ReaderWriterLockSlim Constructors

Definition

Namespace:

System.Threading

Assemblies:

netstandard.dll, System.Threading.dll

Initializes a new instance of the ReaderWriterLockSlim class.

Overloads

ReaderWriterLockSlim()	Initializes a new instance of the ReaderWriterLockSlim class with default property values.
ReaderWriterLockSlim(LockRecursionPolicy)	Initializes a new instance of the ReaderWriterLockSlim class, specifying the lock recursion policy.

ReaderWriterLockSlim()

Source:

ReaderWriterLockSlim.cs

Initializes a new instance of the ReaderWriterLockSlim class with default property values.

C#

public ReaderWriterLockSlim();

Examples

The following example shows a simple synchronized cache that holds strings with integer keys. An instance of ReaderWriterLockSlim is used to synchronize access to the Dictionary<TKey,TValue> that serves as the inner cache. The parameterless constructor is used to create the lock.

The example includes simple methods to add to the cache, delete from the cache, and read from the cache. To demonstrate time-outs, the example includes a method that adds to the cache only if it can do so within a specified time-out.

To demonstrate upgradeable mode, the example includes a method that retrieves the value associated with a key and compares it with a new value. If the value is unchanged, the method returns a status indicating no change. If no value is found for the key, the key/value pair is inserted. If the value has changed, it is updated. Upgradeable mode allows the thread to upgrade from read access to write access as needed, without the risk of deadlocks.

The example includes a nested enumeration that specifies the return values for the method that demonstrates upgradeable mode.

The example uses the parameterless constructor to create the lock, so recursion is not allowed. Programming the ReaderWriterLockSlim is simpler and less prone to error when the lock does not allow recursion.

C#

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

C#

public class SynchronizedCache 
{
    private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
    private Dictionary<int, string> innerCache = new Dictionary<int, string>();

    public int Count
    { get { return innerCache.Count; } }

    public string Read(int key)
    {
        cacheLock.EnterReadLock();
        try
        {
            return innerCache[key];
        }
        finally
        {
            cacheLock.ExitReadLock();
        }
    }

    public void Add(int key, string value)
    {
        cacheLock.EnterWriteLock();
        try
        {
            innerCache.Add(key, value);
        }
        finally
        {
            cacheLock.ExitWriteLock();
        }
    }

    public bool AddWithTimeout(int key, string value, int timeout)
    {
        if (cacheLock.TryEnterWriteLock(timeout))
        {
            try
            {
                innerCache.Add(key, value);
            }
            finally
            {
                cacheLock.ExitWriteLock();
            }
            return true;
        }
        else
        {
            return false;
        }
    }

    public AddOrUpdateStatus AddOrUpdate(int key, string value)
    {
        cacheLock.EnterUpgradeableReadLock();
        try
        {
            string result = null;
            if (innerCache.TryGetValue(key, out result))
            {
                if (result == value)
                {
                    return AddOrUpdateStatus.Unchanged;
                }
                else
                {
                    cacheLock.EnterWriteLock();
                    try
                    {
                        innerCache[key] = value;
                    }
                    finally
                    {
                        cacheLock.ExitWriteLock();
                    }
                    return AddOrUpdateStatus.Updated;
                }
            }
            else
            {
                cacheLock.EnterWriteLock();
                try
                {
                    innerCache.Add(key, value);
                }
                finally
                {
                    cacheLock.ExitWriteLock();
                }
                return AddOrUpdateStatus.Added;
            }
        }
        finally
        {
            cacheLock.ExitUpgradeableReadLock();
        }
    }

    public void Delete(int key)
    {
        cacheLock.EnterWriteLock();
        try
        {
            innerCache.Remove(key);
        }
        finally
        {
            cacheLock.ExitWriteLock();
        }
    }

    public enum AddOrUpdateStatus
    {
        Added,
        Updated,
        Unchanged
    };

    ~SynchronizedCache()
    {
       if (cacheLock != null) cacheLock.Dispose();
    }
}

The following code then uses the SynchronizedCache object to store a dictionary of vegetable names. It creates three tasks. The first writes the names of vegetables stored in an array to a SynchronizedCache instance. The second and third task display the names of the vegetables, the first in ascending order (from low index to high index), the second in descending order. The final task searches for the string "cucumber" and, when it finds it, calls the EnterUpgradeableReadLock method to substitute the string "green bean".

C#

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

C#

public class Example
{
   public static void Main()
   {
      var sc = new SynchronizedCache();
      var tasks = new List<Task>();
      int itemsWritten = 0;

      // Execute a writer.
      tasks.Add(Task.Run( () => { String[] vegetables = { "broccoli", "cauliflower",
                                                          "carrot", "sorrel", "baby turnip",
                                                          "beet", "brussel sprout",
                                                          "cabbage", "plantain",
                                                          "spinach", "grape leaves",
                                                          "lime leaves", "corn",
                                                          "radish", "cucumber",
                                                          "raddichio", "lima beans" };
                                  for (int ctr = 1; ctr <= vegetables.Length; ctr++)
                                     sc.Add(ctr, vegetables[ctr - 1]);

                                  itemsWritten = vegetables.Length;
                                  Console.WriteLine("Task {0} wrote {1} items\n",
                                                    Task.CurrentId, itemsWritten);
                                } ));
      // Execute two readers, one to read from first to last and the second from last to first.
      for (int ctr = 0; ctr <= 1; ctr++) {
         bool desc = ctr == 1;
         tasks.Add(Task.Run( () => { int start, last, step;
                                     int items;
                                     do {
                                        String output = String.Empty;
                                        items = sc.Count;
                                        if (!desc) {
                                           start = 1;
                                           step = 1;
                                           last = items;
                                        }
                                        else {
                                           start = items;
                                           step = -1;
                                           last = 1;
                                        }

                                        for (int index = start; desc ? index >= last : index <= last; index += step)
                                           output += String.Format("[{0}] ", sc.Read(index));

                                        Console.WriteLine("Task {0} read {1} items: {2}\n",
                                                          Task.CurrentId, items, output);
                                     } while (items < itemsWritten | itemsWritten == 0);
                             } ));
      }
      // Execute a red/update task.
      tasks.Add(Task.Run( () => { Thread.Sleep(100);
                                  for (int ctr = 1; ctr <= sc.Count; ctr++) {
                                     String value = sc.Read(ctr);
                                     if (value == "cucumber")
                                        if (sc.AddOrUpdate(ctr, "green bean") != SynchronizedCache.AddOrUpdateStatus.Unchanged)
                                           Console.WriteLine("Changed 'cucumber' to 'green bean'");
                                  }
                                } ));

      // Wait for all three tasks to complete.
      Task.WaitAll(tasks.ToArray());

      // Display the final contents of the cache.
      Console.WriteLine();
      Console.WriteLine("Values in synchronized cache: ");
      for (int ctr = 1; ctr <= sc.Count; ctr++)
         Console.WriteLine("   {0}: {1}", ctr, sc.Read(ctr));
   }
}
// The example displays the following output:
//    Task 1 read 0 items:
//
//    Task 3 wrote 17 items
//
//
//    Task 1 read 17 items: [broccoli] [cauliflower] [carrot] [sorrel] [baby turnip] [
//    beet] [brussel sprout] [cabbage] [plantain] [spinach] [grape leaves] [lime leave
//    s] [corn] [radish] [cucumber] [raddichio] [lima beans]
//
//    Task 2 read 0 items:
//
//    Task 2 read 17 items: [lima beans] [raddichio] [cucumber] [radish] [corn] [lime
//    leaves] [grape leaves] [spinach] [plantain] [cabbage] [brussel sprout] [beet] [b
//    aby turnip] [sorrel] [carrot] [cauliflower] [broccoli]
//
//    Changed 'cucumber' to 'green bean'
//
//    Values in synchronized cache:
//       1: broccoli
//       2: cauliflower
//       3: carrot
//       4: sorrel
//       5: baby turnip
//       6: beet
//       7: brussel sprout
//       8: cabbage
//       9: plantain
//       10: spinach
//       11: grape leaves
//       12: lime leaves
//       13: corn
//       14: radish
//       15: green bean
//       16: raddichio
//       17: lima beans

Remarks

A ReaderWriterLockSlim that is initialized with this constructor does not allow recursion. That is, the RecursionPolicy property returns LockRecursionPolicy.NoRecursion.

For more information about recursion policy and its effects, see the LockRecursionPolicy enumeration and the ReaderWriterLockSlim class.

See also

• LockRecursionPolicy

ReaderWriterLockSlim(LockRecursionPolicy)

Source:

ReaderWriterLockSlim.cs

Initializes a new instance of the ReaderWriterLockSlim class, specifying the lock recursion policy.

C#

public ReaderWriterLockSlim(System.Threading.LockRecursionPolicy recursionPolicy);

Parameters

recursionPolicy

LockRecursionPolicy

One of the enumeration values that specifies the lock recursion policy.

Examples

The following example shows two exception scenarios, one that depends on the LockRecursionPolicy setting and one that does not.

In the first scenario, the thread enters read mode and then tries to enter read mode recursively. If the ReaderWriterLockSlim is created by using the parameterless constructor, which sets recursion policy to LockRecursionPolicy.NoRecursion, an exception is thrown. If LockRecursionPolicy.SupportsRecursion is used to create the ReaderWriterLockSlim, no exception is thrown.

In the second scenario, the thread enters read mode and then tries to enter write mode. LockRecursionException is thrown regardless of the lock recursion policy.

C#

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

C#

public class SynchronizedCache 
{
    private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
    private Dictionary<int, string> innerCache = new Dictionary<int, string>();

    public int Count
    { get { return innerCache.Count; } }

    public string Read(int key)
    {
        cacheLock.EnterReadLock();
        try
        {
            return innerCache[key];
        }
        finally
        {
            cacheLock.ExitReadLock();
        }
    }

    public void Add(int key, string value)
    {
        cacheLock.EnterWriteLock();
        try
        {
            innerCache.Add(key, value);
        }
        finally
        {
            cacheLock.ExitWriteLock();
        }
    }

    public bool AddWithTimeout(int key, string value, int timeout)
    {
        if (cacheLock.TryEnterWriteLock(timeout))
        {
            try
            {
                innerCache.Add(key, value);
            }
            finally
            {
                cacheLock.ExitWriteLock();
            }
            return true;
        }
        else
        {
            return false;
        }
    }

    public AddOrUpdateStatus AddOrUpdate(int key, string value)
    {
        cacheLock.EnterUpgradeableReadLock();
        try
        {
            string result = null;
            if (innerCache.TryGetValue(key, out result))
            {
                if (result == value)
                {
                    return AddOrUpdateStatus.Unchanged;
                }
                else
                {
                    cacheLock.EnterWriteLock();
                    try
                    {
                        innerCache[key] = value;
                    }
                    finally
                    {
                        cacheLock.ExitWriteLock();
                    }
                    return AddOrUpdateStatus.Updated;
                }
            }
            else
            {
                cacheLock.EnterWriteLock();
                try
                {
                    innerCache.Add(key, value);
                }
                finally
                {
                    cacheLock.ExitWriteLock();
                }
                return AddOrUpdateStatus.Added;
            }
        }
        finally
        {
            cacheLock.ExitUpgradeableReadLock();
        }
    }

    public void Delete(int key)
    {
        cacheLock.EnterWriteLock();
        try
        {
            innerCache.Remove(key);
        }
        finally
        {
            cacheLock.ExitWriteLock();
        }
    }

    public enum AddOrUpdateStatus
    {
        Added,
        Updated,
        Unchanged
    };

    ~SynchronizedCache()
    {
       if (cacheLock != null) cacheLock.Dispose();
    }
}

The following code then uses the SynchronizedCache object to store a dictionary of vegetable names. It creates three tasks. The first writes the names of vegetables stored in an array to a SynchronizedCache instance. The second and third task display the names of the vegetables, the first in ascending order (from low index to high index), the second in descending order. The final task searches for the string "cucumber" and, when it finds it, calls the EnterUpgradeableReadLock method to substitute the string "green bean".

C#

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

C#

public class Example
{
   public static void Main()
   {
      var sc = new SynchronizedCache();
      var tasks = new List<Task>();
      int itemsWritten = 0;

      // Execute a writer.
      tasks.Add(Task.Run( () => { String[] vegetables = { "broccoli", "cauliflower",
                                                          "carrot", "sorrel", "baby turnip",
                                                          "beet", "brussel sprout",
                                                          "cabbage", "plantain",
                                                          "spinach", "grape leaves",
                                                          "lime leaves", "corn",
                                                          "radish", "cucumber",
                                                          "raddichio", "lima beans" };
                                  for (int ctr = 1; ctr <= vegetables.Length; ctr++)
                                     sc.Add(ctr, vegetables[ctr - 1]);

                                  itemsWritten = vegetables.Length;
                                  Console.WriteLine("Task {0} wrote {1} items\n",
                                                    Task.CurrentId, itemsWritten);
                                } ));
      // Execute two readers, one to read from first to last and the second from last to first.
      for (int ctr = 0; ctr <= 1; ctr++) {
         bool desc = ctr == 1;
         tasks.Add(Task.Run( () => { int start, last, step;
                                     int items;
                                     do {
                                        String output = String.Empty;
                                        items = sc.Count;
                                        if (!desc) {
                                           start = 1;
                                           step = 1;
                                           last = items;
                                        }
                                        else {
                                           start = items;
                                           step = -1;
                                           last = 1;
                                        }

                                        for (int index = start; desc ? index >= last : index <= last; index += step)
                                           output += String.Format("[{0}] ", sc.Read(index));

                                        Console.WriteLine("Task {0} read {1} items: {2}\n",
                                                          Task.CurrentId, items, output);
                                     } while (items < itemsWritten | itemsWritten == 0);
                             } ));
      }
      // Execute a red/update task.
      tasks.Add(Task.Run( () => { Thread.Sleep(100);
                                  for (int ctr = 1; ctr <= sc.Count; ctr++) {
                                     String value = sc.Read(ctr);
                                     if (value == "cucumber")
                                        if (sc.AddOrUpdate(ctr, "green bean") != SynchronizedCache.AddOrUpdateStatus.Unchanged)
                                           Console.WriteLine("Changed 'cucumber' to 'green bean'");
                                  }
                                } ));

      // Wait for all three tasks to complete.
      Task.WaitAll(tasks.ToArray());

      // Display the final contents of the cache.
      Console.WriteLine();
      Console.WriteLine("Values in synchronized cache: ");
      for (int ctr = 1; ctr <= sc.Count; ctr++)
         Console.WriteLine("   {0}: {1}", ctr, sc.Read(ctr));
   }
}
// The example displays the following output:
//    Task 1 read 0 items:
//
//    Task 3 wrote 17 items
//
//
//    Task 1 read 17 items: [broccoli] [cauliflower] [carrot] [sorrel] [baby turnip] [
//    beet] [brussel sprout] [cabbage] [plantain] [spinach] [grape leaves] [lime leave
//    s] [corn] [radish] [cucumber] [raddichio] [lima beans]
//
//    Task 2 read 0 items:
//
//    Task 2 read 17 items: [lima beans] [raddichio] [cucumber] [radish] [corn] [lime
//    leaves] [grape leaves] [spinach] [plantain] [cabbage] [brussel sprout] [beet] [b
//    aby turnip] [sorrel] [carrot] [cauliflower] [broccoli]
//
//    Changed 'cucumber' to 'green bean'
//
//    Values in synchronized cache:
//       1: broccoli
//       2: cauliflower
//       3: carrot
//       4: sorrel
//       5: baby turnip
//       6: beet
//       7: brussel sprout
//       8: cabbage
//       9: plantain
//       10: spinach
//       11: grape leaves
//       12: lime leaves
//       13: corn
//       14: radish
//       15: green bean
//       16: raddichio
//       17: lima beans

Remarks

Recursion policy determines the restrictions on threads that enter the lock more than once. For example, if a lock was created with LockRecursionPolicy.NoRecursion and a thread has entered the lock in read mode, LockRecursionException is thrown if the thread tries to reenter the lock in read mode. Similarly, if a thread has entered the lock in write mode, LockRecursionException is thrown if the thread tries to reenter the lock in any mode.

Note

A thread in upgradeable mode can upgrade to write mode or downgrade to read mode regardless of the lock recursion policy setting.

Regardless of recursion policy, a thread that initially entered read mode is not allowed to upgrade to upgradeable mode or write mode, because that pattern creates a strong probability of deadlocks.

For more information about recursion policy and its effects, see the LockRecursionPolicy enumeration and the ReaderWriterLockSlim class.

See also

• LockRecursionPolicy
• RecursionPolicy