[DllImportGenerator] ICustomMarshaler ~support

[SupinePandora43]

Note that this example doesn't make use of cookie and marshaler instances, and need additional code to work.

Input

internal class NullMarshaler : ICustomMarshaler {
    public static ICustomMarshaler GetInstance(string pstrCookie){
        return new NullMarshaler();
    }
    public IntPtr MarshalManagedToNative(object obj){
        return IntPtr.Zero;
    }
    public object MarshalNativeToManaged(IntPtr native){
        return null;
    }
}
public static class Methods {
    [GeneratedDllImport("example")]
    [return: MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(NullMarshaler))]
    public static partial string ExampleMethod([MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(NullMarshaler))]string input);
}

Example Generated output

public static partial string ExampleMethod(string input){
    IntPtr __input_gen_native = default;
    string __retVal = default;
    IntPtr __retVal_gen_native = default;
    //
    // Marshal
    //
    ICustomMarshaler marshaler = NullMarshaler.GetInstance(null);
    __input_gen_native = marshaler.MarshalManagedToNative(input);
    //
    // Invoke
    //
    __retVal_gen_native = ExampleMethod__PInvoke(__input_gen_native);
    //
    // Unmarshal
    //
    __retVal = (string)marshaler.MarshalNativeToManaged(__retVal_gen_native);
    return __retVal;
}
[DllImport("example", EntryPoint = "ExampleMethod")]
private static IntPtr ExampleMethod__PInvoke__(IntPtr input);

Probably MarshalUsing should be used instead of custom marshalers.

[SupinePandora43]

the only thing how it relates to NativeAot is that NativeAot doesn't support Custom Marshalers.

[jkoritzinsky]

We've decided that for V1 of the DllImportGenerator that we plan to not support ICustomMarshaler-based marshalling due to its limitations around value types.