Description

While HttpWebRequest is a deprecated API, it is still in use by legacy code being ported from .NET Framework to .NET 9.

Under .NET, the HttpWebRequest.AddRange method fails to generate a valid header if called twice.

Reproduction Steps

Run the following code on .NET (I've tested with 6.0 and later):

var wr = HttpWebRequest.CreateHttp("http://example.com");
wr.AddRange(10, 20);
wr.AddRange(50, 60);
Console.WriteLine(wr.Headers["Range"]);

Expected behavior

It should print:

bytes=10-20,50-60

Actual behavior

It prints:

50-60

Note that this is an invalid header value because it's missing the bytes= range specifier.

Regression?

This works as expected under .NET Framework 4.7.2.

Known Workarounds

As a workaround, construct the full Range: header yourself (and call wr.Headers.Add("Range", "bytes=10-20,50-60");).

Configuration

.NET SDK:
 Version:           10.0.100-preview.2.25164.34
 Commit:            374efa6cf4
 Workload version:  10.0.100-manifests.d1fec061
 MSBuild version:   17.14.0-preview-25155-01+d0cb70a0d

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.26100
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\10.0.100-preview.2.25164.34\

Also tested with:

{
  "sdk": {
    "version": "6.0.100",
    "rollForward": "latestFeature"
  }
}

.NET SDK (reflecting any global.json):
 Version:   6.0.428
 Commit:    ef6f5ce48c

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.26100
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.428\

Host:
  Version:      10.0.0-preview.2.25163.2
  Architecture: x64
  Commit:       c81f403737

Other information

In .NET Framework, AddRange https://referencesource.microsoft.com/#System/net/System/Net/HttpWebRequest.cs,02f08fa9123e2ff5 calls SetAddVerified https://referencesource.microsoft.com/#System/net/System/Net/WebHeaderCollection.cs,d91ebc3a42cb6026, which adds the new range value to an ArrayList of ranges.

In .NET, AddRange trims off the range specifier (if there is a current Range value) then overwrites the existing header value: