If your class has a different namespace, then this would not work. What you need to do is to reference the namespace of your class on your codebehind. Consider the following example.
using MyApp.Objects1;
namespace MyApp.Objects2
{
public class Class2
{
private Class1 _myClass1;
public Class2()
{
_myClass1 = new Class1();
}
}
}
namespace MyApp.Objects1
{
public class Class1
{
public Class1(){ }
}
}