Consider adding strongly-typed Vtbl property to COM interface structs (if trimmable)

[rickbrew]

In implementing my wrapper for IDWriteTextLayout, there are 12 methods that are easily made generic, which relies on passing in a function pointer from the vtbl. These are 6 getters and 6 setters for simple unmanaged value types.

Here's an example of how I'm currently implementing it. A strongly-typed Vtbl property on IDWriteTextLayout would remove the need for the casting in my Get____() and Set____() methods. As long as the property is trimmable, the Vtbl<TSelf> struct should still be trimmable ... right?

// This is repeated for GetFontWeight, GetFontStyle, GetFontStretch, GetUnderline, and GetStrikethrough
// Similar pattern for Set___() methods
public float GetFontSize(int currentPosition, out TextRange textRange)
{
    return GetProperty<float>(
        // This next line of code could be just: this.pDWriteTextLayout->Vtbl->GetFontSize1
        ((IDWriteTextLayout.Vtbl<IDWriteTextLayout>*)this.pDWriteTextLayout->lpVtbl)->GetFontSize1,
        currentPosition,
        out textRange);
}

private T GetProperty<T>(
    delegate* unmanaged<IDWriteTextLayout*, uint, T*, DWRITE_TEXT_RANGE*, int> pGetFunction,
    int currentPosition,
    out TextRange textRange)
    where T : unmanaged
{
    T value;
    DWRITE_TEXT_RANGE dwTextRange;
    HRESULT hr = pGetFunction(
        this.pDWriteTextLayout,
        (uint)currentPosition,
        &value,
        &dwTextRange);
    hr.ThrowOnError();

    textRange = Unsafe.As<DWRITE_TEXT_RANGE, TextRange>(ref dwTextRange);
    return value;
}

Implementation in TerraFX could be:

public unsafe struct IDWriteTextLayout
{
    ...
    public Vtbl<IDWriteTextLayout> Vtbl => ((Vtbl<IDWriteTextLayout>)this.lpvtbl);
    ...
}

[rickbrew]

Hmm, looks like my attempt to use generics here is not supported by the runtime, I get BadImageFormatException with "Bad element type in SizeOf" for the setters, or "Illegal or unimplemented ELEM_TYPE in signature" for the getters. As per dotnet/runtime#9136 this is just how it is right now.

Having the strongly-typed Vtbl property would probably be useful at some point, but not yet for me unfortunately :)

[rickbrew]

Alright I did find a use for this, in my wrappers for IWICComponentInfo and the derived interfaces (e.g. IWICBitmapCodecInfo). Strongly-typed vtbl would help shorten this code, although unfortunately because I can't go generic on the type, I still need the delegate cast (AFAIC -- it still gave BadImageFormatException if I tried generic).

There are many methods on these interfaces that are HRESULT GetFoo(uint cchBuffer, ushort* pszBuffer, uint* cchCopied). You call it once with (0, null, &count) to get the size, then again with the buffer to get the string.

public string Author
{
    get
    {
        if (this.author == null)
        {
            this.author = GetString(
                this.pWICComponentInfo,
                (delegate* unmanaged<void*, uint, ushort*, uint*, int>)
                    ((IWICComponentInfo.Vtbl<IWICComponentInfo>*)this.pWICComponentInfo->lpVtbl)->GetAuthor);
                    // ^^^ that could be simplified to `this.pWICComponentInfo->Vtbl->GetAuthor`
        }

        return this.author;
    }
}

// I couldn't spec this as `GetString<T>(T* pObject, ...) where T : unmanaged, IWICComponentInfo.Interface`, 
// I just get BadImageFormatException
protected static string GetString(void* pObject, delegate* unmanaged<void*, uint, ushort*, uint*, int> getStringFn)
{
    uint cch;
    HRESULT hr = getStringFn(
        pObject,
        0,
        null,
        &cch);
    hr.ThrowOnError();

    if (cch == 0)
    {
        return string.Empty;
    }
    else
    {
        return string.Create(
            (int)cch - 1,
            ((Ptr)pObject, (Ptr)getStringFn),
            static delegate (Span<char> dst, (Ptr pObject, Ptr getStringFn) e)
            {
                fixed (char* pDst = dst)
                {
                    uint cch2;
                    HRESULT hr = ((delegate* unmanaged<void*, uint, ushort*, uint*, int>)(void*)e.getStringFn)(
                        e.pObject.Get(),
                        (uint)(dst.Length + 1),
                        (ushort*)pDst,
                        &cch2);
                    hr.ThrowOnError();

                    if (cch2 != (uint)(dst.Length + 1))
                    {
                        throw new InternalErrorException();
                    }
                }
            });
    }
}

[tannergooding]

This is non-trivial due to there already being a member named Vtbl (the vtbl struct) nested in the type.

There would need to be a different non-conflicting name available for use here. It ultimately would just be a short circuit for the user manually doing:

var lpVtbl = (IUnknown.Vtbl<IUnknown>*)pUnknown.lpVtbl;
lpVtbl->QueryInterface = &QueryInterface

[rickbrew]

We can close this, I found a reasonable solution a long time ago for this particular issue