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Cover image for Propagate headers with .NET Core Web API
Nuno Reis
Nuno Reis

Posted on • Originally published at Medium

Propagate headers with .NET Core Web API

Welcome. Today we are going to learn how to propagate any header you receive onto another HTTP service using .NET Core Web API.

Let’s say I have a pipeline which consists of three services, like this:

Alt Text

And I want my service at the end of the line to receive the headers that were sent from the client that requested information from the service at the beginning of the line.

How is this useful in real life? Well, imagine your service down the line needs the accept-language sent from the client to access some information that is language-specific. Propagating the headers is one way to achieve that the service down the line will receive that header.

Let’s see how we can achieve this with a simple solution that requires no workarounds.

Step 1: Create your Web API

You can create it using the command:

dotnet new webapi --name HeaderPropagationDemo --language "C#"
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Step 2: Install HeaderPropagation package

If you’re already using .NET Core 3.1, add this package:

dotnet add package Microsoft.AspNetCore.HeaderPropagation --version 3.1.8
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If you’re using a lower version, add this one:

dotnet add package HeaderPropagation --version 3.0.2
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Step 3: Setup Dependency Injection

Enable middleware on Startup.Configure using:

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Then add the service onto Startup.ConfigureServices:

services.AddHeaderPropagation(o =>  
    // Add Default   
    o.Headers.Add("User-Agent", context => "HeaderPropagationDemo");  
    // Propagate if header exists  
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I’ve only added two specific examples. One for User-Agent where I can specifically define it so it can be added if there is no User-Agent present and the Accept-Language header with no default value.

This is pretty much all you need for a basic setup to propagate the headers you want.

Step 4: Setup a Client (optional)

If you are using HttpClientFactory, you need to add a delegating handler to every client, like this:

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Step 5: See it actually working

Let’s create a controller with an endpoint capable of making a request to one of the GET method which in turn will send us the headers we propagated onto him.

public class PropagationController : ControllerBase  
    private readonly IHttpClientFactory _factory;  

    public PropagationController(IHttpClientFactory factory)  
        _factory = factory;  

    public async Task<IActionResult> Get()  
        var client = _factory.CreateClient("DefaultClient");  

        var result = await client.GetAsync("");  

        return Ok(await result.Content.ReadAsStringAsync());  
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Run your API and make a request to this endpoint. You should receive a response with the headers. Try not passing any User-Agent and if you’ve followed everything correctly, you should be able to see the default value provided on the code.

Using the browser may be a little tricky to modify the headers, you can use an extension for Chrome for example, but I personally use Postman.

Wrap up

That’s it. Hope this has helped you in any way.

Have a nice day!

Top comments (1)

gjaryczewski profile image
Gerard Jaryczewski

Hello, Nuno! Thank you for your explanation. Does it make a difference where app.UseHeaderPropagation is placed? Sometimes the order of middleware usage is critical.