C# is object valid

Question

You can check if an object is null but can you check if an object is valid?

Assert.IsValid(object_name);

For example, the object has been deleted by the garbage collector or someone has done dispose on it. But the pointer is still pointing to that object.

Answer 1

If the object has been freed by the garbage collector, you won't have a reference to it, by definition.

If it's been disposed and that's important to the object's validity, the type ought to provide a way of determining that. (In some cases Dispose can just mean "reset", for example.)

It's rarely appropriate to even allow the possibility of having a reference to a disposed object though - if you use:

using (Foo foo = new Foo())
{
    ...
}

then the object will be disposed at the same time that foo goes out of scope, so this isn't an issue.

Answer 2

If the object has been disposed, there isn't any "live" reference of it, so you can't access it (it's guaranteed that there is no reachable code that can read/write the object) (this in "safe" code... In "unsafe" code there isn't any guarantee of anything. But it's "unsafe" for a reason :-) )

For the IDisposable objects, classes "correctly done" keep a flag that they check (bool isDisposed = false at the beginning, isDisposed = true; in the Dispose()) in every method/property and if the object is already disposed they throw new ObjectDisposedException().

Note that there isn't anything in the C# language/.NET runtime that forbids for a "disposed" object to be "reignited", reused and "re-disposed" again, but it is bad code writing (and to support this "anti-pattern" there is even a GC.ReRegisterForFinalize to balance the GC.SuppressFinalize often done in Dispose())

If you have a WeakReference and you want to check "only for statistical purpose" if the object is still reachable, you use WeakReference.IsValid. If you want a reference to the object to use it, you use WeakReference.Target and check if the return value is null. This is very important!!

var wr = new WeakReference(new List<int>());

// Right!!
var target = (List<int>)wr.Target;
if (target != null)
{
    target.Clear();
}

// Wrong!! The GC could kick in after the if and before the target = 
if (wr.IsAlive)
{
    target = (List<int>)wr.Target;
    target.Clear();
}