README.md

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. WASM instructions are mapped to their .NET equivalents and converted to native machine language by the .NET JIT compiler.

A preview is available via NuGet at https://www.nuget.org/packages/WebAssembly . Incremental updates to the preview are ongoing to deliver the last missing features. The 1.0 release marks when 100% of the "MVP" spec level of WebAssembly features are covered.

Getting Started

• The base namespace is simply WebAssembly.
• Use the Module class to create, read, modify, and write WebAssembly (WASM) binary files.
• Use the Compile class to execute WebAssembly (WASM) binary files using the .NET JIT compiler.

Compiler limitations are discussed in the Development Status section after the sample.

Sample: Create and execute a WebAssembly file in memory

using WebAssembly; // Acquire from https://www.nuget.org/packages/WebAssembly
using WebAssembly.Instructions;
using System;

// 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 WebAssembly.Type // The first added type gets index 0.
        {
            Parameters = new[]
            {
                WebAssembly.ValueType.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.
				WebAssembly.ValueType.Int32,
            },
        });
        // 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 GetLocal(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 number 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, but it's now the result of Int32CountOneBits.
				new End(), // All functions must end with "End".
				// The final "End" also delivers the returned value, if the type says that a value is returned.
			},
        });

        // 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.
        using (var instance = instanceCreator())
        {
            // 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

The Module class is feature-complete; it should be compatible with any "MVP"-level WASM file.

The Compile class coverage of the "MVP"-level WebAssembly spec is still missing the following capabilities:

• Import and export tables, needed for interoperable virtual functions.
• Import delegates, needed to connect WASM files to each other, with the added benefit of making .NET usage more flexible.

Note that although delivering the above capabilities will enable this library to run any "MVP"-level WASM file, it's up to you to satisfy the application-specific imports.

The subsections below contain the list of things I plan to do and the order I'm likely to do them. All development is being done by one person in his spare time for free, and is therefore subject to the associated motivation (and health) risks as any other single-developer project.

1.0

The current objective is to offer the same ("MVP"-level) WebAssembly compatibility as popular web browsers and make final API changes.

• Support importing tables.
• Support exporting tables.
• API changes to support use of delegates for all import types.
• Move all types needed to support compilation (other than the Compile class itself) to the WebAssembly.Runtime namespace.
• Any other API changes deemed necessary to reduce the risk of future breaking changes and make the library more intuitive.

1.1

• Support delegates for all import types.

After 1.1

• Provide WebAssembly streaming binary reader.
• Provide WebAssembly streaming binary writer.
• Make the compiler extensible: in particular, provide a mechanism to replace the System.Reflection.Emit.AssemblyBuilder-affiliated methods with replacements.
• Support saving generated assemblies as DLLs on .NET Framework 4.5+
• If https://github.com/dotnet/corefx/issues/4491 is fixed, enable saving compiled DLLs on .NET Core builds.
• Remove the compiler's Data section segment size limit of 4128768 bytes.
• As they become available, leverage intrinsics added to the .NET runtime where emulation is used now.
• As new feature levels of WebAssembly are released, support them as best as possible and document cases where .NET doesn't provide the necessary fundamental infrastructure.

Under consideration

• Use the known custom section "name" to provide human-readable names to the generated functions. Since this section is required to be at the end of the file by the WebAssembly standard, its use will be disabled by default for more efficient streaming compilation.
• ☣ Option to remove remove range check on linear memory access, for confident users desiring maximum performance.
• 🤔 Add support for automatic implementation of interfaces as an alternative to existing abstract class code.
• 🚀 Extensible optimization framework.
• Validation of Module instances.
• Parse WAT files.
• Parse WAST files.