In .NET 6 we have introduced new type called RandomAccess that allows for reading and writing to specific file offset.

As of today, all it's Read* and Write* methods throw when given handle points to a non-seekable file like socket or pipe:

But it's not a problem for it's internal implementation (used by FileStream):

And we use it's internal API surface to workaround this limitation is CoreLib (mind the call to RandomAccess.WriteAtOffset (internal), not RandomAccess.Write (public):

We should relax the public requirements and make RandomAccess work with non-seekable files.

This is going to require:

• a breaking change doc
• implementation change (stop throwing exception)
• test changes (this test shows how to easily create a handle that points to non-seekable pipe)

Everything motioned above should be a single PR. In the same or separate PR, the Unix implementation of overloads that accept multiple buffers should start using readv and writev sys-calls. This should be relatively easy (just search for preadv and pwritev and reuse the patterns)

But it's going to allow to:

• switch from heavy FileStream to a very lightweight RandomAccess everywhere where we don't need buffering in BCL (and ofc outside of BCL). Sample perf gains: File.*AllText* optimizations #58167 (comment)
• remove a lot of code duplication. Example: https://github.com/dotnet/runtime/blob/main/src/libraries/System.IO.Pipes/src/System/IO/Pipes/PipeStream.ValueTaskSource.cs