Marshalling “EGLRenderResolutionScaleProperty” to ANGLE from C# using P/Invoke

Question

I am trying to get ANGLE working in C# using P/Invoke. Basically, I am creating a simple 2D surface, and then passing that off to skia (using SkiaSharp). Everything is working and all that, but I am having a problem marshalling PropertySet to the unmanaged code.

This bit works fine:

// the properties
var props = new PropertySet();
props.Add("EGLNativeWindowTypeProperty", swapChainPanel);
// the surface attributes
int[] surfaceAttrs = {
    EGL_ANGLE_SURFACE_RENDER_TO_BACK_BUFFER, EGL_TRUE,
    EGL_NONE
};

// create the surface
surface = eglCreateWindowSurface(eglDisplay, eglConfig, props, surfaceAttrs);

My import looks like this:

[DllImport("libEGL.dll")]
public static extern IntPtr eglCreateWindowSurface(
    IntPtr dpy, 
    IntPtr config, 
    [MarshalAs(UnmanagedType.IInspectable)] object win, 
    int[] attrib_list);

The problem comes in when I am trying to set my scaling for high resolution screens. This "should" work:

var scale = PropertyValue.CreateSingle(2);
props.Add("EGLRenderResolutionScaleProperty", scale);

It works when using C++, but not in C#. I think I have got it down to the fact that the actual value is not being marshalled correctly. This is because when debugging in ANGLE code, it dies over here:

ComPtr<ABI::Windows::Foundation::IPropertyValue> propertyValue;
ABI::Windows::Foundation::PropertyType propertyType;
// ... 
result = propertyValue->get_Type(&propertyType);

https://github.com/Microsoft/angle/blob/54b1fd01f7fddcd7011d5a04e9259edace8a13da/src/libANGLE/renderer/d3d/d3d11/winrt/InspectableNativeWindow.cpp#L242

The runtime exception is:

Run-Time Check Failure #0 - The value of ESP was not properly saved across a function call. This is usually a result of calling a function declared with one calling convention with a function pointer declared with a different calling convention.

Are there any tips, or solutions?

Here is all my code for reference: https://gist.github.com/mattleibow/eacb9c9e87f306b218d99c713c532a82

Original ANGLE issue: https://github.com/Microsoft/angle/issues/89

EDIT

After further investigation, I found that for some reason PropertyValue in C# is different to the same type in C++ (via Windows Runtime Component).

I tried this:

static PropertySet^ CreateSurface(SwapChainPanel^ panel, float scale)
{
    PropertySet^ surfaceCreationProperties = ref new PropertySet();
    surfaceCreationProperties->Insert(L"EGLNativeWindowTypeProperty", panel);
    Object^ scaleVal = PropertyValue::CreateSingle(scale);
    surfaceCreationProperties->Insert(L"EGLRenderResolutionScaleProperty", scaleVal);
    return surfaceCreationProperties;
}

This method created a PropertySet in C++ and then returns that to C#. This works fine and my ANGLE session is all good.

Now, if I change the code to return the PropertyValue::CreateSingle(scale) directly, this fails again. If I pass the PropertyValue.CreateSingle(scale) from C# into this C++ method, I find that the objects are not similar.

In the debugger locals I get this for the object constructed in C#:

0x09f10ba4 <Information not available, no symbols loaded for coreclr.dll>
Platform::Object ^

If the object is constructed in C++, I get this:

0x019162e8 2.00000000
Platform::Object ^ {WinTypes.dll!Windows::Foundation::Value<Windows::Foundation::ValueScalar<float> >}

Shouldn't the object be the same, since they are the same type? What is more scary is that if I pass this value across the boundary, the type changes.

Removed edit 2 and set it as an answer.

Answer 1

I'm not answering your question directly.

But a proven-working alternative way is to create a C++ WindowsRuntime Component wrapper project, aka, use C++/CLI(or CXX, should I say?)to do the interop.

You can expose methods like below, expose APIs depending on your needs.

void InitializeEGL(Windows::UI::Core::CoreWindow^ window);
void InitializeEGL(Windows::UI::Xaml::Controls::SwapChainPanel ^ window);

or, if you want to have finer granularity, declare something like below in the C++ WinRT component,

void EglCreateWindowSurface(Platform::IntPtr display, Platform::IntPtr config, Windows::Foundation::Collections::PropertySet^ propertySet);

This little trick will enable you to get around the C# marshaling issue, and I have verfied it to work.

And I'll continue to investigate the C# interop approach.

Answer 2

After doing a bit of searching, I found the best (and only) way to do this was to create a Windows Runtime Component.

As I could not have a "hard" reference to the component - this library must work without ANGLE present. I opted for a small component with a C API so that I could just P/Invoke it when I needed it. This was my method in C++:

void PropertySetInterop_AddSingle(ABI::Windows::Foundation::Collections::IPropertySet* propertySet, HSTRING key, float scale)
{
    using namespace Microsoft::WRL;
    using namespace Microsoft::WRL::Wrappers;
    using namespace ABI::Windows::Foundation;
    using namespace ABI::Windows::Foundation::Collections;

    ComPtr<IPropertySet> propSet = propertySet;
    ComPtr<IMap<HSTRING, IInspectable*>> map;
    propSet.As(&map);

    ComPtr<IPropertyValueStatics> propValueFactory;
    GetActivationFactory(HStringReference(RuntimeClass_Windows_Foundation_PropertyValue).Get(), &propValueFactory);

    ComPtr<IInspectable> valueInspectable;
    propValueFactory->CreateSingle(scale, &valueInspectable);

    boolean replaced;
    map->Insert(key, valueInspectable.Get(), &replaced);
}

I am using COM here so that I can use this in C/C++ code. My P/Invoke code is this:

internal static class PropertySetInterop
{
    public static void AddSingle(PropertySet properties, string key, float value)
    {
        PropertySetInterop_AddSingle(properties, key, value);
    }

    public static void AddSize(PropertySet properties, string key, float width, float height)
    {
        PropertySetInterop_AddSize(properties, key, width, height);
    }

    private const string libInterop = "SkiaSharp.Views.Interop.UWP.dll";

    [DllImport(libInterop)]
    private static extern void PropertySetInterop_AddSingle(
        [MarshalAs(UnmanagedType.IInspectable)] object properties,
        [MarshalAs(UnmanagedType.HString)] string key,
        float value);

    [DllImport(libInterop)]
    private static extern void PropertySetInterop_AddSize(
        [MarshalAs(UnmanagedType.IInspectable)] object properties,
        [MarshalAs(UnmanagedType.HString)] string key,
        float width, float height);
}