Native dependencies in Blazor WebAssembly - Issue when try use a callback from C++

[harveytriana]

Excellent and amazing progress in Blazor. I illustrate some experience with this new feature. Also, the purpose is to show that there is an issue when using a C++ callback.

To get into context, I have first written the experiment with a C# net6 console and a C++ library. The same code from this C++ library is then copied into a Blazor WASM project, in a folder and referenced in project file as native reference.

When run in Blazor project then following images show the result. Perfect.

When try to run the code with callback, an exception occurs.

Assertion at /__w/1/s/src/mono/mono/metadata/loader.c:1806, condition '<disabled>' not met

Code details in following repository: BlazorCrank

Thanks in advance

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to 'arch-wasm': @lewing
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Issue Details

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Excellent and amazing progress in Blazor. I illustrate some experience with this new feature. Also, the purpose is to show that there is an issue when using a C++ callback.

To get into context, I have first written the experiment with a C# net6 console and a C++ library. The same code from this C++ library is then copied into a Blazor WASM project, in a folder and referenced in project file as native reference.

When run in Blazor project then following images show the result. Perfect.

When try to run the code with callback, an exception occurs.

Assertion at /__w/1/s/src/mono/mono/metadata/loader.c:1806, condition '<disabled>' not met

Code details in following repository: BlazorCrank

Thanks in advance

Author:	harveytriana
Assignees:	-
Labels:	arch-wasm, untriaged, area-Interop-mono
Milestone:	-

[danroth27]

@lewing @radical

[lambdageek]

@harveytriana To use native code make sure that:

1. You install the wasm-tools workload
2. You build with the property WasmBuildNative set to true. (ie: dotnet build and dotnet run with -p:WasmBuildNative=true or set it in your .csproj)
3. You must label managed methods that are passed to C++ with a [UnmanagedCallersOnly] attribute.
4. The method marked with the UnmanagedCallersOnly attribute must be static. That means to call an instance method on a razor page you will need to pass a GCHandle for the instance to C++, and then pass it back to native. Or use some other method to identify the instance of the Razor page.
5. The method marked with [DllImport] must use a C# 9.0 function pointer rather than a delegate type for the callback argument.
6. But there's currently an issue with using C# function pointer types in [DllImport] methods on WebAssembly. So we have to use IntPtr in the method signature on the managed side instead of delegate *unmanaged<int, void>

Here is a patch for the BlazorCrank project that shows how to make it work.
https://gist.github.com/lambdageek/ebba06a8a5786694e14658d2633cb99f

Result:

[harveytriana]

Excelent. Thanks.