即使将项目导入目录中，Mef 也不会导出

Question

尝试在代码中进行导入，所以我尝试这样做：

IServiceBehavior behavior = container.GetExport<IServiceBehavior>().Value;

但是得到这个异常：

{"No valid exports were found that match the constraint '((exportDefinition.ContractName == \"System.ServiceModel.Description.IServiceBehavior\") AndAlso (exportDefinition.Metadata.ContainsKey(\"ExportTypeIdentity\") AndAlso \"System.ServiceModel.Description.IServiceBehavior\".Equals(exportDefinition.Metadata.get_Item(\"ExportTypeIdentity\"))))', invalid exports may have been rejected."}

但是当我检查容器 MefServiceBehavior（IServiceBehavior 的实现）的本地变量时，目录中是否存在：

container   {System.ComponentModel.Composition.Hosting.CompositionContainer}    System.ComponentModel.Composition.Hosting.CompositionContainer
    -       Catalog {MefContrib.Hosting.Generics.GenericCatalog}    System.ComponentModel.Composition.Primitives.ComposablePartCatalog  {MefContrib.Hosting.Generics.GenericCatalog}
        -       Parts   Count = 6   System.Collections.ObjectModel.ReadOnlyCollection<System.ComponentModel.Composition.Primitives.ComposablePartDefinition>
            -       [4] {MyProjectServiceLibrary.Hosting.MefServiceBehavior}    System.ComponentModel.Composition.Primitives.ComposablePartDefinition {System.ComponentModel.Composition.ReflectionModel.ReflectionComposablePartDefinition}
                -       ExportDefinitions   {System.ComponentModel.Composition.Primitives.ExportDefinition[1]}  System.Collections.Generic.IEnumerable<System.ComponentModel.Composition.Primitives.ExportDefinition> {System.ComponentModel.Composition.Primitives.ExportDefinition[]}
                    -       [System.ComponentModel.Composition.Primitives.ExportDefinition[]]   {System.ComponentModel.Composition.Primitives.ExportDefinition[1]}  System.ComponentModel.Composition.Primitives.ExportDefinition[]
                        -       [0] {MyProjectServiceLibrary.Hosting.MefServiceBehavior (ContractName="System.ServiceModel.Description.IServiceBehavior")}  System.ComponentModel.Composition.Primitives.ExportDefinition {System.ComponentModel.Composition.ReflectionModel.ReflectionMemberExportDefinition}
                            -       [System.ComponentModel.Composition.ReflectionModel.ReflectionMemberExportDefinition]    {MyProjectServiceLibrary.Hosting.MefServiceBehavior (ContractName="System.ServiceModel.Description.IServiceBehavior")}  System.ComponentModel.Composition.ReflectionModel.ReflectionMemberExportDefinition
                                -       Metadata    Count = 1   System.Collections.Generic.IDictionary<string,object> {Microsoft.Internal.Collections.ReadOnlyDictionary<string,object>}
                                    -       [0] {[ExportTypeIdentity, System.ServiceModel.Description.IServiceBehavior]}    System.Collections.Generic.KeyValuePair<string,object>
                                                Key "ExportTypeIdentity"    string
                                                Value   "System.ServiceModel.Description.IServiceBehavior"  object {string}

为什么容器找不到导出。 ExportTypeIdentity 与异常消息中提到的相同。这是怎么回事？