无法在运行时强制转换对象

Question

I am learning ASP.NET MVC 5 (vNext). In an effort to do this, I'm migrating an existing app. In that app, I get a list of classes that implement a specific interface. In an attempt to do this, I am using the following code:

// Find all classes that implement IMyInterface
var type = typeof(IMyInterface);
var classes = AppDomain.CurrentDomain.GetAssemblies()
                .SelectMany(x => x.GetTypes())
                .Where(y => type.IsAssignableFrom(y) && y.GetInterfaces().Contains(type))
                .ToList();

if (classes == null)
  Console.WriteLine("None found");
else            
  Console.WriteLine(classes.Count + " found."); 

try {
  foreach (var c in classes)
  {
    Console.WriteLine(c.GetType().FullName);
    var converted = (IMyInterface)(c);
    // Never gets here. Exception gets thrown.  
  }
}
catch (Exception ex)
{
  Console.WriteLine(ex.Message);
  // Prints: Cannot cast from source type to destination type.
}       

Unfortunately, an exception is thrown that says: "Cannot cast from source type to destination type.". When I print out the full name of the type, it is System.MonoType . What am I doing wrong?

Answer 1

In your code, classes is a List<Type> , that is to say these are not instances of your class, they are instances of the class Type which describes the class implementing your interface.

Hence this line

var converted = (IMyInterface)(c);

Will always throw an exception, as Type does not implement IMyInterface . I suspect what you actually wanted to do is instantiate an instance of your class using a Type which can be achieved with the static methods on Activator such as

var converted = (IMyInterface)Activator.CreateInstance(c);

---

Expanding on a comment I made on your question

if (classes == null)

The above line will never evaluate true, you probably wanted to check whether the list is empty

if (classes.Count == 0)

or, in actual fact you do not need a list here at all, remove the .ToList() and consider using

// classes now an IEnumerable<Type> - no need for a list here.
var classes = AppDomain.CurrentDomain.GetAssemblies()
            .SelectMany(x => x.GetTypes())
            .Where(y => type.IsAssignableFrom(y) && y.GetInterfaces().Contains(type));
if (!classes.Any())