In both of your questions, you are overloooking a critical detail: there
is no consistent definition of an object's "contents".

You can come the closest, IMHO, for objects that can be serialized.
Then you can serialize them and use the resulting stream of data
(whether it's XML or binary) for your purposes.

But not all objects can be serialized, and unless you're talking about a
struct or class that only has value-type fields as data, there's not a
single block of data that represents the object.

If you can define "contents" in a way that allows you to address your
first question ("memory stream"), then the answer to the second ("hash")
can be discussed (though I think that answering "fastest" is
problematic, since any hash is going to have tradeoffs with respect to
speed versus how you intend to use it...the method that's truly
"fastest" may not work for your purposes).

Without a better definition and criteria for how you want to do this, I
don't think it's really possible to answer your question in a reasonable
way.

Pete

AFAIK

MD5 Hash is fast and pretty reliable for its purpose  CRC is more reliable
but also much slower

MD5 hash is often used to quickly  compare 2 binary`s ( are they the same or
not , idea; for update/ file synchronization  programs etc etc )
CRC is often used by compression program`s ( winzip , winrar etc etc ) to
check if they are exactly the same on byte level ( to check if the file not
has gone corrupted during deflation )

HTH

Michel