What are the fundamental differences between garbage collection in C# and Java?

Question

I had some very wrong sounding advice recently from a "senior" developer/coworker regarding the C# garbage collector such as...

• "You need to use destructors everywhere in C# because the garbage collector cannot be relied upon."

• "The C# garbage collector cannot be thought of like the Java garbage collector".

This sounds extremely fishy to me, as far as I know the differences between the C# and Java garbage collectors are as follows...

• The C# is a Generational garbage collector, Java is concurrent mark sweep in 1.6 with G1 being the new default (generational) garbage collector featuring Java 7 and has been optional since ~1.6.21. As far as I know
• C# as a language has the ability to manually dispose of objects that implement IDisposable. Java must always use garbage collection, although some frameworks like SWT require you manually call methods to release memory in the underlying native code.

I realize that Java and C# are just the languages and the garbage collectors are a component of the runtime, however for this case I am specifically speaking about the Sun/Oracle JVM and the Microsoft .NET Runtime.

Does anybody have feedback?

Answer 1

The advice you've been given is, broadly speaking, a load of hooey.

Both C# and Java have GCs that attempt to optimise the fast recovery of lots of small objects. They're designed to solve the same problem, they do it in slightly different ways but as a user the technical differences in your approach to using them is minimal, even non-existent for the majority of users.

IDisposable is nothing to do with the GC as such. It's a standard way of naming methods that would otherwise be called close, destroy, dispose, etc., and often are called that in Java. There is a proposal for Java 7 to add something very similar to the using keyword that would call a similar close method.

"Destructors" in C# refers to finalizers - this was done deliberately to confuse C++ programmers. :) The CLR spec itself calls them finalizers, exactly as the JVM does.

There are many ways in which Java and C#/CLR differ (user value types, properties, generics and the whole family of related features known as Linq), but the GC is one area where you can develop a substantial amount of software before you need to worry much about the difference between them.