When will MemoryStream.TryGetBuffer return a useful ArraySegment?

Question

bool MemoryStream.TryGetBuffer(out ArraySegment<byte> buffer) is a new API in .NET 4.6 that can be used to get access to the valid bytes stored in the MemoryStream without copying them. This is very exciting! It returns a bool that is "true if the conversion was successful; otherwise, false" and populates the out param.

When does it return true, indicating that the out ArraySegment<byte> buffer now contains valid information? This isn't documented today.

I know that if it returns false, I can use .ToArray() to get a copy of the bytes. And, we've have .GetBuffer(), but sometimes MemoryStreams are created with an offset into the buffer, and this information is hard (well, sort of) to get later on, not to mention the try ... catch needed for robustness.

Answer 1

For TryGetBuffer to perform a successful conversion and populate the out param with useful information, the buffer must be visible. The buffer is visible if any of these constructors are used:

• MemoryStream()
• MemoryStream(int capacity)
• MemoryStream(byte[] buffer, int index, int count, bool writable, bool publiclyVisible) with publiclyVisible: true.

Check the source code for more details.

Answer 2

GetBuffer return all bytes in memory, independent of use,

ex: Capacity = 100000, Length = 200

GetBuffer (and TryGetBuffer !?) return bytes( Capacity )

ToArray return bytes( Length )