How to correctly handle adding new item to dictionary?

问题

I have an ASP.NET application where I use static class as cache. Inside that static class is internal dictionary, which holds cached objects. Of course, in static class are methods like Add/Remove/Clear... It looks as follows:

public static class CacheManager
{
    private static Dictionary<string, object> cacheItems = new Dictionary<string, object>();

    private static ReaderWriterLockSlim locker = new ReaderWriterLockSlim();

    public static Dictionary<string, object> CacheItems
    {
        get
        {
            return cacheItems;
        }
    }

    public static void AddCacheItem(string key, object data)
    {
        locker.EnterWriteLock();
        try
        {
            cacheItems.Add(key, data);
        }
        finally
        {
            locker.ExitWriteLock();
        }
    }

    ...
}

The items was added to the cache (dictionary) when ASP.NET application runs. I just want to ask should I check for example in Add method if key is already added in this way:

    public static void AddCacheItem(string key, object data)
    {
        locker.EnterWriteLock();
        try
        {
            if (!cacheItems.ContainsKey(key))
            {
                cacheItems.Add(key, data);
            }
        }
        finally
        {
            locker.ExitWriteLock();
        }
    }

Or leave it as is in first code snippet?

Thank you in advance.

回答1:

You can implement one more method TryAdd

public static bool TryAddCacheItem(string key, object data)
{
    locker.EnterWriteLock();
    try
    {
        if (cacheItems.ContainsKey(key))
        {
            return false;
        }
        cacheItems.Add(key, data);
        return true;
    }
    finally
    {
        locker.ExitWriteLock();
    }
}

Also I would suggest to use ConcurrentDictionary. Here is the code of the ConcurrentDictionary.

回答2:

Just execute twice:

cacheItems.Add(key, data);

...and you will know the answer. Basically the exception will be thrown and caught by the try/finally block. I would personally use the if statement rather than putting all the logic responsibility on the try block, but its a question of preference...

来源：https://stackoverflow.com/questions/11027369/how-to-correctly-handle-adding-new-item-to-dictionary