MEF编程-【1.为导出附加元数据】

通用示例接口(比如计算器功能接口)

public interface ICalculator
{
    int GetNumber(int num1, int num2);
}

1.为导出附加元数据

使用 ExportMetadataAttribute 特性，应该这样（这里是加法运算）

[Export(typeof(ICalculator))]
[ExportMetadata("CalciMetaData", "Add")]
public class Add:ICalculator
{
    #region Interface members
    public int GetNumber(int num1, int num2)
    {
        return num1 + num2;
    }
    #endregion
}

我们再添加一个减法运算

[Export(typeof(ICalculator))]
[ExportMetadata("CalciMetaData", "Subtract")]
public class Subtract : ICalculator
{
    #region Interface members
    public int GetNumber(int num1, int num2)
    {
        return num1 - num2;
    }
    #endregion
}

=== [下面我们再看看如何导入使用] ===

public class CalciCompositionHelper
{
        [ImportMany]
        public System.Lazy<ICalculator, IDictionary<string, object>>[] CalciPlugins { get; set; }

        /// <summary>
        /// Assembles the calculator components
        /// </summary>
        public void AssembleCalculatorComponents()
        {
            try
            {
                //Step 1: Initializes a new instance of the 
                //        System.ComponentModel.Composition.Hosting.AssemblyCatalog class with the 
                //        current executing assembly.
                var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());

                //Step 2: The assemblies obtained in step 1 are added to the CompositionContainer
                var container = new CompositionContainer(catalog);

                //Step 3: Composable parts are created here i.e. the Import and Export components 
                //        assembles here
                container.ComposeParts(this);
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        /// <summary>
        /// Sends the result back to the client
        /// </summary>
        /// <param name="num1"></param>
        /// <param name="num2"></param>
        /// <returns></returns>
        public int GetResult(int num1, int num2, string operationType)
        {
            int result = 0;
            foreach (var CalciPlugin in CalciPlugins)
            {
                if ((string)CalciPlugin.Metadata["CalciMetaData"] == operationType)
                {
                    result = CalciPlugin.Value.GetNumber(num1, num2);
                    break;
                }                               
            }
            return result;           
        }
}