Importing other attributes to metadata using C# MEF

Question

Using some tutorials on the web, I created a metadata class for my needs using the MEF in C#

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class)]
public class ActionMetadataAttribute : Attribute
{
    public bool HasSettingsDialog { get; set; }
    public bool UseThreadedProcessing { get; set; }
}

public interface IActionMetadata
{
    [DefaultValue(false)]
    bool HasSettingsDialog { get; }

    [DefaultValue(false)]
    bool UseThreadedProcessing { get; }
}

I've got different kind of plugin types, so there is e. g. IHandlerMetadata and HandlerMetadataAttribute. Now I load it via the Lazy helper to allow strictly typed metadata.

[ImportMany(typeof(IActionPlugin))]
private IEnumerable<Lazy<IActionPlugin, IActionMetadata>> _plugins = null;

public ActionManager()
{
    var catalog = new DirectoryCatalog(".");
    var container = new CompositionContainer(catalog);
    container.ComposeParts(this);

    foreach (var contract in _plugins)
    {
        Debug.WriteLine(contract.Metadata.HasSettingsDialog);
    }
}

Works perfectly. Now, I also like to have some information about the plugins. So I've created an PluginInformationAttribute.

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class)]
public class PluginInformationAttribute : Attribute
{
    public string Name { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public string Url { get; set; }
    public string Contact { get; set; }
    public string Description { get; set; }
    public Version Version { get; set; }
}

The question is now: how can I access this attribute for example in the for loop in the above code snippet? Is there any way or is my design wrong? I don't want to include the PluginInformation stuff to IActionMetadata because I'd like to use it on different types of plugins, e. g. on the IHandlerMetadata.

Answer 1

If you specify the type of your exporting class as a property of ActionMetadataAttribute, then you can access every attribute of the class by reflection with GetCustomAttributes method

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class)]
public class ActionMetadataAttribute : Attribute
{
    public bool HasSettingsDialog { get; set; }
    public bool UseThreadedProcessing { get; set; }
    public Type ClassType { get; set; }
}

var attributes = contract.Metadata.ClassType.GetCustomAttributes(typeof(PluginInformationAttribute), true)