HttpRequest.Headers returns odd result in multiple headers of same type

[shawnwildermuth]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When accepting a HTTP request with multiple headers of the same type, the collection returns a single element with values separated by a comma.

For example:

GET http://localhost:8888/weatherforecast/
Accept: application/json
Accept: application/xml

app.MapGet("/weatherforecast", (HttpContext ctx) =>
{
  var accepts = ctx.Request.Headers["Accept"];

  return accepts;
})

The result is that the variable accepts returns a single element with both headers concatenated:

Recording.2025-07-15.211805.mp4

Expected Behavior

If the HeaderValue returns a collection, it should return a collection of strings, not one string.

Steps To Reproduce

testheaders.zip

This is a repro case

Exceptions (if any)

No response

.NET Version

10.0.100-preview.5.25277.114

Anything else?

No response

[marcominerva]

ctx.Request.Headers["Accept"] returns a StringValues object. What you're seeing is just its ToString() representation, but you can still access each individual value:

This behavior is the same in both .NET 9.0 and .NET 10 Preview 6.

[shawnwildermuth]

Is this also true for custom types or only standard headers?

[marcominerva]

For all the headers.

[shawnwildermuth]

I am not seeing this with the demo I shared:

Recording.2025-07-17.174802.mp4

NOTE: I've tried this on two machines, with .NET 10 Beta and .NET 9 - Visual Studio and Code. My project files has nothing in it:

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

  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <ItemGroup>
  </ItemGroup>

</Project>

Just in case, here are the SDKs on this machine:

C:\clients\nexcor\Core>dotnet --list-sdks
8.0.412 [C:\Program Files\dotnet\sdk]
9.0.300 [C:\Program Files\dotnet\sdk]

Ideas on how to reproduce?

[marcominerva]

Could you use the .Count property instead of the extension method on the accepts variable? Also, have you tried executing lines 17 and 18 anyway?

[shawnwildermuth]

No difference. Line 18 throws an exception.

[shawnwildermuth]

[marcominerva]

After digging a bit, I understood the behavior. Interestingly, when I use Fiddler to make the request, the Accept headers are split into an array. However, when I use an .http file to send the request, they are concatenated into a single string:

GET http://localhost:5021/api/headers
Accept: application/json
Accept: application/xml
Host: localhost:5021

In fact, the HTTP specification (RFC 7230) states that multiple headers with the same name should be treated as a single header with comma-separated values.

So, the correct way to handle this scenario seems to be something like the following:

var acceptHeader = httpContext.Request.Headers["Accept"];

var acceptTypes = acceptHeader.ToString()?
    .Split(',', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);

In the specific context of standard headers, you can use the GetTypedHeaders() extension method to get a strongly typed representation with the values split into an array:

/*
 Accept: application/json
 Accept: application/xml
 */
var acceptHeader = httpContext.Request.GetTypedHeaders().Accept;
var count = acceptHeader.Count; // Returns 2

[shawnwildermuth]

Great. Thanks for the clarification.