Working with WebAssembly and C#: From A to Z in .NET

Introduction: Why WebAssembly is a Game Changer for C# Developers

For a long time, web development was a monopoly of JavaScript. However, with the advent of WebAssembly (Wasm), the situation has changed dramatically. Now, C# developers can compile their code into a binary format that runs in the browser at near-native speed. This opens up huge opportunities: from porting heavy algorithms to creating full-fledged SPA applications on Blazor.

In this article, we will break down how WebAssembly works with C# and .NET, explore key tools (Blazor, Wasmtime, Mono) and write practical examples. By the end, you will be able to independently run C# code in the browser or on the server using Wasm.

Basics of WebAssembly and Its Integration with C#

What is WebAssembly in Simple Terms

WebAssembly is a low-level bytecode that runs in a browser's virtual machine. It does not replace JavaScript but works alongside it. Key advantages:

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• Performance: binary code runs 30-50% faster than JS.
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• Security: isolated execution environment.
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• Portability: the same .wasm file works in Chrome, Firefox, Safari, and Edge.
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For C#, the WebAssembly ecosystem is implemented through several projects: Blazor WebAssembly (UI framework), Mono Wasm Runtime (executes IL code), and Wasmtime (for server-side scenarios).

How C# Compiles to WebAssembly

The process looks like this:

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1. You write code in C#.
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2. The Roslyn compiler turns it into IL (Intermediate Language).
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3. Mono Runtime (or NativeAOT) translates IL into WebAssembly.
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4. The browser loads the .wasm file and executes it.
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Important: there are two approaches — interpretation via Mono (slower but simpler) and AOT compilation (faster but requires configuration).

Practice: Three Ways to Run C# in WebAssembly

1. Blazor WebAssembly — for Web Interfaces

Blazor allows you to write the client side in C# instead of JavaScript. All UI code and business logic are compiled into Wasm.

Example of a simple component:

@page "/counter"\n<h3>Counter: @currentCount</h3>\n<button @onclick="IncrementCount">+1</button>\n\n@code {\n    private int currentCount = 0;\n\n    private void IncrementCount()\n    {\n        currentCount++;\n    }\n}

This code runs in the browser. Blazor automatically loads the Mono Wasm Runtime and your .wasm file. To get started, create a project:

dotnet new blazorwasm -o MyBlazorApp\ncd MyBlazorApp\ndotnet run

Result: a full-fledged SPA application without a single line of JavaScript.

2. Direct Compilation to .wasm with NativeAOT

If you need to execute a specific function (not UI), use NativeAOT — it compiles C# directly into native WebAssembly without the Mono Runtime.

Steps:

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1. Install .NET 8+ SDK and workload: dotnet workload install wasm-tools
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2. Create a console application:
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dotnet new console -n WasmDemo\ncd WasmDemo

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3. Modify .csproj to add Wasm support:
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<PropertyGroup>\n    <OutputType>Exe</OutputType>\n    <TargetFramework>net8.0</TargetFramework>\n    <RuntimeIdentifier>browser-wasm</RuntimeIdentifier>\n    <UseNativeAot>true</UseNativeAot>\n</PropertyGroup>

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4. Write the code:
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using System;\n\npublic class Program\n{\n    public static void Main()\n    {\n