c# - MEF 组合问题，多线程

Question

我有以下代码:

public class Temp<T, TMetadata>
{
    [ImportMany]
    private IEnumerable<Lazy<T, TMetadata>> plugins;

    public Temp(string path)
    {
        AggregateCatalog aggregateCatalog = new AggregateCatalog();
        aggregateCatalog.Catalogs.Add(new DirectoryCatalog(path));
        CompositionContainer container = new CompositionContainer(aggregateCatalog);
        container.ComposeParts(this);
    }

    public T GetPlugin(Predicate<TMetadata> predicate)
    {
        Lazy<T, TMetadata> pluginInfo;

        try
        {
            pluginInfo = plugins.SingleOrDefault(p => predicate(p.Metadata));
        }
        catch
        {
            // throw some exception
        }

        if (pluginInfo == null)
        {
            // throw some exception
        }

        return Clone(pluginInfo.Value); // -> this produces errors
    }
}

我有一个 Temp 对象，我从多个线程调用 GetPlugin()。有时我会遇到奇怪的构图错误，但我找不到重现的方法。例如:

"System.InvalidOperationException: Stack empty.
    at System.Collections.Generic.Stack`1.Pop()
    at System.ComponentModel.Composition.Hosting.ImportEngine.TrySatisfyImports(PartManager partManager, ComposablePart part, Boolean shouldTrackImports)
    at System.ComponentModel.Composition.Hosting.ImportEngine.SatisfyImports(ComposablePart part)
    at System.ComponentModel.Composition.Hosting.CompositionServices.GetExportedValueFromComposedPart(ImportEngine engine, ComposablePart part, ExportDefinition definition)
    at System.ComponentModel.Composition.Hosting.CatalogExportProvider.GetExportedValue(CatalogPart part, ExportDefinition export, Boolean isSharedPart)
    at System.ComponentModel.Composition.ExportServices.GetCastedExportedValue[T](Export export)
    at System.Lazy`1.CreateValue()
    at System.Lazy`1.LazyInitValue()
    at Temp`2.GetPlugin(Predicate`1 predicate)..."

可能是什么原因以及如何修复此代码？

Answer 1

CompositionContainer 类有一个 little-known constructor它接受一个 isThreadSafe 参数(出于性能原因默认为 false)。如果您将创建容器并将此值设置为 true，我相信您的问题将得到解决:

CompositionContainer container = new CompositionContainer(aggregateCatalog, true);

附带说明，与原始问题无关，您可以使用 an export factory 而不是在插件上调用 Clone()相反 - 这样您就不必实现自己的克隆方法，因为 MEF 将为您创建一个新实例。