Lock that will allow multiple readers in C#

Question

I have the following code:

private static object _dbLock = new object();

public static void LoadData()
{
   lock (_dbLock)
   {
      //Load data from the database
   }
}

public static string ReadData(Guid key)
{
   lock (_dbLock)
   {
      //Lookup key in data and return value
   }
}

I don't want to allow people to read the data while it's being loaded from the database, thus I put a lock in ReadData. However, right now if multiple people call ReadData at the same time, only one call can run at once.

Is there a way I can allow simultaneous calls to ReadData but block readers when LoadData is being run?

Answer 1

The ReaderWriterLock and ReaderWriterLockSlim classes support that use case. Use the 'Slim' version unless you need pre-3.5 support.

private static ReaderWriterLockSlim _cacheLock = new ReaderWriterLockSlim();

public static void LoadData()
{
    _cacheLock.EnterWriteLock();
    try
    {
        // Load data from the database
    }
    finally
    {
        _cacheLock.ExitWriteLock();
    }
}

public static string ReadData(Guid key)
{
    _cacheLock.EnterReadLock();
    try
    {
        // Lookup key in data and return value
    }
    finally
    {
        _cacheLock.ExitReadLock();
    }
}

Answer 2

Try using the ManualResetEvent:

private static object _dbLock = new object();
private static ManualResetEvent _mrse = new ManualResetEvent(true);

public static void LoadData()
{

   lock (_dbLock)
   {
      _mrse.Reset();
      //Load data from the database
      _mrse.Set();
   }
}

public static string ReadData(Guid key)
{
    _mrse.Wait();
    //Lookup key in data and return value
}

This allows multiple readers to wait for the data loader to finish its work and then simultaneously read. If the collection is not thread safe, then you need to add another locking object or use another synchronization construct.