Dynamically load dlls

Question

I have following Main method. LocalCabelTV and LocalDishTV classes are in the main application program. The program works fine.

I want to keep LocalCabelTV and LocalDishTV in separate dll files. I wonder then how will I load those classes at runtime? I understand then we'll not use switch but for loop to look for all dll files that implements IVideoSource interface in a particular directory and load those...

Need to know how to dynamically load dlls and create objects and use their methods ?

foreach (string dll in Directory.GetFiles("C:\DLLs\*.dll"))
{
    Assembly assemb = Assembly.LoadFrom(dll);
    ??
}

Following works fine:

static void Main(string[] args)
{
   SmartTv myTv = new SmartTv();

   Console.WriteLine("Select A source to get TV Guide and Play");
   Console.WriteLine("1. Local Cable TV\n2. Local Dish TV");

   ConsoleKeyInfo input = Console.ReadKey();

   switch (input.KeyChar)
   {
      case '1':
          myTv.VideoSource = new LocalCabelTv();
          break;

      case '2':
          myTv.VideoSource = new LocalDishTv();
          break;
   }

   Console.WriteLine(); 

   myTv.ShowTvGuide();

   myTv.PlayTV();

   Console.ReadKey();
}

class SmartTv
    {
        IVideoSource currentVideoSource = null;

        public IVideoSource VideoSource
        {
            get
            {
                return currentVideoSource;
            }
            set
            {
                currentVideoSource = value;
            }
        }

        public void ShowTvGuide()
        {
            if (currentVideoSource != null)
            {
                Console.WriteLine(currentVideoSource.GetTvGuide());
            }
            else
            {
                Console.WriteLine("Please select a Video Source to get TV guide from");
            }
        }

        public void PlayTV()
        {
            if (currentVideoSource != null)
            {
                Console.WriteLine(currentVideoSource.PlayVideo());
            }
            else
            {
                Console.WriteLine("Please select a Video Source to play");
            }
        }

class LocalCabelTv : IVideoSource
    {
        const string SOURCE_NAME = "Local Cabel TV";

        string IVideoSource.GetTvGuide()
        {
            return string.Format("Getting TV guide from - {0}", SOURCE_NAME);
        }

        string IVideoSource.PlayVideo()
        {
            return string.Format("Playing - {0}", SOURCE_NAME);
        }
    }

class LocalDishTv : IVideoSource
    {
        const string SOURCE_NAME = "Local DISH TV";

        string IVideoSource.GetTvGuide()
        {
            return string.Format("Getting TV guide from - {0}", SOURCE_NAME);
        }

        string IVideoSource.PlayVideo()
        {
            return string.Format("Playing - {0}", SOURCE_NAME);
        }
    }

Does the same user always have the choice of Dish or Cable? If so, why don't you just include both as references to the project? No need for dynamic detection. — Darren Young, Sep 23 '15 at 08:33
You probably want to learn about MEF (https://msdn.microsoft.com/en-us/library/dd460648%28v=vs.110%29.aspx) — jeroenh, Sep 23 '15 at 08:39

score 1 · Accepted Answer · edited May 23 '17 at 12:08

To load this assembly at runtime and create an object:

Assembly MyDALL = Assembly.Load("DALL"); // DALL is name of my dll
Type MyLoadClass = MyDALL.GetType("DALL.LoadClass"); // LoadClass is my class
object obj = Activator.CreateInstance(Type.GetType("DALL.LoadClass, DALL", true));

For your dynamic Method you can also use Dynamic Method . Its faster than reflection (This method takes only 1/10th time needed by Activator.)

Here is a sample code for creating Object using Dynamic Method.

void CreateMethod(ConstructorInfo target)
{
    DynamicMethod dynamic = new DynamicMethod(string.Empty,
                typeof(object),
                new Type[0],
                target.DeclaringType);

    methodHandler = (MethodInvoker)dynamic.CreateDelegate(typeof(MethodInvoker));
}

Check out these link for more info: Load Assembly at runtime and create class instance

EDIT: As user @taffer mentioned the DynamicMethod.CreateDelegate is much more slower than reflection. So I would use this only if the created delegate is invoked hundreds or thousands of times. Using Activator with a cache is faster. Secondly, Activator is really fast for parameterless constructors, unless you instantiate so many types, which renders the inner small cache useless.

You DO NOT want to generate code with `Reflection.Emit` unless you really have to... think about the poor programmer that will replace you and have to learn this code... — Tamir Vered, Sep 23 '15 at 08:17

Tamir Vered · Answer 2 · 2015-09-25T10:20:40.900

You need to load the DLLs with the desire classes and iterate over their types, than look for those that implement IVideoSource and activate them:

public static IEnumerable<IVideoSource> GetVideoSources(List<string> assemblyPathes)
{
    IEnumerable<Assembly> yourAssembliesWithClasses = assemblyPathes.Select(x => Assembly.Load(x));

    IEnumerable<Type> implementingTypes = yourAssembliesWithClasses
        .GetTypes()
        .Where(x => x.IsAssignableFrom(IVideoSource));

    return implementingTypes.Select(x => Activator.CreateInstance(x));
}

Note that Activator.CreateInstance() requires the types to have an empty constructor, if they don't have one you can use, Type.GetUniGetUninitializedObject(Type type) from FormatterServices to initialize them.

Dynamically load dlls

2 Answers2