What is garbage collection in C#?

I can help you with that. Here are some common .NET/C# programmer interview questions:

1. What are the key features of C#?
2. What is the difference between a class and an interface in C#?
3. What is the difference between an abstract class and an interface?
4. What is a delegate in C#?
5. What is the difference between a value type and a reference type in C#?
6. What is the difference between an exception and an error in C#?
7. How do you handle exceptions in C#?
8. What is LINQ and how do you use it?
9. What is the difference between the == operator and the Equals() method in C#?
10. What is the difference between the var and dynamic keywords in C#?
11. What is the difference between an assembly and a namespace in C#?
12. What is the difference between a thread and a process in C#?
13. What is garbage collection in C#?
14. How do you optimize performance in a .NET application?
15. What is the difference between an HTTP GET and POST request in ASP.NET?
16. What is the role of Global.asax in ASP.NET?
17. How do you implement caching in ASP.NET?
18. What is the difference between session and application state in ASP.NET?
19. What is MVC and how do you use it in ASP.NET?
20. How do you secure a .NET application?

These are just some common interview questions, but the interviewer may also ask about your experience, previous projects, or ask you to solve coding problems. It's always a good idea to practice coding problems beforehand and be prepared to explain your thought process as you work through the problem.

 

13.What is garbage collection in C#?

Garbage collection is an automated memory management process in C# (and other managed languages) that helps developers to write programs without worrying about managing memory allocation and deallocation manually. The garbage collector (GC) in C# periodically frees up the memory occupied by objects that are no longer being used by the application.

When an object is created in C#, the runtime keeps track of its memory usage. The GC periodically checks the memory and identifies any objects that are no longer being used or referenced by the program. The GC then frees up the memory occupied by these objects, allowing it to be used for other purposes.

C# implements a mark-and-sweep garbage collection algorithm. In this algorithm, the GC scans the object graph from the root and marks all the objects that are still in use. It then sweeps through the memory and deallocates the memory occupied by the unmarked objects.

Garbage collection in C# is automatic, which means that developers don't need to manually allocate or deallocate memory. This can simplify programming and reduce the chances of memory-related errors, such as memory leaks and dangling pointers. However, it also means that the GC can introduce some overhead and potentially affect the performance of the application.

Explain the generation in garbage collection of C#

Garbage collection in C# is managed through a generational model, where objects are grouped into different generations based on their age. The C# garbage collector is optimized to take advantage of the fact that most objects die young, meaning they are allocated and released quickly, and so the garbage collector can be more efficient by performing garbage collection more frequently on younger objects.

The heap, which is where objects are allocated, is divided into three generations: 0, 1, and 2. When an object is first allocated, it is placed in generation 0. If it survives a garbage collection, it is promoted to generation 1. If it survives another collection, it is promoted to generation 2. The idea is that objects that survive multiple collections are likely to be longer-lived, and so they are given more space in higher generations.

Garbage collection is performed on a per-generation basis. The garbage collector first collects objects in generation 0, which are the youngest objects, and then moves on to older generations if necessary. Because most objects die young, the garbage collector can often collect all the garbage it needs to in generation 0, which is called a "minor" collection. If it needs to collect objects from older generations, it performs a "major" collection, which is more expensive.

The garbage collector also uses different algorithms for different generations. For example, it uses a "mark and sweep" algorithm for generation 0, which is simpler and faster, while it uses a more complex "mark and compact" algorithm for older generations, which is slower but more thorough.

Overall, the generational model helps make garbage collection in C# more efficient by taking advantage of the fact that most objects die young, and by performing garbage collection more frequently on younger objects.

 

When will the first garbage collection occurs?

The first garbage collection in a .NET application typically occurs when the system determines that memory is running low. The garbage collector runs automatically on a separate thread, and it periodically checks the memory usage of the application. When it detects that the amount of available memory is becoming low, it triggers a garbage collection to free up unused memory.

The exact timing of the first garbage collection depends on various factors, such as the amount of memory allocated by the application, the frequency and duration of object allocations, and the size of the managed heap. In general, the garbage collector aims to balance the performance of the application with the efficient use of memory.

The system? you mean the operation system or the application itself?

When I said "the system," I meant the .NET runtime environment. The .NET runtime is responsible for managing memory and triggering garbage collection when necessary. Garbage collection is not handled by the operating system, but by the .NET runtime that is running the application.

The memory is running low, whose memory? os or .net runtime?

When the memory is running low, it refers to the memory available to the .NET runtime, not the operating system. The .NET runtime manages memory allocation and deallocation for the managed code that is executing on it. When the memory is running low, it means that the available memory within the .NET runtime is becoming scarce, which can lead to performance issues or even crashes if the application continues to try to allocate more memory than is available. At this point, the garbage collector may be triggered to reclaim unused memory and free up space for the application to continue executing.

How many memory will .net runtime occupy in operation system? Is it a fixed size?

The amount of memory that the .NET runtime occupies in the operating system is not fixed and can vary depending on factors such as the version of the runtime, the application's configuration, and the size and complexity of the application itself.

When an application starts, the .NET runtime is loaded into memory along with the application's code and any necessary dependencies such as libraries and frameworks. The runtime also creates several system-level objects such as the garbage collector, JIT compiler, and type system.

The size of the .NET runtime itself can vary depending on the version being used. For example, the size of the runtime for .NET Framework 4.8 is larger than that of .NET Core 3.1 due to differences in architecture and functionality.

Additionally, the amount of memory used by an application depends on the amount of data it needs to store, the number of objects it creates, and the complexity of its code. As the application runs, the .NET garbage collector manages the memory by periodically freeing up memory that is no longer needed.

Who is responsible for allocating memory to .net runtime?

The operating system is responsible for allocating memory to .NET runtime. When the .NET runtime needs memory, it requests it from the operating system. The operating system then provides the requested memory to the .NET runtime. The .NET runtime then manages the memory using the garbage collector.

Does two .net app use the same .net runtime?

Each .NET application runs on its own instance of the .NET runtime. While two .NET applications can run simultaneously on the same machine, each application will have its own instance of the .NET runtime.

However, it's important to note that multiple instances of the .NET runtime can run concurrently on the same machine. For example, if you have multiple versions of the .NET Framework installed, each version will have its own instance of the .NET runtime. Additionally, in some cases, an application may load multiple versions of the .NET runtime into memory at the same time.

The first garbage collection in a .NET application typically occurs when the system determines that memory is running low. Why the .net runtime do garbage collection instead of requesting more memory from operation system?

Garbage collection is a memory management technique used in modern programming languages like C# to automatically free up memory that is no longer being used by the program. When an application requests memory from the operating system, the operating system gives the application a block of memory to use. As the application runs, it allocates and deallocates memory as necessary, but it may not return all of the memory it has allocated when it is done with it. Over time, this can result in the application using more and more memory, even if it no longer needs all of it.

When the system determines that memory is running low, it is because the application has used up all the memory that the operating system has allocated to it. At this point, the .NET runtime performs a garbage collection to free up memory that is no longer being used by the program. By doing this, the .NET runtime can reclaim memory that is no longer needed by the program, allowing it to be used by other applications on the system.

It is important to note that the .NET runtime does not always perform a garbage collection when memory is running low. Instead, it uses a number of heuristics and algorithms to determine when to perform garbage collections. These heuristics take into account factors like the amount of memory being used by the application, the rate at which memory is being allocated and deallocated, and the current state of the garbage collector. By using these heuristics, the .NET runtime can optimize the garbage collection process to balance the need for freeing up memory with the need to avoid impacting the performance of the application.

Then the problem is, how .net runtime detect the memory is low? 

The .NET runtime tracks the amount of memory that an application is using, including both managed and unmanaged memory. When the runtime detects that the amount of available memory is running low, it will trigger a garbage collection in an attempt to reclaim memory that is no longer being used.

There are different ways that the .NET runtime can determine that memory is running low. One common approach is to use a heuristic based on the amount of free physical memory available on the system, as well as the amount of virtual memory being used by the application. The runtime can also monitor the rate at which memory is being allocated and freed by the application, and trigger a garbage collection if it detects that memory usage is increasing rapidly or has spiked suddenly.

The specific approach used by the .NET runtime to manage memory can depend on various factors, including the version of the runtime, the operating system, and the specific configuration of the application. In general, however, the goal is to ensure that the application is using memory efficiently and that it is not causing the system to become bogged down due to excessive memory usage.

Why there are only three generations: 0, 1, and 2 in garbage collection of .net runtime?

The .NET runtime uses a generational garbage collection algorithm to manage memory. The idea behind generational garbage collection is that most objects are short-lived and become unused relatively quickly. Therefore, the garbage collector focuses on collecting these short-lived objects more frequently, while allowing long-lived objects to survive for a longer period of time. By dividing objects into different generations, the garbage collector can optimize its collection algorithm and reduce the time spent on garbage collection.

The three generations in the .NET garbage collection algorithm are:

1. Generation 0: This generation contains short-lived objects that are allocated and deallocated quickly. When garbage collection occurs, only this generation is collected.

2. Generation 1: This generation contains objects that have survived one garbage collection cycle. These objects are more likely to be long-lived than objects in Generation 0.

3. Generation 2: This generation contains objects that have survived multiple garbage collection cycles. These objects are likely to be long-lived and are not collected as frequently as objects in Generation 0 or 1.

By dividing objects into these three generations, the .NET runtime can optimize its garbage collection algorithm and reduce the time spent on garbage collection.

 

What's the frequency of gc collector to do collection for each generation?

The .NET garbage collector uses different algorithms and thresholds for collecting objects in each generation, so the frequency of garbage collection can vary based on the state of the application and the type of objects being created and destroyed.

In general, the garbage collector tries to minimize the number of collections performed by maximizing the amount of memory available to the application, and it adjusts the frequency of collections dynamically based on the needs of the application.

For example, in the early stages of an application's lifecycle, the garbage collector may perform more frequent collections of Generation 0, since this generation tends to contain short-lived objects. Once the application has stabilized and the amount of surviving objects in Generation 1 and 2 has increased, the garbage collector may perform less frequent collections of these generations to minimize the impact on application performance.

In addition, you can adjust the frequency of garbage collection by modifying the values of certain configuration settings or by manually invoking garbage collection through the GC.Collect() method. However, it's generally not recommended to manually invoke garbage collection unless you have a specific reason to do so, as this can have unintended consequences and may impact the performance of your application.

 

作者:Chuck Lu    GitHub