MEF GetExports＆LT; T，TMetaDataView＆GT;用的AllowMultiple没有返回= TRUE [英] MEF GetExports<T, TMetaDataView> returning nothing with AllowMultiple = True

问题描述

我不明白MEF非常好，所以希望这是我怎么想它的工作原理简单的修复。

I don't understand MEF very well, so hopefully this is a simple fix of how I think it works.

我试图使用MEF获取一类的一些信息，应该如何使用。我使用的元数据的选项来尝试实现这一目标。我的接口和属性如下：

I'm trying to use MEF to get some information about a class and how it should be used. I'm using the Metadata options to try to achieve this. My interfaces and attribute looks like this:

public interface IMyInterface
{
}

public interface IMyInterfaceInfo
{
    Type SomeProperty1 { get; }
    double SomeProperty2 { get; }
    string SomeProperty3 { get; }
}

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class ExportMyInterfaceAttribute : ExportAttribute, IMyInterfaceInfo
{
    public ExportMyInterfaceAttribute(Type someProperty1, double someProperty2, string someProperty3)
        : base(typeof(IMyInterface))
    {
        SomeProperty1 = someProperty1;
        SomeProperty2 = someProperty2;
        SomeProperty3 = someProperty3;
    }

    public Type SomeProperty1 { get; set; }
    public double SomeProperty2 { get; set; }
    public string SomeProperty3 { get; set; }
}

这是装饰与属性的类看起来是这样的：

The class that is decorated with the attribute looks like this:

[ExportMyInterface(typeof(string), 0.1, "whoo data!")]
[ExportMyInterface(typeof(int), 0.4, "asdfasdf!!")]
public class DecoratedClass : IMyInterface
{
}

这是试图使用导入的方法是这样的：

The method that is trying to use the import looks like this:

private void SomeFunction()
{
    // CompositionContainer is an instance of CompositionContainer
    var myExports = CompositionContainer.GetExports<IMyInterface, IMyInterfaceInfo>();
}

在我的情况下 myExports 总是空空的。在我CompositionContainer中，我有我的目录中的部分有两个 ExportDefinitions ，都与以下 ContractName ：myNameSpace对象.IMyInterface。在元还装载正确按我的出口。

In my case myExports is always empty. In my CompositionContainer, I have a Part in my catalog that has two ExportDefinitions, both with the following ContractName: "MyNamespace.IMyInterface". The Metadata is also loaded correctly per my exports.

如果我删除的AllowMultiple 二传手，只包括一个输出属性，在 myExports 变量现在有一个出口其装载的元数据。

If I remove the AllowMultiple setter and only include one exported attribute, the myExports variable now has the single export with its loaded metadata.

我在做什么错了？

编辑：如果我使用弱类型的元数据，我出口突然感到满意：

var myExports = CompositionContainer.GetExports<IMyInterface, IDictionary<string, object>>();

任何想法，为什么？

Any ideas why?

推荐答案

据了解，MEF与的AllowMultiple = TRUE打交道时有一些问题。有关完整的说明，你可以，例如看<一个href=\"http://msmvps.com/blogs/kathleen/archive/2009/09/07/a-fix-we-really-need-in-mef-for-net-4-0.aspx\">here,反正从事实的元数据保存在一个字典，其中值是数组时的AllowMultiple是真的，这样的事情不能在你的IMyInterfaceInfo映射派生的。

It is known that MEF has some problems when dealing with AllowMultiple = true. For a complete explanation you could for instance look here, anyway it derives from the fact that the metadata are saved in a Dictionary where the values are arrays when AllowMultiple is true, and such a thing can't be mapped on your IMyInterfaceInfo.

这是我用的解决方法。首先，你的属性应该从属性中获得，而不是从ExportAttribute：

This is the workaround I use. First of all your attribute should derive from Attribute, not from ExportAttribute:

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class ExportMyInterfaceAttribute : Attribute, IMyInterfaceInfo
{
    public ExportMyInterfaceAttribute(Type someProperty1, double someProperty2, string someProperty3)

    {
        SomeProperty1 = someProperty1;
        SomeProperty2 = someProperty2;
        SomeProperty3 = someProperty3;
    }

    public Type SomeProperty1 { get; set; }
    public double SomeProperty2 { get; set; }
    public string SomeProperty3 { get; set; }
}

这意味着，出口类应该有3个属性，一个标准的出口和自定义属性：

This means that the exported class should have 3 attributes, a standard export and your custom attributes:

[Export(typeof(IMyInterface))]
[ExportMyInterface(typeof(string), 0.1, "whoo data!")]
[ExportMyInterface(typeof(int), 0.4, "asdfasdf!!")]
public class DecoratedClass : IMyInterface

然后，你必须定义将要导入的元数据的视图。这必须有一个构造函数的IDictionary的作为参数。事情是这样的：

Then you have to define a view for the metadata which will be imported. This has to have a constructor that takes the IDictionary as parameter. Something like this:

public class MyInterfaceInfoView
{
    public IMyInterfaceInfo[] Infos { get; set; }

    public MyInterfaceInfoView(IDictionary<string, object> aDict)
    {
        Type[] p1 = aDict["SomeProperty1"] as Type[];
        double[] p2 = aDict["SomeProperty2"] as double[];
        string[] p3 = aDict["SomeProperty3"] as string[];

        Infos = new ExportMyInterfaceAttribute[p1.Length];
        for (int i = 0; i < Infos.Length; i++)
            Infos[i] = new ExportMyInterfaceAttribute(p1[i], p2[i], p3[i]);
    }
}

现在，你应该能够成功调用

Now you should be able to successfully call

var myExports = CompositionContainer.GetExports<IMyInterface, MyInterfaceInfoView>();

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