Question

I got all the classes that implement my interface. I want to call a method when the instance of the object is created

var types =AppDomain.CurrentDomain.GetAssemblies().ToList()
            .SelectMany(s => s.GetTypes())
            .Where(t => typeof(IManagerReport).IsAssignableFrom(t));

Console.WriteLine("Processing manager reports..");
foreach(var TheType in types)
{
              //error here
    var temptype = Activator.CreateInstance(TheType) as IManagerReport;

    temptype.Load();
    temptype.Save();
    Console.WriteLine("Saved to: " + temptype.SavePath);
}

The error that is produced is here:

Cannot create an instance of an interface

Answer 1

Make sure you exclude IManagerReport from the selection of types you’re fetching.

var types =AppDomain.CurrentDomain.GetAssemblies().ToList()
            .SelectMany(s => s.GetTypes())
            .Where(t => typeof(IManagerReport).IsAssignableFrom(t)
                        && typeof(IManagerReport) != t
                        && !t.IsInterface
                        && !t.IsAbstract
                        && !t.IsGenericTypeDefinition);

The problem is, in your types enumerable, you’ve got not only the derived types of IManagerReport, but also the IManagerReport itself. You can’t create an instance of an interface, only a class. Use the code I posted to exclude most trouble items, but I would still add a try/catch on the Activator.CreateInstance. You may have derived types that have no parameterless public constructor. These would also fail.

Wrap the call in a try/catch and continue. I would say you should account for all possibilities here, but there’s just simply too many of them. Account for a few, then also account for the possibility that the creation simply won’t work for another reason.

try {
    var temptype = Activator.CreateInstance(TheType) as IManagerReport;
} catch {
    continue;
}