为什么是线程?这里使用MemoryBarrier

浏览MEF源代码时，我发现了这篇文章。谁能解释一下为什么锁里需要MemoryBarrier ?

整个方法是:

public void SatisfyImportsOnce(ComposablePart part)
{
    this.ThrowIfDisposed();
    if (this._importEngine == null)
    {
        ImportEngine importEngine = new ImportEngine(this, this._compositionOptions);
        lock(this._lock)
        {
            if (this._importEngine == null)
            {
                Thread.MemoryBarrier();
                this._importEngine = importEngine;
                importEngine = null;
            }
        }
        if(importEngine != null)
        {
            importEngine.Dispose();
        }
    }
    this._importEngine.SatisfyImportsOnce(part);
}