C# MEF：导出一种类型的多个对象，并导入特定对象

Question

我有一个应用程序可以导出同一个类的多个对象，以及只导入特定对象的插件。例如

public class Part
{
  string name;
  public Part(string nm)
  {
    name = nm;
  }
}

public class Car //Exports ALL Parts of the Car
{
  [Export(typeof(Part))]
  public Part steeringWheel = new Part("SteeringWheel");

  [Export(typeof(Part))]
  public Part engine = new Part("Engine");

  [Export(typeof(Part))]
  public Part brakes = new Part("Brakes");
}

public class SystemMonitorPlugin //Imports only SOME Parts from the Car
{
  [Import(typeof(Part))]
 public Part engine;

  [Import(typeof(Part))]
  public Part brakes;
}

有人可以解释我如何实现这种行为吗？

Answer 1

你需要一个合约（接口）和一个元数据合约（接口）：

public interface ICarPart{
    int SomeMethodFromInterface();
}

public interface ICarPartMetadata{
    string /*attention to this!!!*/ NameCarPart { get; } /* Only for read. */
}

然后你导出你的零件：

[Export(typeof (ICarPart))]
[ExportMetadata("NameCarPart","SteeringWheel")] /* is string!! */

public class SteeringWheel : ICarPart {

    public int SomeMethodFromInterface(){
        ... //your stuff
    }
}
[Export(typeof (ICarPart))]
[ExportMetadata("NameCarPart","Engine")] /* is string!! */

public class Engine : ICarPart {

    public int SomeMethodFromInterface(){
        //Each method have diferent behavior in each part.
        ... //your stuff
    }
}
[Export(typeof (ICarPart))]
[ExportMetadata("NameCarPart","Brakes")] /* is string!! */

public class Brakes : ICarPart {

    public int SomeMethodFromInterface(){
        //Each method have diferent behavior in each part.
        ... //your stuff
    }
}

然后你可以使用 ImportMany 和 Lazy 导入：

    [ImportMany()]
    IEnumerable<Lazy<ICarPart, ICarPartMetadata>> carParts = null;
    public void Importing(){
    ...
    ...

    foreach (Lazy<ICarPart,ICarPartMetadata> item in carParts){
        switch (item.Metadata.ICarPartMetadata.ToString()){
            case "SteeringWheel":
                item.Value.SomeMethodFromInterface();
            break;
            case "Engine":
                item.Value.SomeMethodFromInterface();
            break;
            case "Brakes":
                item.Value.SomeMethodFromInterface();
            break;
            default:
            ...
            break;
        }
    }

Answer 2

您可以命名导出：

[Export("SteeringWheel", typeof(Part))]

当你想要一个特定的，

[Import("Engine", typeof(Part))]

如果不指定名称，仍然可以导入许多类型的 Part。