在.NET中将项目添加到Dictionary <>对象时,如何避免双重检查锁定？

using System.Collections.Concurrent;

public class MyClass
{
    private ConcurrentDictionary<string, object> _dictionary = new ConcurrentDictionary<string, object>();

    public void AddItem(string key, object value)
    {
        _dictionary.TryAdd(key, value);
    }

    public bool ContainsKey(string key)
    {
        return _dictionary.ContainsKey(key);
    }

    public object GetItem(string key)
    {
        if (_dictionary.TryGetValue(key, out object value))
        {
            return value;
        }
        return null;
    }
}