manski's blog

C# and GC.KeepAlive()

Today, while browsing some C++/CLI code, I stumbled upon several calls to GC.KeepAlive(someObj).

Immediately I thought memory leak – because I thought KeepAlive() would keep the object alive indefinitely.

Fortunately, this turned out to be wrong, though.

After reading the documentation of GC.KeepAlive() (couldn’t really figure it out), I did some decompiling and found out that GC.KeepAlive() looks like this:

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[MethodImpl(MethodImplOptions.NoInlining)]
public static void KeepAlive(object obj)
{
}

It just does nothing. So what’s its purpose?

It’s there to fake an access to a variable.

Why? The .NET garbage collector may collect a variable directly after its last use – and not necessarily, contrary to common belief, at the end of the variable’s scope.

Consider this code:

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class SomeClass
{
  // This field is initialized somewhere 
  // in the constructor (not shown here).
  public SomeOtherClass Value;
 
  ...
}
 
...
 
void MyMethod()
{
  SomeClass obj = new SomeClass();
  SomeOtherMethod(obj.Value);
  YetAnotherMethod();
  // obj still alive here? Possibly not.
}

The garbage collector may collect obj just after the runtime has retrieved obj.Value (line 15), i.e. before SomeOtherMethod() is even called.

Note: The exact line where obj will be marked for collection is up to the JIT compiler. The behavior describe above seems to be called “lookahead optimization”.

Usually this optimization not a problem. It becomes a problem, however, if SomeClass has a finalizer:

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class SomeClass
{
  public SomeOtherClass Value;
 
  ~SomeClass()
  {
     // "Value" can't be used anymore 
     // after Dispose() has been called.
     this.Value.Dispose();
  }
}

So, if obj‘s finalizer is executed before SomeOtherMethod() is called, SomeOtherMethod() won’t be able to use obj.Value anymore.

To solve this problem, add GC.KeepAlive() after the call to SomeOtherMethod(), like this (line 5):

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void MyMethod()
{
  SomeClass obj = new SomeClass();
  SomeOtherMethod(obj.Value);
  GC.KeepAlive(obj);
  YetAnotherMethod();
}

This way, the garbage collector won’t collect obj (and thus run its finalizer) before line 5 has been reached.

Notes:

  • The implementation of the finalizer of SomeClass is flawed – as the examples in this article show. The user shouldn’t need to worry about Value being disposed too early.

    • Rule of thumb: A finalizer should only dispose the resources of its own class, not resources of some member (by calling Dispose() on members).
    • The problem with the finalizer persists if Value is an unmanaged resource/pointer that’s being passed to SomeOtherMethod(). This is always possible in C++/CLI. In C# Value could be of type IntPtr.
    • In the examples above, consider implementing and using IDisposable for SomeClass instead of GC.KeepAlive(), if you need a finalizer.
    • You still need to use GC.KeepAlive() if you can’t change the implementation of SomeClass.
  • Using GC.KeepAlive() is like using GCHandle, just more light-weight and faster.
  • GC.KeepAlive() only works because it can’t be inlined by the compiler (MethodImplOptions.NoInlining).

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