你如何使用ExportFactory <T>

Question

我是MEF的新手,正在尝试使用ExportFactory.我可以使用ExportFactory根据用户插入对象创建列表吗？样本将类似于下面显示的内容.我可能不理解ExportFactory的使用,因为在运行时我在组合期间得到如下所示的错误.

1)没有找到符合约束'((exportDefinition.ContractName =="System.ComponentModel.Composition.ExportFactory(CommonLibrary.IFoo)")AndAlso(exportDefinition.Metadata.ContainsKey("ExportTypeIdentity")AndAlso"System"的有效导出). ComponentModel.Composition.ExportFactory(CommonLibrary.IFoo)".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))',无效导出可能已被拒绝.

 class Program
{
    static void Main(string[] args)
    {
        Test mytest = new Test();
    }
}

public class Test : IPartImportsSatisfiedNotification
{
    [Import]
    private ExportFactory<IFoo> FooFactory { get; set; }

    public Test()
    {
        CompositionInitializer.SatisfyImports(this);
        CreateComponent("Amp");
        CreateComponent("Passive");
    }

    public void OnImportsSatisfied()
    {
        int i = 0;
    }

    public void CreateComponent(string name)
    {
        var componentExport = FooFactory.CreateExport();
        var comp = componentExport.Value;
    }
}

public interface IFoo
{
    double Name { get; set; }
}

[ExportMetadata("CompType", "Foo1")]
[Export(typeof(IFoo))]
[PartCreationPolicy(System.ComponentModel.Composition.CreationPolicy.NonShared)]
public class Foo1 : IFoo
{
    public double Name { get; set; }
    public Foo1()
    {

    }
}

[ExportMetadata("CompType", "Foo2")]
[Export(typeof(IFoo))]
[PartCreationPolicy(System.ComponentModel.Composition.CreationPolicy.NonShared)]
public class Foo2 : IFoo
{
    public double Name { get; set; }
    public Foo2()
    {
    }
}

Answer 1

问题似乎是您希望导入单个ExportFactory<IFoo>,但您已导出两个不同的IFoo实现.在您的示例中,MEF无法在两个实现之间做出决定.

您可能想要导入多个工厂,包括如下元数据:

[ImportMany]
private IEnumerable<ExportFactory<IFoo,IFooMeta>> FooFactories 
{ 
    get;
    set;
}

IFooMeta将在哪里声明如下:

public interface IFooMeta
{
    string CompType { get; }
}

然后你可以CreateComponent像这样实现:

public IFoo CreateComponent(string name, string compType)
{
    var matchingFactory = FooFactories.FirstOrDefault(
        x => x.Metadata.CompType == compType);
    if (matchingFactory == null)
    {
        throw new ArgumentException(
            string.Format("'{0}' is not a known compType", compType),
            "compType");
    }
    else
    {
        IFoo foo = matchingFactory.CreateExport().Value;
        foo.Name = name;
        return foo;
    }
}