MEF : Discover plugin using ExportMetaData attribute - mef

Question

In the below code I discover all the plugins using the contract ICalculator and then pick the plugin depending on the metat data attribute.
I need to discover the plugin depending on the ExportMetadata attribute directly and not on the Interface contract type "ICalculator".
Is there any way I can do that?
Thanks!
public abstract class MathOperation : ICalculator
{
public abstract int GetNumber(int num1, int num2);
}
[Export(typeof(ICalculator))]
[ExportMetadata("CalciMetaData", "Add")]
public class Add : MathOperation
{
#region Interface members
public override int GetNumber(int num1, int num2)
{
return num1 + num2;
}
#endregion
}
[Export(typeof(ICalculator))]
[ExportMetadata("CalciMetaData", "Divide")]
public class Divide : MathOperation
{
#region Interface members
public override int GetNumber(int num1, int num2)
{
return num1 / num2;
}
#endregion
}
// Composition.
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.LoadFrom(#"D:\Study\MEF_Program_Files\Files\SimpleCalculator-Part2\CalculatorUI\CalculatorFactory\bin\Debug\CalculatorFactory.dll"));
//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;
}
}

Answer 1

You should be able to use CompositionContainer.GetExports(ImportDefinition definition). In the ImportDefinition, you can set the Constraint property to match the particular metadata you are looking for. Typically you will need to know the interface type, otherwise how will you interact with the value you get from the CompositionContainer?