Question

错误：

The composition produced a single composition error, with 2 root causes. The root causes are provided below. Review the CompositionException.Errors property for more detailed information.

1) No exports were found that match the constraint: 
    ContractName    MyNonMefInterface
    RequiredTypeIdentity    MyNonMefInterface

Resulting in: Cannot set import 'MyMefClass..ctor (Parameter="myNonMefClass", ContractName="MyNonMefInterface")' on part 'MyMefClass'.

有没有告诉MEF NOT 尝试导入“myInterface”？它是一个可选的参数，如果没有传递给构造函数，它就会被默认：

public MyMefClass() : (new MyNonMefClass()) {}
public MyMefClass(myNonMefInterface myNonMefClass) {
    _myNonMefClass = myNonMefClass;
}

我正在使用MEF 2的新属性配置，并像我这样使用我的RegistrationBuilder：

var builder = new RegistrationBuilder();
builder.ForTypesDerivedFrom<IMyMefClass>().ExportInterfaces();
...
var aggregateCatalog = new AggregateCatalog();

aggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(MyMefClass).Assembly, builder));

var container = new CompositionContainer(aggregateCatalog, CompositionOptions.DisableSilentrejection);

基本上我总是在默认构造函数中设置默认的MyNonMefClass，但允许自己重载它以进行测试。它没有为MEF设置并且来自另一个dll，所以如果我可以告诉MEF忽略它并使用没有params的构造函数，那将是最简单的解决方案。有谁知道怎么做？

编辑：我能够通过在空构造函数上设置[ImportingConstructor]标记来解决它，但这意味着我必须在该项目中引用MEF，我是否能够通过新的流利来做到这一点以某种方式使用RegistrationBuilder的API？

Answer 1

要在MEF中获取可选导入，您应该使用Optional Imports。例如，在您的代码中：

public MyMefClass([Import(AllowDefault=true)]myNonMefInterface myNonMefClass) {
    _myNonMefClass = myNonMefClass;
}

Answer 2

好的我找到了答案，有SelectConstructor方法：

builder.ForTypesDerivedFrom<IMyMefClass>().SelectConstructor(ctors =>
{
    var minParams = ctors.Min(ctor => ctor.GetParameters().Length);
    return ctors.First(ctor => ctor.GetParameters().Length == minParams);
}).ExportInterfaces();

这将选择最少量的参数，默认选择最多。希望能帮助别人！：）

http://blogs.msdn.com/b/bclteam/archive/2011/11/01/getting-started-with-convention-based-part-registration-in-mef-version-2.aspx

MEF 2 - 未找到与具有多个重载的构造函数的约束匹配的导出

2 个答案: