Using MEF with asp.net web form application (SharePoint)

Question

I'm trying to use MEF in my ASP.NET web form (SharePoint) application to load some controls from a directory at runtime. I'm not getting any error but the controls are not getting loaded.

Here is my code -

aspx.cs

    public partial class SampleMEF : System.Web.UI.Page
    {
        [ImportMany(typeof(IControlLoader))]
        IEnumerable<Lazy<IControlLoader, IControlLoaderMetaData>> controls;
        private CompositionContainer _partsContainer;
        private AggregateCatalog _catalog;
        private string _partsPath;

        /// <summary>
        /// default constructor
        /// </summary>
        public SampleMEF()
        {
        }

        /// <summary>
        /// Initialize the page
        /// </summary>
        /// <param name="e"></param>
        protected override void OnInit(EventArgs e)
        {
            _partsPath = SPUtility.GetGenericSetupPath(@"TEMPLATE\LAYOUTS\MEFProtoType\Parts");
            _catalog = new AggregateCatalog();
            DirectoryCatalog c = new DirectoryCatalog(_partsPath, "*.dll");
            _partsContainer = new CompositionContainer(c);
            _partsContainer.ComposeParts(this);
        }

        protected void Page_Load(object sender, EventArgs e)
        {
            foreach(Lazy<IControlLoader, IControlLoaderMetaData> i in controls)
            {
                SPPControl ctrl = i.Value.LoadControl();
                lbxControls.Items.Add(new ListItem(ctrl.Name, ctrl.ControlID.ToString()));
            }
        }
    }

Contracts

    /// <summary>
    /// Contract for Imports and Exports
    /// </summary>
    public interface IControlLoader
    {
        SPPControl LoadControl();
    }

    /// <summary>
    /// Exports metadata
    /// </summary>
    public interface IControlLoaderMetaData
    {
        string ControlID { get; }
    }

Sample Export

    [Export(typeof(IControlLoader))]
    [ExportMetadata("ControlID", "7a6c6288-ab52-4010-8c56-79959843ec7c")]
    public class ctrlAccordion : IControlLoader
    {
        #region IControlLoader Members

        public SPPControl LoadControl()
        {
            SPPControl ctrl = new SPPControl("Accordion", new Guid("7a6c6288-ab52-4010-8c56-79959843ec7c"), 5);
            return ctrl;
        }

        #endregion
    }

I'm able to see the DLLs copied in the parts directory. But I'm not able to load the parts. The import is not filled and it is empty.

I'm using.Net Framework 3.5 and downloaded the MEF dll from MEF Codeplex and signed the assembly myself.

Any ideas?

Answer 1

Okay, I eventually found the issue. I'm able to load the part dlls only if I sign them and add the dlls to global assembly. But I don't understand what DirectoryCatalog does loading the folder.

Bottom line: If the dlls are just in the parts folder and not in the GAC they are not getting loaded.

Answer 2

There can be problems when trying to load assemblies from arbitrary locations. The "Assembly Load Issues" section of this blog post has a bit of information on this and links to more information.

You can also inspect the catalog in the debugger to see if it loaded any parts at all.