PointerToolkit.TerraFX.Interop.Windows 10.0.20348-preview1

PointerToolkit.TerraFX.Interop.Windows

Provides methods ("operators") that enable compile-time type-safety when casting COM interface pointers from the TerraFX.Interop.Windows package.

C# and .NET do not have struct inheritance. The COM interface structs defined in TerraFX.Interop.Windows project the native COM interfaces defined in the Windows SDK. These COM interfaces have inheritance, but this cannot be expressed in .NET, and the C# compiler knows nothing about it. This gives us the problem where you may have a pointer to a COM object that should be implicitly castable to its base interface, but C# won't let you do it without an explicit cast.

For example, let's say you're doing some drawing with Direct2D and you want to fill a rectangle with a solid color brush:

ID2D1SolidColorBrush* pBrush;
HRESULT hr = pFactory->CreateSolidColorBrush(..., &pBrush); // creates a brush
...
pRenderTarget->FillRectangle(..., pBrush); // error: ID2D1SolidColorBrush* isn't an ID2D1Brush*

To fix this you can of course insert a cast to ID2D1Brush*:

ID2D1SolidColorBrush* pBrush;
HRESULT hr = pFactory->CreateSolidColorBrush(..., &pBrush); // creates a brush
...
pRenderTarget->FillRectangle(..., (ID2D1Brush*)pBrush); // works fine, but error prone

In a large code base, this is error-prone. If you cast to the wrong type you will get errors at runtime -- hopefully. In the worst case you'll have heap corruption, smashed stacks, crashes, and security vulnerabilities. We developers are imperfect beings, and we need the compiler's help at every step along the way.

PointerToolkit, among other things, introduces a suite of ref structs called CastPtr<T, TBase1, ..., TBaseN>. They wrap a pointer and provide implicit casting operators to the provided "base" types.

This package then defines a suite of generated __cast() methods ("operators") that take pointers to these COM objects and return CastPtr<...> to permit you to use a derived pointer type when a base pointer type is needed.

For example:

ID2D1SolidColorBrush* pBrush;
HRESULT hr = pFactory->CreateSolidColorBrush(..., &pBrush); // creates a brush
...
pRenderTarget->FillRectangle(..., __cast(pBrush)); // works great, not error prone!

When using this package, I highly recommend adding global using statics for the various Pointers classes from this package:

global using static TerraFX.Interop.DirectX.Pointers;
global using static TerraFX.Interop.Gdiplus.Pointers;
global using static TerraFX.Interop.Windows.Pointers;
global using static TerraFX.Interop.WinRT.Pointers;

You can then use __cast(p) as if it were one of those ugly-but-useful vendor-specific extensions that C++ compilers have loads of.

Oh, I also recommend using trimming. I use illink in a custom post-build event rather than relying on the built-in facilities that MSBuild and Visual Studio provide. This way I can trim only the DLLs I need trimmed, such as TerraFX.Interop.Windows.dll and PointerToolkit.TerraFX.Interop.Windows.dll.