R – NHibernate and a REST Service Layer

string is an alias in C# for System.String.
So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, it's generally recommended to use string any time you're referring to an object.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

string greet = String.Format("Hello {0}!", place);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:

public void CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

and explicitly as:

void ICollection.CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.

MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.

I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.

I am sure there are more reasons to use/not use explicit that others will post.

See the next post in this thread for excellent reasoning behind each.