Question

我正在尝试测试MEF的单实例合成功能。导出使用[PartCreationPolicy(CreationPolicy.Shared)]，而导入使用[ImportMany(typeof(IPartOfThePuzzle), RequiredCreationPolicy = CreationPolicy.Shared)]。

我正在运行测试，并从控制台应用程序中的两个不同类中导入相同的零件。但是，IPartOfThePuzzle实现的两个实例似乎并不相同。看来MEF正在第二次调用中重新实例化该对象。

下面是我所拥有的。

Part1.DLL：

[PartCreationPolicy(CreationPolicy.Shared)]
[Export(typeof(IPartOfThePuzzle))]
class Piece1 : IPartOfThePuzzle
{
    string _prop1;
    public string SomeProp { get => _prop1; set => _prop1 = value; }
    public Piece1()
    {
        Console.WriteLine("Piece1 instantiated, setting Prop value to 'First Part " +
            "Original Value'");
        _prop1 = "First Part Original Value";
    }
}

Part2.DLL：

[PartCreationPolicy(CreationPolicy.Shared)]
[Export(typeof(IPartOfThePuzzle))]
class Piece2 : IPartOfThePuzzle
{
    string _prop1;
    public string SomeProp { get => _prop1; set => _prop1 = value; }
    public Piece2()
    {
        Console.WriteLine("Piece2 instantiated, setting Prop value to 'Second Part " +
            "Original Value'");
        _prop1 = "Second Part Original Value";
    }
}

Console.exe> GetParts.cs

public class GetParts
{
    [ImportMany(typeof(IPartOfThePuzzle))]
    public IPartOfThePuzzle[] Pieces { get; set; }

    public GetParts()
    {
        var catalog = new AggregateCatalog();
        catalog.Catalogs.Add(
            new DirectoryCatalog(
                @"C:\PuzzlePieces"
            )
        );
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
    }
}

Console.exe> GetPartsAgain.cs

public class GetPartsAgain
{
    [ImportMany(typeof(IPartOfThePuzzle))]
    public IPartOfThePuzzle[] Pieces { get; set; }
    public GetPartsAgain()
    {
        var catalog = new AggregateCatalog();
        catalog.Catalogs.Add(
            new DirectoryCatalog(
                @"C:\PuzzlePieces"
            )
        );
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
    }
}

最后是Console.exe> Program.cs

static void Main(string[] args)
{
    GetParts firstImport = new GetParts();
    Console.WriteLine($"Initial value from First Import: {firstImport.Pieces[0].SomeProp}");
    firstImport.Pieces[0].SomeProp = "First Import Overwritten Value";
    Console.WriteLine($"Overwritten from First Import: {firstImport.Pieces[0].SomeProp}");
    Console.WriteLine("-  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -");
    GetPartsAgain secondImport = new GetPartsAgain();
    Console.WriteLine($"Value from Second Import: {secondImport.Pieces[0].SomeProp}");

    Console.ReadLine();
}

控制台输出：

输出显示碎片被实例化了两次（对于GetParts一次，对于GetPartsAgain一次）。文档，博客和在线论坛表示，这是防止多个实例的方法，但我认为我的理解不足。你能帮我理解那是什么吗？

MEF未导入同一实例

0 个答案: