how to pass null in c#?

Hi,

I am trying to pass a null value as string to a function that will send to a sp.

            String v1;
            String v2;
            AssignedName = e.CommandArgument.ToString();
            Activity = null;

            loadActivity(sqlConnection, v1, v2);

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I know my sp works. also, if v2 is not null, it works fine. Once I use null, I get the error v2 was not supplied. Any ideas? thanks