Using reflection to get all classes of certain base type in dll

I have a dll that contains a number of classes that all inherit from a CommandBase class. I'm trying to get instances of all of these classes (CommandA, CommandB, CommandC, etc...) using reflection in C# so that I can call a specific method on each one. Here is what I have so far:

//get assemblies in directory.
string folder = Path.Combine(HttpContext.Current.Server.MapPath("~/"), "bin");
var files = Directory.GetFiles(folder, "*.dll");
//load each assembly.
foreach (string file in files)
{
  var assembly = Assembly.LoadFile(file);
  if (assembly.FullName == "MyCommandProject")
  {
    foreach (var type in assembly.GetTypes())
    {
      if (!type.IsClass || type.IsNotPublic) continue;
      if(type is CommandBase)
      {
        var command = Activator.CreateInstance(type) as CommandBase;
      }
    }
  }
}

I'm having 2 issues. The 1st issue is that the line "if(type is CommandBase") gives the following warning:

The given expression is never of the provided type CommandBase.

The 2nd issue is that I can't figure out how to get an instance of the actual object (CommandA, CommandB, etc...), just converting it to CommandBase isn't enough.