API Proposal: Memory<byte> apis for MemoryStream

[huoyaoyuan]

Background and Motivation

Saw code in roslyn creating MemoryStream from underlying array of ImmutableArray.

https://github.com/dotnet/roslyn/blob/a6f654bb03596c2c3392bb16f082cc1615fac425/src/Compilers/CSharp/Test/Emit/Emit/CompilationEmitTests.cs#L921-L922

Proposed API

namespace System.IO
{
    public class MemoryStream
    {
+        public MemoryStream(Memory<byte> memory);
+        public MemoryStream(ReadOnlyMemory<byte> memory);
+        public MemoryStream(Memory<byte> memory, bool writable);
+        public MemoryStream(Memory<byte> memory, bool writable, bool publicVisible);

        public bool TryGetBuffer(out ArraySegment<byte> buffer);
        // Use a different name to not break `out var`
+       public bool TryGetMemoryBuffer(out Memory<byte> buffer);
    }
}

Usage Examples

new MemoryStream(image1.AsMemory())

Alternative Designs

A new class named ReadOnlyMemoryStream? The current MemoryStream is actually "array stream", but takes the name.

Risks

Unlike span, memory is slower than array AFAIK. Delegating array implementation to Memory may cause performance regression.
Memory and ReadOnlyMemory may require 2 versions of fields, or some casting hacks.

[stephentoub]

Dup of #22838

[huoyaoyuan]

Sure, that issue covers the scenario. It's a bit old and needs work, please don't forget it.