C# error System.NullReferenceException

Question

I have some problems with System.NullReferenceException. When server do things and we first check user is not null and then he disconnect and server progress something and it tr

Answer 1

SendMessage is throwing null exception, check code in your SendMessage Method

if User goes null before call dont call SendMessage:-

 if (User != null)
 {
       User.SendMessage("crash"); //<-- No More System.NullReferenceException... -.-
 }

Answer 2

It looks like the value of User is changing after the test but before the call to SendMessage. If your application is multithreaded I suspect that another thread is setting User to null whilst you are sleeping.

One option would be to grab User and check against it:

var user = User;
if (user != null)
{
    //do some things
    System.Threading.Thread.Sleep(1000);get error)
    user.SendMessage("crash"); // Need to be sure user is in a valid state to make call
}

This way you can be sure that you've got a valid reference. What you'll now need to do is ensure that user is in a valid state for you to call SendMessage.

UPDATE: Since you're keep to avoid adding try/catch blocks you could write a helper function:

void WithUser(Action<User> action)
{
  try
  {
     var user = User;
     if (user != null) action(user);
  }
  catch(Exception e)
  {
    // Log it...
  }
}

Now you can say:

WithUser(user=>
{
  System.Threading.Thread.Sleep(1000);
  user.SendMessage("crash");
});

Answer 3

Who is setting User to null and where? While your thread sleeps, somebody is obviously setting the User variable to null, causing the crash.

Besides: "Magical sleeps" will not solve any problems. You need proper locking here.