.NET Reference Guide

New Memory Management Functions in .NET 2.0

Last updated May 4, 2006.

Several new memory management facilities have been added to .NET 2.0. These facilities give you slightly more control over how the garbage collector works, and the ability to check for sufficient memory before continuing in a program.

New Garbage Collector Functionality

The runtime garbage collector is implemented in the System.GC class. Normally, your programs don’t have to do much if anything with this class. Most programs, if they access the GC class at all, only call GC.SuppressFinalize to prevent the garbage collector from calling Object.Finalize on a disposed object that doesn’t need it. See the .NET SDK topic Implementing a Dispose Method for more information.

The .NET 1.1 version of the GC class allowed programs to force garbage collection if necessary, but it didn’t give programs the ability to determine if garbage collection was needed. In 2.0, the GC.CollectionCount method returns the number of collections that have occurred for a given generation. If your application is forcing collections, it can save the value returned by GC.Collect and compare it later with the value returned by GC.CollectionCount. If the numbers are the same, then no collection has occurred since the last time your code called Collect, meaning that you might want to force a collection.

Note that it is not recommended that you force garbage collections unless you know exactly what you’re doing. Unnecessary collections do nothing but slow your program.

The other new functionality exposed by the GC class is the ability to inform the garbage collector of external--outside of the framework--memory allocations. Doing so can help the garbage collector run more efficiently because it has a better idea of how much of the system’s total memory is available.

In determining when to schedule garbage collections, the .NET runtime takes into account the amount of managed memory allocated. That’s all it can know about. However, if you have a small object that allocates a large amount of unmanaged memory, the garbage collector will underestimate the urgency of scheduling a collection. By giving you a way to notify the garbage collector of memory pressures, you help the system run more efficiently.

To notify the garbage collector of a large unmanaged memory allocation, call GC.AddMemoryPressure, passing to the function the incremental number of unmanaged bytes allocated. You should do this for any large allocation, or for small allocations if there will be many of them.

It’s very important that you make a corresponding call to GC.RemoveMemoryPressure whenever you deallocate memory for which you called GC.AddMemoryPressure. Failure to do so will almost certainly lead to decreased performance in applications that run for long periods of time.