why I couldn't reflect over private assemblies?

I made a private assembly callled CarLibrary (CarLibrary.dll). Then I created a client console application and added the reference of CarLibrary , below is the main method of console application

using CarLibrary; 
... 
static void Main(string[] args)
{
   SportsCar s = new SportsCar();  //CarLibrary contains a type-SportsCar class
   Type t = s.GetType();
   MethodInfo[] mi = t.GetMethods();
   foreach (MethodInfo m in mi)
   {
      ...
   }
}

it worked, but if I change the main method to :

static void Main(string[] args)
{
   string typeName = "CarLibrary.SportsCar";
   Type t = Type.GetType(typeName);
   // Reflection doesn't work
}

why it doesn't work in this way, I did add reference of CarLibrary.dll, and the manifest of client console application does have

.assembly extern CarLibrary
{ ... }

so in runtime, the CLR should find CarLibrary.dll and load it into memory, Refelction should work!

and if I change it as:

string typeName = "System.Int32";
Type t = Type.GetType(typeName);

it does work. I'm really confused.