MEF

Question

System.ComponentModel.Composition.ImportCardinalityMismatchException‘没有发现与约束相匹配的出口: ContractName IntroToMEF.DigitalMonitor RequiredTypeIdentity IntroToMEF.IMonitor’

我的类型是对的。如果只有一个从IMonitor继承的实例，我就能够得到导出的值。每当我需要使用合同名称时，我都会得到一个错误。我没有主意了。

清除Visual缓存不起作用.

[Export(typeof(IMonitor))]
public class DigitalMonitor : IMonitor
{
    public void start()
    {
        Console.WriteLine("Digital Monitor: Started monitoring");
    }
}

internal class HostMefInApp
{
    [Import]
    public ILogger logger { get; set; }

    [Import]
    public IMonitor monitor { get; set; }

    private void ComposeMultiple()
    {
        Console.WriteLine("DEBUG:" + GetType().Assembly);
        var assemblyCatalog = new AssemblyCatalog(GetType().Assembly);
        var container = new CompositionContainer(assemblyCatalog);

        //  var contractName = AttributedModelServices.GetContractName(typeof(FileLogger));

        try
        {
            var instancesOfFileLogger = container.GetExports<ILogger>(); //"IntroToMEF.FileLogger"
        }
        catch (ImportCardinalityMismatchException ex)
        {
            Console.WriteLine(ex);
        }

        CompositionBatch batch = new CompositionBatch();
        batch.AddPart(AttributedModelServices.CreatePart(new FileLogger()));
        batch.AddPart(AttributedModelServices.CreatePart(new DigitalMonitor()));
        batch.AddPart(AttributedModelServices.CreatePart(new AnalogMonitor()));


        container.Compose(batch);

        var digitalMonitorContractName = AttributedModelServices.GetContractName(typeof(DigitalMonitor));
        var currentLogger = container.GetExportedValue<ILogger>();
        var digitalMonitor = container.GetExportedValue<IMonitor>(digitalMonitorContractName);


        logger = currentLogger;
        monitor = digitalMonitor;         
    }

Answer 1

我认为你应该：

[Export(typeof(IMonitor))]
[Export(typeof(DigitalMonitor))]
public class DigitalMonitor : IMonitor
{
    public void start()
    {
        Console.WriteLine("Digital Monitor: Started monitoring");
    }
}

意思是将DigitalMonitor导出为IMonitor和DigitalMonitor。这将为您提供使用两种方式导入它的可能性。当然，您的代码将使用：

var digitalMonitor = container.GetExportedValue<DigitalMonitor>();