C# Winforms timers / object reference not set to an instance of an object

Question

I have this code:

System.Timers.Timer ti1, ti2, ti3;

private void button1_Click(object sender, EventArgs e)
{
   if(_1 == true)
{
    ti1 = new System.Timers.Timer(1000);
    ti1.Interval = 1000;
    ti1.Elapsed += new System.Timers.ElapsedEventHandler(tiel1);
    ti1.Enabled = true;
}
else
{
    ti1.Stop();
}
}

It successfully works, but because I do not want to use the same code, I created a helper method:

private void my_timer(System.Timers.Timer tim, System.Timers.ElapsedEventHandler Tick)
        {
            tim = new System.Timers.Timer(1000);
            tim.Interval = 1000;
            tim.Elapsed += new System.Timers.ElapsedEventHandler(Tick);
            tim.Enabled = true;
        }

and then I call it my_timer(ti1, tiel1)

but when ti1.Stop() is called it gives me this error "object reference not set to an instance of an object".

Any idea why? Thanks.

It looks like they should be, but can you verify that the created timer(s) in the helper are actually running? — SteveFerg, Jul 03 '15 at 18:24
possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Dmitry, Jul 03 '15 at 19:08
@Dmitry it is not a duplicate so please remove the downvote if you downvoted. :/ — coldfire, Jul 04 '15 at 16:04

Ilya Kogan · Answer 1 · 2015-07-03T18:27:28.580

In C#, parameters are generally passed by value.

Here's what it means.

When you assign a new value to a method parameter, like this:

tim = new System.Timers.Timer(1000);

it means that from now on tim will be a reference to a new Timer, but it does not affect the reference you passed from outside (ti1). So in your case ti1 is still a null reference, because you never assigned any object to it.

ti1 and tim are not the same reference. If you assign (=) something to one, it does not affect the other.

Possible solution

Instead of declaring tim as an argument, you should return the new object from the method:

var tim = ...

...

return tim

and assign the result of the method to ti1:

ti1 = my_timer( ... )

Thanks, It worked perfectly. I have two choices now. :D Thanks again. :) — coldfire, Jul 04 '15 at 16:03

PaulShovan · Accepted Answer · 2015-07-03T18:54:10.800

It is because in c# method parameters are passed by value not by reference.

In case of your code System.Timers.Timer ti1, ti2, ti3; all 3 are reference of System.Timers.Timer containing null values.

When you have passed ti1 as parameter, it passed the value null of ti1 so, after executing the following expression : tim = new System.Timers.Timer(1000); it has the reference of System.Timers.Timer while ti1 is containing null value still

So, Rearrange your my_timer method as following to achieve your expected behavior

private System.Timers.Timer my_timer(System.Timers.Timer tim, System.Timers.ElapsedEventHandler Tick)
    {
        tim.Interval = 1000;
        tim.Elapsed += new System.Timers.ElapsedEventHandler(Tick);
        tim.Enabled = true;
        return tim;
    }

and then trigger the method and use the returned value as following

System.Timers.Timer ti1, ti2, ti3;

private void button1_Click(object sender, EventArgs e)
{
   ti1 = new System.Timers.Timer(1000);
   if(_1 == true)
   {
      ti1 = my_timer(ti1, tiel1)
   }
   else
   {
     ti1.Stop();
   }
}

perhaps you should include in your answer that if you use `ref` the op will get the behavior they want — Conrad Frix, Jul 03 '15 at 18:40

C# Winforms timers / object reference not set to an instance of an object

2 Answers2