Multithreading is the ability of a CPU, or a single core in a multi-core processor, to execute multiple threads concurrently. It enables the execution of multiple parts of a program simultaneously. Parallelism refers to performing multiple operations simultaneously, typically using multiple threads or processes.

In .NET, you can achieve multithreading and parallelism using various classes and libraries, such as Thread, ThreadPool, Task, and Parallel.

Key Concepts

1. Thread Class: Represents a thread of execution.
2. ThreadPool Class: Provides a pool of threads that can be used to execute tasks, minimizing the overhead of creating and destroying threads.
3. Task Class: Represents an asynchronous operation and is part of the Task Parallel Library (TPL).
4. Parallel Class: Provides support for parallel loops and regions, making it easier to run parallel tasks.

Commonly Asked Interview Questions with Answers

Question 1: What is the difference between a thread and a process?

Answer:

• Process: A process is an independent execution unit that contains its own state information, memory space, and has its own address space. It is heavyweight.
• Thread: A thread is the smallest unit of execution within a process. Threads within the same process share the same memory space and can communicate more efficiently. It is lightweight.

Question 2: How do you create and start a new thread in .NET?

Answer: You can create and start a new thread using the Thread class.

Thread thread = new Thread(new ThreadStart(MyMethod));
thread.Start();

void MyMethod()
{
    // Code to be executed on the new thread
}

Question 3: What is the ThreadPool, and why is it useful?

Answer: The ThreadPool class provides a pool of worker threads that can be used to execute tasks. This reduces the overhead of creating and destroying threads and improves performance.

ThreadPool.QueueUserWorkItem(new WaitCallback(MyMethod));

void MyMethod(object state)
{
    // Code to be executed on a thread pool thread
}

Question 4: Explain the Task class and how it is used.

Answer: The Task class represents an asynchronous operation. It is part of the Task Parallel Library (TPL) and provides a higher-level abstraction than threads.