Load a .DLL file and access methods from class within?

Use Assembly.GetTypes() to get a collection of all the types, or Assembly.GetType(name) to get a particular type.

You can then create an instance of the type with a parameterless constructor using Activator.CreateInstance(type) or get the constructors using Type.GetConstructors and invoke them to create instances.

Likewise you can get methods with Type.GetMethods() etc.

Basically, once you've got a type there are loads of things you can do - look at the member list for more information. If you get stuck trying to perform a particular task (generics can be tricky) just ask a specific question an I'm sure we'll be able to help.

This is how you can get the classes if you know the type.

Assembly assembly = Assembly.LoadFrom("C:\\dll\\test.dll");

// Load the object
string fullTypeName = "MyNamespace.YourType";

YourType myType = assembly.CreateInstance(fullTypeName);

The full type name is important. Since you aren't adding the .dll you can't do a Using because it is not in your project.

If you want all I would just Jon Skeet answer.

If you want to dynamically load an assembly, and then invoke methods from classes therein, you need to perform some form of dynamic invoke.

Check here for basic advice on that.

The only bit missing is how to get the type itself, which can easily be retrieved wth code like this:

foreach (Type t in assemblyToScan.GetTypes())
        {
            if(condition)
                //do stuff
        }

And if you simply want to use the assembly statically (by having the assembly available at compile time), then the answer fom Launcy here on this page is the way to go.