Below example will show you how to pass null value to stored procedure parameter from c#
Passing null value for integer type parameter
//If user id is grater than 0 then assign user id other wise pass null value
if (intUserID > 0)
SqlCmd.Parameters.AddWithValue("@UserID", intUserID);
else
SqlCmd.Parameters.AddWithValue("@UserID", DBNull.Value);
Common Syntax
SqlCmd.Parameters.AddWithValue("@ParameterName", DBNull.Value);