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当我更改部件时,如何让MEF重新编译?

mef .net c#

Matt · 技术社区 · 14 年前

我试图让MEF重新编译它在更新导出的实例时知道的所有部分。本质上,我希望MEF在更改连接字符串配置值时更新导入该值的所有部分。在我想更改实例的地方,一切都看起来很好。如果我尝试用更新的值组合部件,它似乎会向容器中添加第二个部件实例,然后我的导入将被更新,但为空。

有人能指出我哪里出错了吗?或者我应该尝试用这种方式来使用MEF吗?

我正在使用MEF Preview 9和Targetting.NET Framework 3.5以及WPF。

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Linq;
using System.Text;
using Shouldly;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            MainClass main = new MainClass();
            main.RunTest();
        }
    }

    public class MainClass
    {
        [ImportMany]
        public IEnumerable<Settings> Settings { get; set; }

        public void RunTest()
        {
            AggregateCatalog catalog = new AggregateCatalog();
            catalog.Catalogs.Add(new AssemblyCatalog(typeof(Settings).Assembly));

            CompositionContainer container = new CompositionContainer(catalog);

            container.SatisfyImportsOnce(this);

            Config cfg = new Config
            {
                Settings = new Settings { ConnectionString = "Value1" },
            };

            // result is returned with a null settings value
            UsesSettings result = container.GetExportedValue<UsesSettings>();

            // this recomposes everything with the new value, result changes to have settings of Value1
            container.ComposeParts(cfg);

            // this line results in my import many enumerable returning 2 parts the Value1 setting and null
            container.SatisfyImportsOnce(this);

            result.TheSettings.ConnectionString.ShouldBe("Value1");

            cfg.Settings = new Settings { ConnectionString = "Value2" };

            // how do I tell the container to recompose now I have changed the config object,
            // or how do I replace the part value with the new value?

            // this line causes the result.Settings to return null
            container.ComposeParts(cfg);

            // this updates the ImportMany to 3 values, Value1, Value2 and null
            container.SatisfyImportsOnce(this);
        }
    }

    public class Settings
    {
        public string ConnectionString = "default value";
    }

    public class Config
    {
        [Export(typeof(Settings))]
        public Settings Settings { get; set; }
    }

    [Export(typeof(UsesSettings))]
    public class UsesSettings
    {
        [Import(typeof(Settings), AllowRecomposition = true)]
        public Settings TheSettings { get; set; }
    }
}

1 回复 | 直到 14 年前

Wes Haggard 14 年前

对于您要完成的场景,您有一些事情要做。

首先:如果要添加/删除/更改特定的导出,则该导出不能位于目录本身,因此应删除具有导出设置属性的配置类。这是由CatalogExportProvider创建的,也是您在集合中看到空值的原因。

请尝试以下操作:

public class Program
{
    [ImportMany(AllowRecomposition=true)]
    public IEnumerable<Settings> Settings { get; set; }

    public void RunTest()
    {
        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(Settings).Assembly));

        CompositionContainer container = new CompositionContainer(catalog);

        // Settings should have 0 values.
        container.ComposeParts(this);
        Contract.Assert(this.Settings.Count() == 0);

        CompositionBatch batch = new CompositionBatch();

        // Store the settingsPart for later removal...
        ComposablePart settingsPart = 
            batch.AddExportedValue(new Settings { ConnectionString = "Value1" });

        container.Compose(batch);

        // Settings should have "Value1"
        UsesSettings result = container.GetExportedValue<UsesSettings>();
        Contract.Assert(result.TheSettings.ConnectionString == "Value1");

        // Settings should have 1 value which is "Value1";
        Contract.Assert(this.Settings.Count() == 1);
        Contract.Assert(this.Settings.First().ConnectionString == "Value1");

        // Remove the old settings and replace it with a new one.
        batch = new CompositionBatch();
        batch.RemovePart(settingsPart);
        batch.AddExportedValue(new Settings { ConnectionString = "Value2" });
        container.Compose(batch);

        // result.Settings should have "Value2" now.
        Contract.Assert(result.TheSettings.ConnectionString == "Value2");

        // Settings should have 1 value which is "Value2"
        Contract.Assert(this.Settings.Count() == 1);
        Contract.Assert(this.Settings.First().ConnectionString == "Value2");
    }
}
public class Settings
{
    public string ConnectionString = "default value";
}

[Export(typeof(UsesSettings))]
public class UsesSettings
{
    [Import(typeof(Settings), AllowRecomposition = true)]
    public Settings TheSettings { get; set; }
}