MEF 组合错误未传播内部异常（通过消息除外）

Question

我正在与 MEF 合作，将来自不同来源的模块加载到我的应用程序中。我有一个示例（下面的代码），我在其中创建了一个可组合的类，该类在构造函数中引发异常。该异常导致组合失败，从而引发一个异常，说明“为什么”......在声明中它说原因是InvalidCastException......这是真的。

如果我打印异常，我知道它为什么会失败，但我如何检索抛出的实际原始异常 (InvalidCastException)，以便我正确处理它——而不仅仅是一个通用的 catch，这将错过了其他异常——在模块中请求所述类的实例？查询CompositionException.Errors 也不会给我原始异常...

代码：

using System.ComponentModel.Composition;
using Microsoft.Practices.ServiceLocation;

[Export(typeof(A))]
Class A
{
    [ImportingConstructor]
    public A(ISomeinterface x, ISomeOtherInterface y)
    {
        //// Throw some exception (in my code it happens to be an InvalidCastException)
        throw new InvalidCastException ("Unable to cast object of type 'Sometype' to type 'Someothertype'.");  
    }
}

Class B
{
    public B(IServiceLocator serviceLocator)
    {
        try
        {
            //// Here is where the exception would be thrown as an "ActivationException"
            A myAInstance = serviceLocator.GetInstance(A);
        }
        catch (ActivationException activationException)
        {
            //// Print original activation exception from trying to get the service
            System.Diagnostics.Debug.WriteLine(activationException);

            if (ex.InnerException is CompositionException)
            {
                CompositionException compositionException = (CompositionException)activationException.InnerException;

                //// *** Here is where I want to retrieve the InvalidCastException in order to handle it properly
                //// *** Also, componentException.InnerException is "null" and so is the componentException.Errors[0].Exception.InnerException 
            }
            else
            {
                throw;
            }
        }
    }
}

输出：

Microsoft.Practices.ServiceLocation.ActivationException: Activation error occured while trying to get instance of type A, key "" ---> System.ComponentModel.Composition.CompositionException: The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) Unable to cast object of type 'Sometype' to type 'Someothertype'.

Resulting in: An exception occurred while trying to create an instance of type 'A'.

Resulting in: Cannot activate part 'A'.
Element: A -->  A -->  DirectoryCatalog (Path="C:\path\to\code\")

Resulting in: Cannot get export 'A (ContractName="A")' from part 'A'.
Element: A (ContractName="A") -->  A -->  DirectoryCatalog (Path="C:\path\to\code\")

   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.ComponentModel.Composition.ExportServices.<>c__DisplayClass10`2.<CreateSemiStronglyTypedLazy>b__d()
   at System.Lazy`1.CreateValue()
   at System.Lazy`1.LazyInitValue()
   at Microsoft.Practices.Prism.MefExtensions.MefServiceLocatorAdapter.DoGetInstance(Type serviceType, String key) in c:\release\WorkingDir\PrismLibraryBuild\PrismLibrary\Desktop\Prism.MefExtensions\MefServiceLocatorAdapter.cs:line 73
   at Microsoft.Practices.ServiceLocation.ServiceLocatorImplBase.GetInstance(Type serviceType, String key)
   --- End of inner exception stack trace ---
   at Microsoft.Practices.ServiceLocation.ServiceLocatorImplBase.GetInstance(Type serviceType, String key)
   at Microsoft.Practices.ServiceLocation.ServiceLocatorImplBase.GetInstance[TService]()
   at <code> line whatever...
System.ComponentModel.Composition.CompositionException: The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

Answer 1

我找到了实际异常嵌套在 compositionException.Errors 层次结构深处的解决方案。有人会认为原始异常会在原始组合异常下，但实际上这是访问它的方法：

它恰好是另一个组合异常的错误之一中的内部异常，它本身就是原始组合异常中的错误之一

CompositionException compositionException = (CompositionException)activationException.InnerException;
CompositionException innerCompositionException = compositionException.Errors[0].Exception;
InvalidCastException castException = (InvalidCastException)innerCompositionException.Errors[0].Exception.InnerException

我不会删除它以防其他人遇到同样的事情。