Question

For some reason the Sqlparameter for my IN() clause is not working. The code compiles fine, and the query works if I substitute the parameter with the actual values

StringBuilder sb = new StringBuilder();
            foreach (User user in UserList)
            {
                sb.Append(user.UserId + ",");
            }

            string userIds = sb.ToString();
            userIds = userIds.TrimEnd(new char[] { ',' });


SELECT userId, username 
FROM Users 
WHERE userId IN (@UserIds) 

Answer 1

You have to create one parameter for each value that you want in the IN clause.

The SQL needs to look like this:

SELECT userId, username 
FROM Users 
WHERE userId IN (@UserId1, @UserId2, @UserId3, ...) 

So you need to create the parameters and the IN clause in the foreach loop.
Something like this (out of my head, untested):

StringBuilder sb = new StringBuilder();
int i = 1;

foreach (User user in UserList)
{
    // IN clause
    sb.Append("@UserId" + i.ToString() + ",");

    // parameter
    YourCommand.Parameters.AddWithValue("@UserId" + i.ToString(), user.UserId);

    i++;
}