Forcing Garbage Collection in .NET

There might be times in your application when you want to force the .NET Garbage Collector (GC) to spin through all unused objects and de-allocate them. The method for accomplishing this task is the GC.Collect method. When you call GC.Collect, the GC will run each object's finalizer on a separate thread. Therefore, another method to keep in mind is GC.WaitForPendingFinalizers. This synchronous method that will not return until the GC.Collect has finished its work.

Here's a simple example of using these two methods:

using System;

namespace GCCollect
{
  class Account
  {
    public Account(string accountNumber)
    {
      this.accountNumber = accountNumber; 
      Console.WriteLine("Account::Acount - c'tor");
    }
    ~Account() 
    {
      Console.WriteLine("Account::~Acount - d'tor");
    }

    protected string accountNumber;
    override public string ToString() { return accountNumber; }
  };

  class Class1
  {
    [STAThread]
    static void Main(string[] args)
    {
      CreateAccount("111006116");

      GC.Collect();
      GC.WaitForPendingFinalizers();

      Console.WriteLine("Application ending");
    }

    public static void CreateAccount(string accountNumber)
    {
      Console.WriteLine("CreateAccount - instantiate Account object");
      Account account = new Account(accountNumber);
      Console.WriteLine("CreateAccount - created account number {0}",
                        account);
    }
  }
}

If you build and run this example, you will receive the following output:

CreateAccount - instantiate Account object
Account::Acount - c'tor
CreateAccount - created account number 111006116
Account::~Acount - d'tor
Application ending

Note a couple of issues here:

  1. I allocated the Account object in a different method than the Main method. This is because if I allocated the Account object in the Main method and then called GC.Collect in the same method, the Account object would technically still be associated with running code and would therefore not be eligible for collection.
  2. While I used a simple single object example here to illustrate calling the GC.Collect method, the GC.Collect method is designed not to control specific object destruction, but to allow for the forcing of collection for all of unused objects. Therefore, it's a very expensive operation and should only be used in cases where you want/need to force global collection.
  3. For situations where you want to force the finalization of a specific object, you should implement the Dispose pattern, which I'll cover in a separate article.

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Download the accompanying code here (17 Kb).



About the Author

Tom Archer - MSFT

I am a Program Manager and Content Strategist for the Microsoft MSDN Online team managing the Windows Vista and Visual C++ developer centers. Before being employed at Microsoft, I was awarded MVP status for the Visual C++ product. A 20+ year veteran of programming with various languages - C++, C, Assembler, RPG III/400, PL/I, etc. - I've also written many technical books (Inside C#, Extending MFC Applications with the .NET Framework, Visual C++.NET Bible, etc.) and 100+ online articles.

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