使用 ImportingConstructor 处理 ctor 抛出的异常

Question

下面是两个类，其中一个使用 ImportingConstructor 导入另一个类，并直接从 ctor 抛出 MyException。所以，我希望在另一个类中捕获 MyException，但我得到了另一个类，即：

 System.InvalidOperationException: GetExportedValue cannot be called before prerequisite import 'Class1..ctor ' has been set. (...)

如何强制 MEF 抛出原始异常，或者至少将原始异常包装到某个异常中，以便我可以从我的代码中处理它？

[Export]
class Class1
{
    public Class1()
    {
       (....)
       throw new MyException("MyException has been thrown");
    }
}

class Class2
{
    [ImportingConstructor]
    public Class2(Class1 class1)
    {
        (....)
    }
}

static void Main(string[] args)
{
    var catalog = new AggregateCatalog(
        new AssemblyCatalog(typeof (Class1).Assembly));
    CompositionContainer container = new CompositionContainer(catalog, true);
    try{
      Class2 class2 = container.GetExportedValue<Class2>();
    }
    catch(MyException ex){...}
}

Answer 1

AFAIK，你不能让 MEF 抛出原始异常，但如果你挖掘，原始异常就在堆栈跟踪中。因此，处理此问题的一种方法是检查 CompositionException 是否存在您预期的异常。如果你找到它，那么你可以继续你需要的错误处理。

try
{
    Class2 class2 = container.GetExportedValue<Class2>();
}
catch (CompositionException ex)
{
    foreach (var cause in ex.RootCauses)
    {
        //Check if current cause is the Exception we're looking for
        var myError = cause as MyException;

        //If it's not, check the inner exception (chances are it'll be here)
        if (myError == null)
            myError = cause.InnerException as MyException;

        if (myError != null)
        {
            //do what you want
        }
    }
}

取决于抛出异常的位置可能取决于您需要在堆栈跟踪中查看的位置，但上面的简单示例应该让您走上正确的道路。如果你有兴趣，这里是MSDN docs

为了完整起见，这里是完整的实现

[Export]
class Class1
{
    public Class1()
    {
        throw new MyException("MyException has been thrown");
    }
}

[Export]
class Class2
{
    [ImportingConstructor]
    public Class2(Class1 class1)
    {

    }
}

static void Main(string[] args)
{
    var catalog = new AggregateCatalog(
        new AssemblyCatalog(typeof (Class1).Assembly));
    CompositionContainer container = new CompositionContainer(catalog, true);
    try
    {
        Class2 class2 = container.GetExportedValue<Class2>();
    }
    catch (CompositionException ex)
    {
        foreach (var cause in ex.RootCauses)
        {
            var myError = cause as MyException;

            if (myError == null)
                myError = cause.InnerException as MyException;

            if (myError != null)
            {
                //do what you want
            }
        }
    }
}