How to measure elapsed time using DateTime class?

Question

I am encountering a strange behavior here. Control never comes out of the do-while loop. On debugging i found that the value of the variable interval decreases during some of the loops, instead of increasing! I'm missing something obvious here, but can't figure it out.

static void Main(string[] args)
{
    int interval = 0;
    DateTime startTime = DateTime.Now;

    //Task1(); //Task1 can take 4-8 seconds to complete

    do
    {
        Thread.Sleep(100);
        interval = (DateTime.Now - startTime).Milliseconds;
    } while (interval < 10000); //wait for at-least ten seconds from program start, before executing task2

    //Task2();
}

Answer 1

Don't use DateTime for measuring time intervals.

Use Stopwatch class, which was designed exactly for this purpose

var clock = new Stopwatch();
clock.Start();
do
{
    // do something
}
while (clock.ElapsedMilliseconds < 10000)
clock.Stop();

Note: Of course you can use DateTime for measuring time, but if you need precision less then second then Stopwatch is right tool for the job.
And Stopwatch have much more readable and easy to use methods for measuring time

In your particular case: "wait for at-least ten seconds from program start, before executing task2" you can use asynchronous approach

var task1 = StartTask1();

await Task.Delay(10000); // next line will be executed approximately after 10 s

var task2 = StartTask2();