Question

我如何将可空值传递给方法

[Import("Default", typeof(ISomeInterface), AllowRecomposition = true, AllowDefault = true)]
public ISomeInterface x { get; set; }
// x is null till now 
void DoWork(ISomeInterface obj) //Not working
{
        if (obj == null)
        {
            //Download and Satisfy
            DeploymentCatalog DC = new DeploymentCatalog("TheXAPfile.xap");
            DC.DownloadCompleted += (s, e) =>
            {
                catalog.Catalogs.Add(f); //catalog is AggregateCatalog
                obj.Show();
            };
            DC.DownloadAsync();
        }
        else
        {
            obj.Show();
        }

}

由于

Answer 1

异常是{null}时使用obj的某些方法或属性的结果。你必须检查它是否为你自己的

void DoWork(ISomeInterface obj) //Not working
{
    if(obj == null)
    {
        return;
    }
    /* do something ... */}
}

Answer 2

这里假设当你启动异步下载时obj为null，并且当触发DownloadCompleted时，它不再是null。有什么东西你没有表明保证你满足这个条件吗？ DownloadAsync之后你在做什么，你怎么能确定在下载完成之前评估它？

    if (obj == null) 
    {
        //// you enter here only if obj is null
        //Download and Satisfy
        DeploymentCatalog DC = new DeploymentCatalog("TheXAPfile.xap");
        DC.DownloadCompleted += (s, e) =>
        {
            catalog.Catalogs.Add(f); //catalog is AggregateCatalog
            //// here you are assuming that obj is not null anymore. Why???
            obj.Show();
        };
        DC.DownloadAsync();
    }

Answer 3

这可以通过放置一个？在函数头中的对象类型之后。

void DoWork(ISomeInterface? obj)

将可空值传递给方法

3 个答案: