V3145. Unsafe dereference of a WeakReference target. The object could have been garbage collected before the 'Target' property was accessed.


The analyzer has detected a case of unsafe use of an object of 'WeakReference' type, which may result in null dereference:

string Foo(WeakReference weak)
{
    Return weak.Target.ToString();
}

Since the object referred to by 'WeakReference' can be garbage collected at any time, there is always a risk that the 'Target' property will return 'null'.

In that case, calling the 'ToString' method will cause a null dereference and a 'NullReferenceException'. One way to prevent potential object removal is to have the object stored in a local variable during the time it is being handled. This is what the fixed version looks like:

string Foo(WeakReference weak)
{
    var weakTarget = weak.Target;
    return weakTarget != null ? weakTarget.ToString() : String.Empty;
}

With the reference stored in the local variable, the garbage collector will not be able to delete that object as long as the reference to it remains on the stack. But even then you must check that local variable for 'null' after writing the reference to it as the object referred to by the 'Target' property could have been already collected by the time the 'Target' is accessed.

Using the 'IsAlive' property for checking if the object is still inside 'WeakReference' will not protect you against 'NullReferenceException' either because the object could as well be collected between the 'IsAlive' check and dereference of the 'Target' property:

char Foo(WeakReference weak)
{
  if (weak.IsAlive)
     return (weak.Target as String)[0];
  return ' ';
}

This is one example of how to use the 'IsAlive' property correctly:

char Foo(WeakReference weak)
{
  var str = weak.Target as String;
  if (weak.IsAlive)
     return str[0];

  return ' ';
}

Bugs Found

Checked Projects
355
Collected Errors
13 303