PropVariant uncontrollable Dispose issues

[smourier]

The PropVarianttype (a class) maps the native PROPVARIANT type.

However, the actual (in version 1.15.0.2 ) version clears its inner data when it's disposed (using native PropVariantClear function)

public void Dispose()
{
    _ = PropVariantClear(this);
    GC.SuppressFinalize(this);
}

The problem is when this type is declared as a parameter (in or out) of a COM method, since the way it will be freed cannot be controlled, it will just happend "at GC collection time" which can cause AV issues difficult to track.

[smourier]

The resolution will cause a breaking change, as we introduce the following changes:

• PropVariantis not sealed anymore
• PropVariant by default still clears itself at Dispose time but it's now overridable
• There's a new .NET PROPVARIANT type deriving from PropVariantthat doesn't clear itself at Dispose time.
• All COM methods will now use PROPVARIANT in lieu of PropVariant.

Relevant code:

class PropVariant
{
  public virtual bool ClearOnDispose => true;

  // Detached is just a raw structure copy (doing nothing special, no AddRef on sub objects, etc.)
  public PROPVARIANT Detached => new PROPVARIANT { <copy internal data>};
  ....
  public void Dispose()
  {
      if (ClearOnDispose)
      {
          Clear(false);
      }
      GC.SuppressFinalize(this);
  }
}

public class PROPVARIANT : PropVariant
{
    public override bool ClearOnDispose => false;
    ...
}

Now, to call a COM native method like this:

HRESULT DoSomething(PROPVARIANT pv);

You'd use this:

using (var pv = new PropVariant(<something>))
{
    myObj.DoSomething(pv.Detached).ThrowOnError();
}