C#中Dictionary的TryGetValue和Contains

一：前言

如果只是判断字典中某个值是否存在，使用Contains和TryGetValue都可以。如果需要判断是否存在之后再得到某个值，尽量使用TryGetValue

if (Dictionary.TryGetValue(key, out value))
{

}

if(Dictionary.ContainsKey(key))
{
	var value = Dictionary[key];
}

---

二：解释

先看一下TryGetValue源码实现

public bool TryGetValue(TKey key, out TValue value)
        {
            if (key == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
            }
            lock(_lock)
            {
                VerifyIntegrity();
                return TryGetValueWorker(key, out value);
            }
        }

        private bool TryGetValueWorker(TKey key, out TValue value)
        {
            int entryIndex = FindEntry(key);
            if (entryIndex != -1)
            {
                Object primary = null;
                Object secondary = null;
                _entries[entryIndex].depHnd.GetPrimaryAndSecondary(out primary, out secondary);
                // Now that we've secured a strong reference to the secondary, must check the primary again
                // to ensure it didn't expire (otherwise, we open a ---- where TryGetValue misreports an
                // expired key as a live key with a null value.)
                if (primary != null)
                {
                    value = (TValue)secondary;
                    return true;
                }
            }
 
            value = default(TValue);
            return false;
        }

再看一下ContainsKey和字典索引的实现

public bool ContainsKey(TKey key) {
            return FindEntry(key) >= 0;
        }

        public TValue this[TKey key] {
            get {
                int i = FindEntry(key);
                if (i >= 0) return entries[i].value;
                ThrowHelper.ThrowKeyNotFoundException();
                return default(TValue);
            }
            set {
                Insert(key, value, false);
            }
        }

想得到某个值，用TryGetValue只调用了1次FindEnty，而ContainKey判断还需要使用字典索引，一共需要调用2次FindEnty才能获取到值