Multipart request sends and then throws NRE

[DemonikGH]

Describe the bug
I'm trying to send request from Unity using RestSharp 110.2.0 - sending normal request works but when I make multipart request with attachment it sends requests and then immediately throws NullReferenceException from System.Net.Http.MultipartContent.Dispose. I tried attaching file using stream creating callback and file name with same results. This is an attempt to send discord webhook request with attachment and message actually appears on the server despite NRE after sending.

To Reproduce
Sending request with this code succeeds but throws exception:

request.Method = Method.Post;
request.AddFile("file1", file.FullName);
request.AddParameter("payload_json", message.ToJsonString(), ParameterType.RequestBody);
// request.AddHeader("Content-Type", "multipart/form-data");
var response = await client.ExecuteAsync(request); // Throws from ExecuteAsync

Expected behavior
Function successfully returning response after request has been sent.

Stack trace

NullReferenceException: Object reference not set to an instance of an object
System.Net.Http.MultipartContent.Dispose (System.Boolean disposing) (at <92a43029e91441ee9df5714eb5dcd209>:0)
System.Net.Http.HttpContent.Dispose () (at <92a43029e91441ee9df5714eb5dcd209>:0)
RestSharp.RequestContent.Dispose () (at <dfd03b4ab8614e549933c37db1618c06>:0)
RestSharp.RestClient.ExecuteRequestAsync (RestSharp.RestRequest request, System.Threading.CancellationToken cancellationToken) (at <dfd03b4ab8614e549933c37db1618c06>:0)
RestSharp.RestClient.ExecuteAsync (RestSharp.RestRequest request, System.Threading.CancellationToken cancellationToken) (at <dfd03b4ab8614e549933c37db1618c06>:0)
TestComponent.TestWebhookSend () (at Assets/Runtime/Scripts/TestComponent.cs:106)

Desktop (please complete the following information):

• OS: Windows 10
• .NET version: Unity 2023.2.16f1 (I think .NET 6 internally / .NET Standard 2.1 level), both il2cpp AOT and mono runtimes
• Version 110.2.0

[DemonikGH]

After debugging with dnspy I can see what's happening - it seems like RequestContent tries to dispose its content after it has been disposed already by HttpClient which I think it does after sending request. When disposing MultipartContent it does this (decompiled):

protected override void Dispose(bool disposing)
{
	if (disposing)
	{
		foreach (HttpContent httpContent in this.nested_content)
		{
			httpContent.Dispose();
		}
		this.nested_content = null;
	}
	base.Dispose(disposing);
}

So second call to that function would throw NRE while trying to enumerate nested_content. This is probably a bug in the System.Net.Http because Dispose is supposed to be safe to be called twice but that's not something I can replace. Can probably be worked around by not calling dispose on request content that has been disposed by HttpClient on .NET versions where bug is present if that is possible. This seems to have been fixed in reference source by replacing = null with .Clear() but that bug is still in version used by latest Unity.

[LiuweiY]

I encountered the same problem and resolved it by rolling back to version 110.0.0.