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JavaScript [JSImport]/[JSExport] interop

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Note

This isn't the latest version of this article. For the current release, see the .NET 8 version of this article.

Warning

This version of ASP.NET Core is no longer supported. For more information, see .NET and .NET Core Support Policy. For the current release, see the .NET 8 version of this article.

Important

This information relates to a pre-release product that may be substantially modified before it's commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

For the current release, see the .NET 8 version of this article.

This article explains how to run .NET from JavaScript (JS) using JS [JSImport]/[JSExport] interop.

For additional guidance, see the Configuring and hosting .NET WebAssembly applications guidance in the .NET Runtime (dotnet/runtime) GitHub repository.

Existing JS apps can use the expanded client-side WebAssembly support to reuse .NET libraries from JS or to build novel .NET-based apps and frameworks.

Note

This article focuses on running .NET from JS apps without any dependency on Blazor. For guidance on using [JSImport]/[JSExport] interop in Blazor WebAssembly apps, see JavaScript JSImport/JSExport interop with ASP.NET Core Blazor.

These approaches are appropriate when you only expect to run on WebAssembly (WASM). Libraries can make a runtime check to determine if the app is running on WASM by calling OperatingSystem.IsBrowser.

Prerequisites

.NET SDK (latest version)

Install the wasm-tools workload in an administrative command shell, which brings in the related MSBuild targets:

dotnet workload install wasm-tools

The tools can also be installed via Visual Studio's installer under the ASP.NET and web development workload in the Visual Studio installer. Select the .NET WebAssembly build tools option from the list of optional components.

Optionally, install the wasm-experimental workload, which contains experimental project templates for getting started with .NET on WebAssembly in a browser app (WebAssembly Browser App) or in a Node.js-based console app (WebAssembly Console App). This workload isn't required if you plan to integrate JS [JSImport]/[JSExport] interop into an existing JS app.

dotnet workload install wasm-experimental

The templates can also be installed from the Microsoft.NET.Runtime.WebAssembly.Templates NuGet package with the following command:

dotnet new install Microsoft.NET.Runtime.WebAssembly.Templates

For more information, see the Experimental workload and project templates section.

Namespace

The JS interop API described in this article is controlled by attributes in the System.Runtime.InteropServices.JavaScript namespace.

Project configuration

To configure a project (.csproj) to enable JS interop:

  • Set the target framework moniker ({TARGET FRAMEWORK} placeholder):

    <TargetFramework>{TARGET FRAMEWORK}</TargetFramework>

    .NET 7 (net7.0) or later is supported.

  • Enable the AllowUnsafeBlocks property, which permits the code generator in the Roslyn compiler to use pointers for JS interop:

    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

    Warning

    The JS interop API requires enabling AllowUnsafeBlocks. Be careful when implementing your own unsafe code in .NET apps, which can introduce security and stability risks. For more information, see Unsafe code, pointer types, and function pointers.

The following is an example project file (.csproj) after configuration. The {TARGET FRAMEWORK} placeholder is the target framework:

<Project Sdk="Microsoft.NET.Sdk.WebAssembly">

  <PropertyGroup>
    <TargetFramework>{TARGET FRAMEWORK}</TargetFramework>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

</Project>

  • Set the target framework moniker:

    <TargetFramework>net7.0</TargetFramework>

    .NET 7 (net7.0) or later is supported.

  • Specify browser-wasm for the runtime identifier:

    <RuntimeIdentifier>browser-wasm</RuntimeIdentifier>

  • Specify an executable output type:

    <OutputType>Exe</OutputType>

  • Enable the AllowUnsafeBlocks property, which permits the code generator in the Roslyn compiler to use pointers for JS interop:

    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

    Warning

    The JS interop API requires enabling AllowUnsafeBlocks. Be careful when implementing your own unsafe code in .NET apps, which can introduce security and stability risks. For more information, see Unsafe code, pointer types, and function pointers.

  • Specify WasmMainJSPath to point to a file on disk. This file is published with the app, but use of the file isn't required if you're integrating .NET into an existing JS app.

    In the following example, the JS file on disk is main.js, but any JS filename is permissible:

    <WasmMainJSPath>main.js</WasmMainJSPath>

Example project file (.csproj) after configuration:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net7.0</TargetFramework>
    <RuntimeIdentifier>browser-wasm</RuntimeIdentifier>
    <OutputType>Exe</OutputType>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <WasmMainJSPath>main.js</WasmMainJSPath>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

JavaScript interop on WASM

APIs in the following example are imported from dotnet.js. These APIs enable you to set up named modules that can be imported into your C# code and call into methods exposed by your .NET code, including Program.Main.

Important

"Import" and "export" throughout this article are defined from the perspective of .NET:

  • An app imports JS methods so that they can be called from .NET.
  • The app exports .NET methods so that they can be called from JS.

In the following example:

  • The dotnet.js file is used to create and start the .NET WebAssembly runtime. dotnet.js is generated as part of the app's build output.

    Important

    To integrate with an existing app, copy the contents of the publish output folder† to the existing app's deployment assets so that it can be served along with the rest of the app. For production deployments, publish the app with the dotnet publish -c Release command in a command shell and deploy the output folder's contents with the app.

    †The publish output folder is the target location of your publish profile. The default for a Release profile in .NET 8 or later is bin/Release/{TARGET FRAMEWORK}/publish, where the {TARGET FRAMEWORK} placeholder is the target framework (for example, net8.0).

  • dotnet.create() sets up the .NET WebAssembly runtime.

  • setModuleImports associates a name with a module of JS functions for import into .NET. The JS module contains a dom.setInnerText function, which accepts and element selector and time to display the current stopwatch time in the UI. The name of the module can be any string (it doesn't need to be a file name), but it must match the name used with the JSImportAttribute (explained later in this article). The dom.setInnerText function is imported into C# and called by the C# method SetInnerText. The SetInnerText method is shown later in this section.

  • exports.StopwatchSample.Reset() calls into .NET (StopwatchSample.Reset) from JS. The Reset C# method restarts the stopwatch if it's running or resets it if it isn't running. The Reset method is shown later in this section.

  • exports.StopwatchSample.Toggle() calls into .NET (StopwatchSample.Toggle) from JS. The Toggle C# method starts or stops the stopwatch depending on if it's currently running or not. The Toggle method is shown later in this section.

  • runMain() runs Program.Main.

  • setModuleImports associates a name with a module of JS functions for import into .NET. The JS module contains a window.location.href function, which returns the current page address (URL). The name of the module can be any string (it doesn't need to be a file name), but it must match the name used with the JSImportAttribute (explained later in this article). The window.location.href function is imported into C# and called by the C# method GetHRef. The GetHRef method is shown later in this section.

  • exports.MyClass.Greeting() calls into .NET (MyClass.Greeting) from JS. The Greeting C# method returns a string that includes the result of calling the window.location.href function. The Greeting method is shown later in this section.

  • dotnet.run() runs Program.Main.

JS module:

import { dotnet } from './_framework/dotnet.js'

const { setModuleImports, getAssemblyExports, getConfig, runMain } = await dotnet
  .withApplicationArguments("start")
  .create();

setModuleImports('main.js', {
  dom: {
    setInnerText: (selector, time) => 
      document.querySelector(selector).innerText = time
  }
});

const config = getConfig();
const exports = await getAssemblyExports(config.mainAssemblyName);

document.getElementById('reset').addEventListener('click', e => {
  exports.StopwatchSample.Reset();
  e.preventDefault();
});

const pauseButton = document.getElementById('pause');
pauseButton.addEventListener('click', e => {
  const isRunning = exports.StopwatchSample.Toggle();
  pauseButton.innerText = isRunning ? 'Pause' : 'Start';
  e.preventDefault();
});

await runMain();

import { dotnet } from './_framework/dotnet.js'

const { setModuleImports, getAssemblyExports, getConfig } = await dotnet
  .withDiagnosticTracing(false)
  .withApplicationArgumentsFromQuery()
  .create();

setModuleImports('main.js', {
  window: {
    location: {
      href: () => globalThis.window.location.href
    }
  }
});

const config = getConfig();
const exports = await getAssemblyExports(config.mainAssemblyName);
const text = exports.MyClass.Greeting();
console.log(text);

document.getElementById('out').innerHTML = text;
await dotnet.run();

import { dotnet } from './dotnet.js'

const is_browser = typeof window != "undefined";
if (!is_browser) throw new Error(`Expected to be running in a browser`);

const { setModuleImports, getAssemblyExports, getConfig } = 
  await dotnet.create();

setModuleImports("main.js", {
  window: {
    location: {
      href: () => globalThis.window.location.href
    }
  }
});

const config = getConfig();
const exports = await getAssemblyExports(config.mainAssemblyName);
const text = exports.MyClass.Greeting();
console.log(text);

document.getElementById("out").innerHTML = text;
await dotnet.run();

To import a JS function so it can be called from C#, use the new JSImportAttribute on a matching method signature. The first parameter to the JSImportAttribute is the name of the JS function to import and the second parameter is the name of the module.

In the following example, the dom.setInnerText function is called from the main.js module when SetInnerText method is called:

[JSImport("dom.setInnerText", "main.js")]
internal static partial void SetInnerText(string selector, string content);

In the following example, the window.location.href function is called from the main.js module when GetHRef method is called:

[JSImport("window.location.href", "main.js")]
internal static partial string GetHRef();

In the imported method signature, you can use .NET types for parameters and return values, which are marshalled automatically by the runtime. Use JSMarshalAsAttribute<T> to control how the imported method parameters are marshalled. For example, you might choose to marshal a long as System.Runtime.InteropServices.JavaScript.JSType.Number or System.Runtime.InteropServices.JavaScript.JSType.BigInt. You can pass Action/Func<TResult> callbacks as parameters, which are marshalled as callable JS functions. You can pass both JS and managed object references, and they are marshaled as proxy objects, keeping the object alive across the boundary until the proxy is garbage collected. You can also import and export asynchronous methods with a Task result, which are marshaled as JS promises. Most of the marshalled types work in both directions, as parameters and as return values, on both imported and exported methods.

Functions accessible on the global namespace can be imported by using the globalThis prefix in the function name and by using the [JSImport] attribute without providing a module name. In the following example, console.log is prefixed with globalThis. The imported function is called by the C# Log method, which accepts a C# string message (message) and marshalls the C# string to a JS String for console.log:

[JSImport("globalThis.console.log")]
internal static partial void Log([JSMarshalAs<JSType.String>] string message);

To export a .NET method so it can be called from JS, use the JSExportAttribute.

In the following example, each method is exported to JS and can be called from JS functions:

  • The Toggle method starts or stops the stopwatch depending on its running state.
  • The Reset method restarts the stopwatch if it's running or resets it if it isn't running.
  • The IsRunning method indicates if the stopwatch is running.
[JSExport]
internal static bool Toggle()
{
    if (stopwatch.IsRunning)
    {
        stopwatch.Stop();
        return false;
    }
    else
    {
        stopwatch.Start();
        return true;
    }
}

[JSExport]
internal static void Reset()
{
    if (stopwatch.IsRunning)
        stopwatch.Restart();
    else
        stopwatch.Reset();

    Render();
}

[JSExport]
internal static bool IsRunning() => stopwatch.IsRunning;

In the following example, the Greeting method returns a string that includes the result of calling the GetHRef method. As shown earlier, the GetHref C# method calls into JS for the window.location.href function from the main.js module. window.location.href returns the current page address (URL):

[JSExport]
internal static string Greeting()
{
    var text = $"Hello, World! Greetings from {GetHRef()}";
    Console.WriteLine(text);
    return text;
}

Experimental workload and project templates

To demonstrate the JS interop functionality and obtain JS interop project templates, install the wasm-experimental workload:

dotnet workload install wasm-experimental

The wasm-experimental workload contains two project templates: wasmbrowser and wasmconsole. These templates are experimental at this time, which means the developer workflow for the templates is evolving. However, the .NET and JS APIs used in the templates are supported in .NET 8 and provide a foundation for using .NET on WASM from JS.

The templates can also be installed from the Microsoft.NET.Runtime.WebAssembly.Templates NuGet package with the following command:

dotnet new install Microsoft.NET.Runtime.WebAssembly.Templates

Browser app

You can create a browser app with the wasmbrowser template from the command line, which creates a web app that demonstrates using .NET and JS together in a browser:

dotnet new wasmbrowser

Alternatively in Visual Studio, you can create the app using the WebAssembly Browser App project template.

Build the app from Visual Studio or by using the .NET CLI:

dotnet build

Build and run the app from Visual Studio or by using the .NET CLI:

dotnet run

Alternatively, install and use the dotnet serve command:

dotnet serve -d:bin/$(Configuration)/{TARGET FRAMEWORK}/publish

In the preceding example, the {TARGET FRAMEWORK} placeholder is the target framework moniker.

Node.js console app

You can create a console app with the wasmconsole template, which creates an app that runs under WASM as a Node.js or V8 console app:

dotnet new wasmconsole

Alternatively in Visual Studio, you can create the app using the WebAssembly Console App project template.

Build the app from Visual Studio or by using the .NET CLI:

dotnet build

Build and run the app from Visual Studio or by using the .NET CLI:

dotnet run

Alternatively, start any static file server from the publish output directory that contains the main.mjs file:

node bin/$(Configuration)/{TARGET FRAMEWORK}/{PATH}/main.mjs

In the preceding example, the {TARGET FRAMEWORK} placeholder is the target framework moniker, and the {PATH} placeholder is the path to the main.mjs file.

Additional resources