Calling C# Code From WASM

Once a WASM module has been instantiated WASM code can be executed in a completely contained "sandbox" - the WASM code cannot interact with anything outside of the box. This is excellent for ensuring security and determinism, but it does significantly limit the capabilities of WASM code. The Linker allows C# methods to be exposed to the sandbox, allowing WASM code to call it directly.

Limitations

WASM supports a very limited set of types, functions defined in the Linker can only use these types:

WASM Type	C# Type
i32	int
i64	long
f32	float
f64	double
externref	Any class/object
funcref	Wasmtime.Function
v128	Wasmtime.V128

Example

To add things to a Linker simply Define them. The simplest use case is directly adding a C# function to call or adding a WASI feature. Alternatively complex systems can be assembled from multiple independent WASM modules by adding an Instance or adding a Module.

Defining Functions

A C# function/method can be defined:

// Lambda function
linker.DefineFunction("module", "add", (int a, int b) => a + b);

// Method
linker.DefineFunction("module", "sub", Subtract);

static int Subtract(int a, int b)
{
    return a - b;
}

Defining WASI Features

A WASI feature (such as a random number generator) can be defined:

var rng = new CryptoRandomSource();
linker.Define(rng);

Defining An Instance

An Instance can be defined, to add all of it's exports for other WASM code to call:

Instance instance;
Linker linker;
linker.DefineInstance(store, "the_name", instance);

tip

This is an advanced use case which requires manually loading and instantiating modules! Read more.

Defining A Module

Module module;
Linker linker;
linker.DefineModule(store, module);

tip

This is an advanced use case which requires manually loading modules! Read more.

WASI

WASI is the WebAssembly System Interface. WASI is an API specification that provides access to several operating-system-like features such as filesystems, clocks, and random numbers which can all be added to the Linker for WASM code to call directly. Read more about using WASI here.