GC.Collect

Learn memory management with GC.Collect and GC.GetTotalMemory.

GC.Collect and GC.GetTotalMemory

GC.Collect() forces the garbage collector to run. It is generally discouraged because it disrupts the GC's own scheduling; use it only when memory must be freed at a known point.

GC.GetTotalMemory(false) returns the approximate number of managed bytes; with true it waits for collection. The value is useful for measurement and debugging.

The WaitForPendingFinalizers pattern

After GC.Collect, call GC.WaitForPendingFinalizers() so finalizers run. This is used when you want finalizer-bearing classes to release their resources right away.

The typical pattern is Collect → WaitForPendingFinalizers → Collect. The second Collect reclaims the memory that the finalizers freed. This pattern is also useful in measurement and test code to obtain consistent memory readings.

Exercise Collect and print a message

In Main first call GC.Collect();, then GC.WaitForPendingFinalizers();, finally Console.WriteLine("Collected");.

Show example solution

Write GC.Collect(); GC.WaitForPendingFinalizers(); Console.WriteLine("Collected");
Example solution:
using System;

class Program
{
    static void Main()
    {
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Console.WriteLine("Collected");
    }
}

Test yourself

Match the GC methods

GC.Collect()

Force collection
GC.GetTotalMemory(false)

Approximate memory
GC.WaitForPendingFinalizers()

Waits for finalizers
GC.KeepAlive(o)

Keeps object alive
When is GC.Collect() recommended?
- After every method call
- Performance measurement or a specific point
- In every loop
- Always at program start
Show answer

Performance measurement or a specific point — Correct. GC.Collect is used only for special cases, like measurement.

Force a collection

GC.____();
GC.WaitForPendingFinalizers();

Show answer

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

Next lesson: Large Object Heap

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