So this line:

return (int)sqlCommand.ExecuteScalar();

throws error

becaue it is trying to cast null to an int in that case. That'll raise a NullReferenceException.

you need to check for null:

object o = sqlCommand.ExecuteScalar();
item = o == null ? 0 : (int)o;

Answer 2

The value from RETURN can be handled by a SqlParameter with a .Direction = ParameterDirection.ReturnValue . The value that .ExecuteScalar() will catch is a single row, single column returned by a SELECT in your stored procedure.

public static int CheckForExistingTimecard(int userId, int payPeriodId)
{
   using (SqlConnection connection = new SqlConnection(dbMaintenanceConnectionString))
   using (SqlCommand sqlCommand = new SqlCommand("spCheckForExistingTimecard", connection))
   {
       sqlCommand.CommandType = CommandType.StoredProcedure;
       sqlCommand.Parameters.AddWithValue("@userId", userId);
       sqlCommand.Parameters.AddWithValue("@payPeriodId", payPeriodId);

       -- define your parameter for the RETURN value
       sqlCommand.Parameters.Add("@ReturnValue").Direction = ParameterDirection.ReturnValue;

       connection.Open();
       sqlCommand.ExecuteNonQuery();

       -- read the value returned
       int returnValue = (int)sqlCommand.Parameters["@ReturnValue"];

       connection.Close();

       return returnValue;
   }
}