Why is my C# code returning a null value when retrieving a return value from a stored procedure?

Troubleshooting Return Value Retrieval from a Stored Procedure in C#

When attempting to retrieve a return value from a stored procedure in C#, you may encounter an issue where the returned value is null. Here's a detailed explanation of the problem and a solution:

Problem Description:

You have defined a Transact-SQL stored procedure that takes two parameters: @a as input and @b as output. The stored procedure sets @b to the result of a query, and you expect to retrieve the value of @b in your C# code. However, when you execute the stored procedure and attempt to read the return value, you get null.

Code Snippet:

SqlConnection SqlConn = ...;
SqlCommand sqlcomm = new SqlCommand("Validate", SqlConn);
sqlcomm.CommandType = CommandType.StoredProcedure;
sqlcomm.Parameters.Add("...", SqlDbType.VarChar, ParameterDirection.Input);
SqlParameter retval = sqlcomm.Parameters.Add("...", SqlDbType.VarChar);
retval.Direction = ParameterDirection.ReturnValue;
sqlcomm.ExecuteNonQuery(); // MISSING
string retunvalue = (string)sqlcomm.Parameters["..."].Value;

Logic Error:

The missing piece of code in your C# routine is the ExecuteNonQuery call. Executing the stored procedure initiates the database query and updates the output parameter @b. Without this call, the stored procedure will not run, and the return value remains unchanged.

Solution:

Include the ExecuteNonQuery method before retrieving the return value:

sqlcomm.ExecuteNonQuery();
string retunvalue = (string)sqlcomm.Parameters["..."].Value;

After making this change, your code will correctly retrieve the return value assigned to @b in the stored procedure.

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