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

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WebAssembly for .NET


A library able to create, read, modify, write and (incomplete) execute WebAssembly (WASM) files from .NET-based applications. Execution does not use an interpreter. 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
    class to create, read, modify, and write WebAssembly (WASM) binary files.
    • There are no known issues with this functionality and the API is stable.
  • Use the
    class to execute WebAssembly (WASM) binary files using the .NET JIT compiler.
    • Missing features may prevent complex WASMs from working:
    • Table imports
    • Handling unreachable code
    • Various edge cases
    • No breaking changes are planned for this API, but may be required for full compatibility.

Please file an issue if you encounter an assembly that works in browsers but not with this library.

Sample: Create and execute a WebAssembly file in memory

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

// We need this later to call the code we're generating. public abstract class Sample { // Sometimes you can use C# dynamic instead of building an abstract class like this. public abstract int Demo(int value); }

static class Program { static void Main() { // 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 = new[]
            WebAssemblyValueType.Int32, // This sample takes a single Int32 as input.
            // Complex types can be passed by sending them in pieces.
        Returns = new[]
            // 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 Instruction[]
            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>();

    // 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.


Development Status

Required for 1.0

  • Leverage the official WebAssembly spec tests to ensure correct behavior.
  • Implement C# 8.0 nullable reference types.

After 1.0

  • Make the compiler extensible: in particular, provide a mechanism to replace the
    -affiliated methods with replacements.
  • Support saving generated assemblies as DLLs on .NET Framework via the above extensibility mechanism.
  • If is fixed, enable saving compiled DLLs on .NET Core builds.
  • Remove the compiler's Data section segment size limit of 4128768 bytes.

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