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Create, read, modify, write and execute WebAssembly (WASM) files from .NET-based applications.


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WebAssembly for .NET



Only WebAssembly 1.0 is supported! Most WASM files target a higher version and will encounter errors if you try to load them with WebAssembly for .NET.

A library able to create, read, modify, write and execute WebAssembly (WASM) files from .NET-based applications. Execution does not use an interpreter or a 3rd party library: WASM instructions are mapped to their .NET equivalents and converted to native machine language by the .NET JIT compiler.

Available on NuGet at .

Getting Started

  • Use the WebAssembly.Module class to create, read, modify, and write WebAssembly (WASM) binary files.
    • Module.ReadFromBinary reads a stream into an instance, which can then be inspected and modified through its properties.
      • Most WASM files use post-1.0 features and will experience errors when you try to load them.
    • WriteToBinary on a module instance writes binary WASM to the provided stream.
  • Use the WebAssembly.Runtime.Compile class to execute WebAssembly (WASM) binary files using the .NET JIT compiler.
    • Most WASM files have many imports and exports--you'll need to cover these yourself.
    • This should work for most WASM 1.0 files, but spec compliance is not perfect.
    • This will not work for any newer-than-1.0 files

You're welcome to report a bug if you can share a WASM file that has a problem, but no one is actively working on this project so a fix may not come.

Sample: Create and execute a WebAssembly file in memory

using System;
using WebAssembly; // Acquire from
using WebAssembly.Instructions;
using WebAssembly.Runtime;

// Module can be used to create, read, modify, and write WebAssembly files.
var module = new Module(); // In this case, we're creating a new one.

// Types are function signatures: the list of parameters and returns.
module.Types.Add(new WebAssemblyType // The first added type gets index 0.
    Parameters =
        WebAssemblyValueType.Int32, // This sample takes a single Int32 as input.
        // Complex types can be passed by sending them in pieces.
    Returns =
        // Multiple returns are supported by the binary format.
        // Standard currently allows a count of 0 or 1, though.
// Types can be re-used for multiple functions to reduce WASM size.

// The function list associates a function index to a type index.
module.Functions.Add(new Function // The first added function gets index 0.
    Type = 0, // The index for the "type" value added above.

// Code must be passed in the exact same order as the Functions above.
module.Codes.Add(new FunctionBody
    Code =
        new LocalGet(0), // The parameters are the first locals, in order.
        // We defined the first parameter as Int32, so now an Int32 is at the top of the stack.
        new Int32CountOneBits(), // Returns the count of binary bits set to 1.
        // It takes the Int32 from the top of the stack, and pushes the return value.
        // So, in the end, there is still a single Int32 on the stack.
        new End(), // All functions must end with "End".
        // The final "End" also delivers the returned value.

// Exports enable features to be accessed by external code.
// Typically this means JavaScript, but this library adds .NET execution capability, too.
module.Exports.Add(new Export
    Kind = ExternalKind.Function,
    Index = 0, // This should match the function index from above.
    Name = "Demo", // Anything legal in Unicode is legal in an export name.

// We now have enough for a usable WASM file, which we could save with module.WriteToBinary().
// Below, we show how the Compile feature can be used for .NET-based execution.
// For stream-based compilation, WebAssembly.Compile should be used.
var instanceCreator = module.Compile<Sample>(); // Sample is defined later.

// Instances should be wrapped in a "using" block for automatic disposal.
// This sample doesn't import anything, so we pass an empty import dictionary.
using (var instance = instanceCreator(new ImportDictionary()))
    // FYI, instanceCreator can be used multiple times to create independent instances.
    Console.WriteLine(instance.Exports.Demo(0)); // Binary 0, result 0
    Console.WriteLine(instance.Exports.Demo(1)); // Binary 1, result 1,
    Console.WriteLine(instance.Exports.Demo(42));  // Binary 101010, result 3
} // Automatically release the WebAssembly instance here.

public abstract class Sample
    // Sometimes you can use C# dynamic instead of building an abstract class like this.
    public abstract int Demo(int value);

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