Questions about ReaderWriterLockSlim. I use readerwriterlockslim to read and write, but only show write at last in console. Why?

Question

this is code:

class program
{
    static ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim();
    static void Main(string[] args)
    {
        for (int i = 0; i < 2; i++)
        {
            Task.Factory.StartNew(Read);
        }
        for (int i = 0; i < 1; i++)
        {
            Task.Factory.StartNew(Write);
        }
        Console.Read();
    }

    static void Read()
    {
        while (true)
        {
            rwLock.EnterReadLock();;
            Thread.Sleep(100);
            Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId}  reading");
            rwLock.ExitReadLock();
        }
    }
    static void Write()
    {
        while (true)
        {
            rwLock.EnterWriteLock();
            Thread.Sleep(3000);
            Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId} writing");
            rwLock.ExitWriteLock();
        }
    }
}

I want to test the ReaderWriterLockSlim, the result what I expect is read and write alternately in console, but at last, there only write in console.

Here the result:

5 reading 2020/2/17 18:24:36
4 reading 2020/2/17 18:24:36
3 reading 2020/2/17 18:24:36
6 writing 2020/2/17 18:24:39
6 writing 2020/2/17 18:24:42
6 writing 2020/2/17 18:24:45
6 writing 2020/2/17 18:24:48
6 writing 2020/2/17 18:24:51
6 writing 2020/2/17 18:24:54
6 writing 2020/2/17 18:24:57
6 writing 2020/2/17 18:25:00
6 writing 2020/2/17 18:25:03
6 writing 2020/2/17 18:25:06

but when use ReaderWriterLock, its alternate display. Here the result:

4 reading 2020/2/17 18:30:22
3 reading 2020/2/17 18:30:22
5 reading 2020/2/17 18:30:22
6 writing 2020/2/17 18:30:25
4 reading 2020/2/17 18:30:25
3 reading 2020/2/17 18:30:25
5 reading 2020/2/17 18:30:25
6 writing 2020/2/17 18:30:28
4 reading 2020/2/17 18:30:29
5 reading 2020/2/17 18:30:29
3 reading 2020/2/17 18:30:29
6 writing 2020/2/17 18:30:32
4 reading 2020/2/17 18:30:32
5 reading 2020/2/17 18:30:32
3 reading 2020/2/17 18:30:32
6 writing 2020/2/17 18:30:35
5 reading 2020/2/17 18:30:35
3 reading 2020/2/17 18:30:35
4 reading 2020/2/17 18:30:35
6 writing 2020/2/17 18:30:38
5 reading 2020/2/17 18:30:45
3 reading 2020/2/17 18:30:45
4 reading 2020/2/17 18:30:45

Why does this happen, someone tell me the reason for this phenomenon？ Hope answer

Answer 1

The problems here is that thread.Sleep() is called whilst inside the critical section. this causes the writer thread to hold the lock for periods of 3 secs each time and releases it for a very small amount of time in which the writer thread re-acquires the lock as, in this case apparently, ReaderWriterLockSlim gives better priority to writers.

I tried calling Thread.Sleep() after releasing the lock and everything works as expected.

class LockProgram
{
    static ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim();
    static void Main(string[] args)
    {
        for (int i = 0; i < 2; i++)
        {
            Task.Factory.StartNew(Read);
        }
        for (int i = 0; i < 1; i++)
        {
            Task.Factory.StartNew(Write);
        }
        Console.Read();
    }

    static void Read()
    {
        while (true)
        {
            rwLock.EnterReadLock(); ;                
            Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId}  reading");
            rwLock.ExitReadLock();
            Thread.Sleep(100);
        }
    }
    static void Write()
    {
        while (true)
        {
            rwLock.EnterWriteLock();                
            Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId} writing");
            rwLock.ExitWriteLock();
            Thread.Sleep(3000);
        }
    }
}

It's worth checking out the .Net reference source for ReaderWriterLockSlim and ReaderWriterLock