A new experiment: Call .NET gRPC services from the browser with gRPC-Web

James Newton-King

I’m excited to announce experimental support for gRPC-Web with .NET. gRPC-Web allows gRPC to be called from browser-based apps like JavaScript SPAs or Blazor WebAssembly apps.

gRPC-Web for .NET promises to bring many of gRPC’s great features to browser apps:

  • Strongly-typed code-generated clients
  • Compact Protobuf messages
  • Server streaming

What is gRPC-Web

It is impossible to implement the gRPC HTTP/2 spec in the browser because there is no browser API with enough fine-grained control over HTTP requests. gRPC-Web solves this problem by being compatible with HTTP/1.1 and HTTP/2.

gRPC-Web is not a new technology. There is a stable gRPC-Web JavaScript client, and a proxy for translating between gRPC and gRPC-Web for services. The new experimental packages allow an ASP.NET Core gRPC app to support gRPC-Web without a proxy, and allow the .NET Core gRPC client to call gRPC-Web services. (great for Blazor WebAssembly apps!)

New opportunites with gRPC-Web

  • Call ASP.NET Core gRPC apps from the browser – Browser APIs can’t call gRPC HTTP/2. gRPC-Web offers a compatible alternative.
    • JavaScript SPAs
    • .NET Blazor Web Assembly apps
  • Host ASP.NET Core gRPC apps in IIS and Azure App Service – Some servers, such as IIS and Azure App Service, currently can’t host gRPC services. While this is actively being worked on, gRPC-Web offers an interesting alternative that works in every environment today.
  • Call gRPC from non-.NET Core platforms – Some .NET platforms HttpClient doesn’t support HTTP/2. gRPC-Web can be used to call gRPC services on these platforms (e.g. Blazor WebAssembly, Xamarin).

Note that there is a small performance cost to gRPC-Web, and two gRPC features are no longer supported: client streaming and bi-directional streaming. (server streaming is still supported!)

Server gRPC-Web instructions

If you are new to gRPC in .NET, there is a simple tutorial to get you started.

gRPC-Web does not require any changes to your services, the only modification is startup configuration. To enable gRPC-Web with an ASP.NET Core gRPC service, add a reference to the Grpc.AspNetCore.Web package. Configure the app to use gRPC-Web by adding AddGrpcWeb(...) and UseGrpcWeb() in the startup file:


public void ConfigureServices(IServiceCollection services)

public void Configure(IApplicationBuilder app)

    // Add gRPC-Web middleware after routing and before endpoints

    app.UseEndpoints(endpoints =>

Some additional configuration may be required to call gRPC-Web from the browser, such as configuring the app to support CORS.

Client gRPC-Web instructions

The JavaScript gRPC-Web client has instructions for setting up a gRPC-Web client to use in browser JavaScript SPAs.

Calling gRPC-Web with a .NET client is the same as regular gRPC, the only modification is how the channel is created. To enable gRPC-Web, add a reference to the Grpc.Net.Client.Web package. Configure the channel to use the GrpcWebHandler:

// Configure a channel to use gRPC-Web
var handler = new GrpcWebHandler(GrpcWebMode.GrpcWebText, new HttpClientHandler());
var channel = GrpcChannel.ForAddress("https://localhost:5001", new GrpcChannelOptions
        HttpClient = new HttpClient(handler)

var client = Greeter.GreeterClient(channel);
var response = await client.SayHelloAsync(new GreeterRequest { Name = ".NET" });

To see gRPC-Web with .NET in action, take a moment to read a great blog post written by Steve Sanderson that uses gRPC-Web in Blazor WebAssembly.

Try gRPC-Web with ASP.NET Core today

Preview packages are on NuGet:

Documentation for using gRPC-Web with .NET Core can be found here.

gRPC-Web for .NET is an experimental project, not a committed product. We want to test that our approach to implementing gRPC-Web works, and get feedback on whether this approach is useful to .NET developers compared to the traditional way of setting up gRPC-Web via a proxy. Please add your feedback here or at the https://github.com/grpc/grpc-dotnet to ensure we build something that developers like and are productive with.



Discussion is closed. Login to edit/delete existing comments.

  • Darren Sandford 0

    I successfully managed to get a grpc-web service working using Kestrel, but then attempted to convert it to using Http.sys (not under IIS), but I’ve tried and failed to get it working, always getting a “The response headers cannot be modified because the response has already starteed.” invalid operation exception.

    Am I completely barking up the wrong tree – does this experiment not support http.sys in this way?

    A little frustrated – this is exactly the use case we need it for!

  • Ben Hayat 0

    James, can you bring us up to date on the status of gRPC-Web and Blazor Client please? Perhaps a new blog as we’re getting closer to Blazor client release?


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