c# - MEF 2 - 未找到与具有多个重载的构造函数的约束相匹配的导出

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'.

是否有办法告诉 MEF 不要尝试导入“myInterface”？它是一个可选参数，如果未传递给构造函数，则该参数已默认为:

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

我正在使用 MEF 2 的新属性较少配置，并像这样执行我的 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 MyNonMefClass 的默认值，但允许自己重载它以进行测试。它不是为 MEF 设置的，而是来自另一个 dll，所以如果我可以告诉 MEF 忽略它并使用不带参数的构造函数，那将是最简单的解决方案。有人知道该怎么做吗？

编辑:我能够通过在空构造函数上设置 [ImportingConstructor] 标记来解决它，但这意味着我也必须在该项目中引用 MEF，我可以这样做吗它通过新的 Fluent API 使用 RegistrationBuilder 以某种方式代替？

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;
}