Why Does Garbage Collection Behavior Differ Between Debug and Release Modes in .NET?

Discuss in -depth .NET garbage recycling mechanism

The garbage recovery mechanism in the .NET framework is critical to memory management and preventing memory leakage. However, in some cases, especially when using a debugger, the garbage recovery mechanism may be misunderstood.

The following code fragment is an example:

Run this code in the debug mode, you may be surprised to find that the Class 1.c output is 0, even if the variable C1 has exceeded the scope and is no longer cited. This is because the debugger modified the garbage recycling behavior.

public class Class1
{
    public static int c;
    ~Class1()
    {
        c++;
    }
}

public class Class2
{
    public static void Main()
    {
        {
            var c1 = new Class1();
            //c1 = null; // 取消此行注释，在Console.WriteLine调用时，输出为1。
        }
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Console.WriteLine(Class1.c); // 输出0
        Console.Read();
    }
}

In the release version without a debugger, the JIT compiler optimizes the code and generates a table to track local variables. This table allows the garbage recycle to determine when to recover the variable, even if the method is still running. In this example, C1 is no longer used after its scope, so it can be recycled before main ().

However, when the additional debugger, the JIT compiler will change the table so that local variables maintain a state of activity during the method execution. This is to prevent the variable from disappearing during debugging. Therefore, the C1 maintains its activity during the full main () method to prevent it from being recycled.

Another factor affecting garbage recovery is to set the variable to NULL. In the debug mode, this is not effective, because the JIT table still believes that variables are in use. However, in the distribution version, the variable is set to NULL to allow the garbage recyrior to recognize that there is no reference to the object, which allows it to be recycled.

Understanding these subtles is essential for avoiding memory leakage and ensuring efficient memory management of .NET applications. By running code and analyzing garbage recycling behaviors in the debugging and distribution mode, you can ensure that the code runs as expected.

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