calculate time difference

Question

I'm try to us the following code

string timespent1 = "SELECT DATEDIFF(minute ,departureTime,entryTime) AS timespent FROM mambo where plateno = '" + TextBox1.Text + "'";

SqlCommand comman = new SqlCommand(timespent1, conn);

comman.Parameters.AddWithValue("@timespent", ToString());

comman.ExecuteNonQuery();

 

where departureTime is the departure time and entryTime is the entry time both in data type of datetime

unfortunately is returnining a null value in the database.