类SessionModel是一个提供多种服务的服务定位器（我将来会详细说明我的系统架构，但是现在我需要这样做）。
我将以下代码部分编辑为短，自包含，正确（可编译），示例（SSCCE）：
using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
namespace ConsoleApplication1
{
internal class Program
{
private static void Main(string[] args)
{
var sessionModel = new SessionModel(3);
// first case (see text down below):
var compositionContainer = new CompositionContainer();
// second case (see text down below):
//var typeCatalog = new TypeCatalog(typeof (SessionModel));
//var compositionContainer = new CompositionContainer(typeCatalog);
compositionContainer.ComposeExportedValue(sessionModel);
var someService = compositionContainer.GetExportedValue<ISomeService>();
someService.DoSomething();
}
}
public class SessionModel
{
private int AValue { get; set; }
[Export]
public ISomeService SomeService { get; private set; }
public SessionModel(int aValue)
{
AValue = aValue;
// of course, there is much more to do here in reality:
SomeService = new SomeService();
}
}
public interface ISomeService
{
void DoSomething();
}
public class SomeService : ISomeService
{
public void DoSomething()
{
Console.WriteLine("DoSomething called");
}
}
}
我希望MEF在编写其他部分时考虑服务定位器导出的部分（即SomeService），但不幸的是，这不起作用。
当我尝试获取ISomeService的导出值时，有System.ComponentModel.Composition.ImportCardinalityMismatchException告诉我没有带有此合约名称和所需类型标识（ConsoleApplication1.ISomeService）的导出。
如果我使用CompositionContainer创建TypeCatalog，则异常略有不同。这是一个System.ComponentModel.Composition.CompositionException告诉我MEF找不到创建ConsoleApplication1.SessionModel的方法（这是正确的，也是我自己做的原因）。
mefx对两种情况都说：
[Part] ConsoleApplication1.SessionModel from: DirectoryCatalog (Path=".")
[Export] ConsoleApplication1.SessionModel.SomeService (ContractName="ConsoleApplication1.ISomeService")
[Part] ConsoleApplication1.SessionModel from: AssemblyCatalog (Assembly="ConsoleApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null")
[Export] ConsoleApplication1.SessionModel.SomeService (ContractName="ConsoleApplication1.ISomeService")
我该怎么办？这可能与MEF有关，还是我必须使用Unity或StructureMap，还是其他什么？这可以通过实现ExportProvider吗？
答案 0 :(得分：1)
好的，我就这样做了：
我在我的SessionModelExportProvider中实现了我自己的SessionModel查找导出（请参阅下面的代码）。类SessionModelExport仅用于保存导出数据，而不是创建服务实例 - 返回SessionModel的属性值。
public class SessionModelExportProvider : ExportProvider
{
private List<Export> Exports { get; set; }
public SessionModelExportProvider(SessionModel sessionModel)
{
// get all the properties of the session model having an Export attribute
var typeOfSessionModel = typeof (SessionModel);
PropertyInfo[] properties = typeOfSessionModel.GetProperties();
var propertiesHavingAnExportAttribute =
from p in properties
let exportAttributes = p.GetCustomAttributes(typeof (ExportAttribute), false)
where exportAttributes.Length > 0
select new
{
PropertyInfo = p,
ExportAttributes = exportAttributes
};
// creating Export objects for each export
var exports = new List<Export>();
foreach (var propertyHavingAnExportAttribute in propertiesHavingAnExportAttribute)
{
var propertyInfo = propertyHavingAnExportAttribute.PropertyInfo;
foreach (ExportAttribute exportAttribute in propertyHavingAnExportAttribute.ExportAttributes)
{
string contractName = exportAttribute.ContractName;
if (string.IsNullOrEmpty(contractName))
{
Type contractType = exportAttribute.ContractType ?? propertyInfo.PropertyType;
contractName = contractType.FullName;
}
var metadata = new Dictionary<string, object>
{
{CompositionConstants.ExportTypeIdentityMetadataName, contractName},
{CompositionConstants.PartCreationPolicyMetadataName, CreationPolicy.Shared}
};
var exportDefinition = new ExportDefinition(contractName, metadata);
var export = new SessionModelExport(sessionModel, propertyInfo, exportDefinition);
exports.Add(export);
}
}
Exports = exports;
}
protected override IEnumerable<Export> GetExportsCore(ImportDefinition definition,
AtomicComposition atomicComposition)
{
return Exports.Where(e => definition.IsConstraintSatisfiedBy(e.Definition));
}
}
public class SessionModelExport : Export
{
private readonly SessionModel sessionModel;
private readonly PropertyInfo propertyInfo;
private readonly ExportDefinition definition;
public SessionModelExport(SessionModel sessionModel, PropertyInfo propertyInfo, ExportDefinition definition)
{
this.sessionModel = sessionModel;
this.propertyInfo = propertyInfo;
this.definition = definition;
}
public override ExportDefinition Definition
{
get { return definition; }
}
protected override object GetExportedValueCore()
{
var value = propertyInfo.GetValue(sessionModel, null);
return value;
}
}
答案 1 :(得分：0)
问题是SomeService是一个实例属性。您的系统中可能有多个SessionModel对象，并且MEF无法知道哪个SessionModel正在返回应该与导入匹配的ISomeService实例。
相反，只需将SessionModel设为静态类，将SomeService设为静态属性即可。或者，使SessionModel成为单例。 SomeService属性仍然是静态的，但会从SessionModel的唯一实例中导出服务。
答案 2 :(得分：0)
using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.ComponentModel.Composition.ReflectionModel;
using System.Reflection;
using System.Linq;
namespace ConsoleApplication1
{
internal class Program
{
private static void Main(string[] args)
{
var catalogs = new AggregateCatalog();
var catalog = new System.ComponentModel.Composition.Hosting.AssemblyCatalog(Assembly.GetExecutingAssembly());
catalogs.Catalogs.Add(catalog);
var sessionModel = new SessionModel(3);
var container = new CompositionContainer(catalog);
ISomeService someService = container.GetExportedValueOrDefault<ISomeService>(sessionModel.cname);
if (someService != null)
{
someService.DoSomething();
}
}
}
public class SessionModel
{
private int AValue { get; set; }
//[Import("One",typeof(ISomeService))]
//public ISomeService SomeService { get; private set; }
public SessionModel(int aValue)
{
AValue = aValue;
// of course, there is much more to do here in reality:
}
public string cname { get { return "One"; } }
}
public class SessionModel1
{
private int AValue { get; set; }
//[Import("Two",typeof(ISomeService))]
//public ISomeService SomeService { get; private set; }
public SessionModel1(int aValue)
{
AValue = aValue;
}
public string cname { get { return "Two"; } }
}
public interface ISomeService
{
void DoSomething();
}
[Export("One",typeof(ISomeService))]
public class SomeService : ISomeService
{
public SomeService()
{
Console.WriteLine("Some Service Called");
}
public void DoSomething()
{
Console.WriteLine("DoSomething called");
Console.ReadKey();
}
}
[Export("Two",typeof(ISomeService))]
public class SomeService1 : ISomeService
{
public SomeService1()
{
Console.WriteLine("Some Service1 Called");
}
public void DoSomething()
{
Console.WriteLine("DoSomething called 1");
Console.ReadKey();
}
}
}
答案 3 :(得分：0)
第一种情况：通过将sessionModel传递给ComposeExportedValue，您可以添加SessionModel类型的部分，而不是ISomeService。要使此案例有效，您需要将服务传递给ComposeExportedValue。
compositionContainer.ComposeExportedValue(sessionModel.SomeService);
第二种情况：在这种情况下，您将零件的创建留给容器。如果存在无参数构造函数或带有ImportingConstructorAttribute参数的构造函数，则容器可以创建新零件。这很可能意味着您需要稍微改变一下设计。
就个人而言，我会选择第一种情况，但要尽量减少这种情况。在MEF的所有正常（和建议）使用之后，让容器创建和处理零件。