What are the consequences (positive/negative) of using the unsafe keyword in C# to use pointers? For example, what becomes of garbage collection, what are the performance gains/losses, what are the performance gains/losses compared to other languages manual memory management, what are the dangers, in which situation is it really justifiable to make use of this language feature, is it longer to compile... ?
Garbage Collection is inefficient with long-lived objects. .Net's garbage collector works best when most objects are released rather quickly, and some objects "live forever." The problem is that longer-living objects are only released during full garbage collections, which incurs a significant performance penalty. In essence, long-living objects quickly move into generation 2.
(For more information, you might want to read up on .Net's generational garbage collector: http://msdn.microsoft.com/en-us/library/ms973837.aspx)
In situations where objects, or memory use in general, is going to be long-lived, manual memory management will yield better performance because it can be released to the system without requiring a full garbage collection.
Implementing some kind of a memory management system based around a single large byte array, structs, and lots of pointer arithmetic, could theoretically increase performance in situations where data will be stored in RAM for a long time.
Unfortunately, I'm not aware of a good way to do manual memory management in .Net for objects that are going to be long-lived. This basically means that applications that have long-lived data in RAM will periodically become unresponsive when they run a full garbage collection of all of the memory.