来自 Galasoft 的带有 MEF 和 IOC 容器的自定义服务定位器

Question

我有一个要求，我必须在我的应用程序中支持两种类型的容器。因此，我创建了一个自定义 ServiceLocator，它为 2 种类型的容器提供服务，即

1. SimpleIoc (Galasoft)

2. MefServiceLocatorAdapter

因此，我创建了一个通用服务定位器类，即

public class CommonServiceLocator : ServiceLocatorImplBase
{
    private IServiceLocator _iocContainer, _mefContainer;

    public CommonServiceLocator(IServiceLocator iocLocator, IServiceLocator mefLocator)
    {
        _iocContainer = iocLocator;
        _mefContainer = mefLocator;
    }

    protected override IEnumerable<object> DoGetAllInstances(Type serviceType)
    {

        IEnumerable<object> services;
        try
        {
            services = _iocContainer.GetAllInstances(serviceType);
        }
        catch (Exception)
        {
            //Current assumption is that if IocContainer doesn't contain an instance then
            //it should look for MefContainer for values
            services = _mefContainer.GetAllInstances(serviceType);
        }
        return services;
    }

    protected override object DoGetInstance(Type serviceType, string key)
    {
        object service;
        try
        {
            service = _iocContainer.GetInstance(serviceType, key);
        }
        catch (Exception)
        {
            //Current assumption is that if IocContainer doesn't contain an instance then
            //it should look for MefContainer for values
            service = _mefContainer.GetInstance(serviceType, key); //<== This fails to get an instance
        }
        return service;
    }

    public override object GetInstance(Type serviceType, string key)
    {
        return DoGetInstance(serviceType, key);
    }

    public override object GetInstance(Type serviceType)
    {
        return GetInstance(serviceType, null);
    }
}

现在，问题是当我尝试使用 GetInstance() 方法通过 IOC 容器获取类实例时，它工作正常。但是，通过 MefContainer 获取类实例会引发一些错误（“System.Core.dll 中发生了“System.InvalidOperationException”类型的第一次机会异常”）。我创建了一个测试类来测试这个场景：

public class ServiceLocatorTest
{
    public void CommonServiceLocatorInstanceTest()
    {
        var serviceLocator = new SimpleIoc();
        serviceLocator.Register<GalaSoftIocData>();

        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(MefCompositionAddin).Assembly));
        CompositionContainer compContainer = new CompositionContainer(catalog);

        CompositionBatch batch = new CompositionBatch();

        batch.AddPart(AttributedModelServices.CreatePart(new MefCompositionAddin()));

        compContainer.Compose(batch);

        var vl = compContainer.GetExportedValue<MefCompositionAddin>(); // <== This gives instance correctly
        var mefserviceLocator = new MefServiceLocatorAdapter(compContainer);
        var commonServiceLocator = new CommonServiceLocator(serviceLocator, mefserviceLocator);
        var galaSoftData = commonServiceLocator.GetInstance(typeof(GalaSoftIocData));
        var mefData = commonServiceLocator.GetInstance(typeof(MefCompositionAddin));
    }
}

[Export]
class MefCompositionAddin
{

    public string MyData { get { return "Mef's Data composed"; } }

    [Import]
    public MefCompositionAddin MyObj { get; set; }
}

class MefCompositionData
{
    [Import]
    public MefCompositionAddin MyAddin { get; set; }
}

class GalaSoftIocData
{
    public string MyData { get { return "Galasoft's Data composed"; } }
}

如果我错过了什么，请告诉我。以及可能的原因是什么。提前感谢大家的帮助。

Answer 1

我相信您正在使用 Prism 库中提供的 MefServiceLocatorAdapter。这是它的 DoGetInstance 方法的代码：

protected override object DoGetInstance(Type serviceType, string key)
{
    IEnumerable<Lazy<object, object>> exports = this.compositionContainer.GetExports(serviceType, null, key);
    if ((exports != null) && (exports.Count() > 0))
    {
        // If there is more than one value, this will throw an InvalidOperationException, 
        // which will be wrapped by the base class as an ActivationException.
        return exports.Single().Value;
    }

    throw new ActivationException(
        this.FormatActivationExceptionMessage(new CompositionException("Export not found"), serviceType, key));
}

正如您在评论中看到的那样，如果您为相应的服务类型和密钥导出了多个类型，则会由于 Single 方法而引发您描述的异常。

您可以查看应用程序中的导出以查看是否有某种类型映射到多个类（或多次导出同一个类）或重写该方法以使其不会引发异常（例如，使用 FirstOrDefault 而不是 Single。）

注意：我没有添加上述注释，它包含在库的代码中。