其实是显式实现的.

object ICollection.SyncRoot
{
    get
    {
        if (this._syncRoot == null)
        {
            Interlocked.CompareExchange(ref this._syncRoot, new object(), null);
        }
        return this._syncRoot;
    }
}

这意味着您必须强制转换为 ICollection 才能使用它.

This means you must cast to ICollection to use it.