Tip - How To Correctly Add Response HttpHeaders In C# & .NET

Our previous post, “Stop Prefixing HTTP Headers With X”, talked about prefixes to to custom headers.

In this post, we will look at how to correctly add headers to responses.

The Headers is a property of the Response class, which is a property of the HttpContext.

In order to add our own headers, we inject the HttpContext into our endpoint and then use that to access the Response, and in turn, the Headers collection, which we can then manipulate.

app.MapGet("/", (HttpContext context) =>
{
  // Manipulate our headers here
});

The next question is which method to call to add the actual header(s) and it’s corresponding value(s).

Here you have options.

Add

You can use the Add method to add headers to the response, like so:

app.MapGet("/", (HttpContext context) =>
{
   context.Response.Headers.Add("YourResponseHeaderNameHere", "YourValueHere");
});

The danger with this method is it will throw an exception if you try to add a header that already exists.

Append

The other, safer method is to use the Append method. This will also add headers and their corresponding values to the Headers collectiom, but it will not throw an exception if the header exists. It will just add the header again with the new value.

app.MapGet("/", (HttpContext context) =>
{
   context.Response.Headers.Append("YourResponseHeaderNameHere", "YourValueHere");
   context.Response.Headers.Append("YourResponseHeaderNameHere", "YourOtherValueHere");
});

This will return the following:

Typically, you would not want this.

Headers Collection

The other option is to set the headers directly via index, like this:

app.MapGet("/", (HttpContext context) =>
{
    context.Response.Headers["YourResponseHeaderNameHere"] = "YourValueHere";
    context.Response.Headers["YourResponseHeaderNameHere"] = "YourOtherValueHere";

    return Results.Ok("Hello World!");
});

This will return the following:

Here we can see that only the latest header value is preserverd.

You would typically use this approach if there is a possibility that subsequent middleware might want to set the header and you don’t want duplicates or exceptions.

TLDR

To add custom headers to your response, can inject the HttpContext into your endpoints and throught that access the Headers object of the Response.

The code is in my GitHub.

Happy hacking!