Bug with multipart boundary field value when re-using the FlurlRequest object about flurl HOT 2 OPEN

A FlurlRequest should never be reused. Much like the HttpRequestMessage that it effectively wraps, it's underpinned by a forward-only stream. The rules mirror those of the HttpClient stack: reuse clients heavily; never reuse requests. Your fix of creating a new one is correct.

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Ah ok, explained like that it makes sense.

With the fluent interface it's actually not that visible that a request is created:

var jiraAddAttachmentUrl = jiraCreateIssueResponse.Self
.AppendPathSegment("attachments");
or
url.WithHeader("Accept", "text/plain")

One might be tempted to reuse a request.

Might be helpful to add this information to the documentation or maybe even throw an Exception in case of PostMultipartAsync as the sent request is not usable.

from flurl.