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.
URIs identify and URLs locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.
Examples
This is my name, which is an identifier.
It is like a URI, but cannot be a URL, as it tells you nothing about my location or how to contact me.
In this case it also happens to identify at least 5 other people in the USA alone.
- 4914 West Bay Street, Nassau, Bahamas
This is a locator, which is an identifier for that physical location.
It is like both a URL and URI (since all URLs are URIs), and also identifies me indirectly as "resident of..".
In this case it uniquely identifies me, but that would change if I get a roommate.
I say "like" because these examples do not follow the required syntax.
Popular confusion
From Wikipedia:
In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI, ... [emphasis mine]
Because of this common confusion, many products and documentation incorrectly use one term instead of the other, assign their own distinction, or use them synonymously.
URNs
My name, Roger Pate, could be like a URN (Uniform Resource Name), except those are much more regulated and intended to be unique across both space and time.
Because I currently share this name with other people, it's not globally unique and would not be appropriate as a URN. However, even if no other family used this name, I'm named after my paternal grandfather, so it still wouldn't be unique across time. And even if that wasn't the case, the possibility of naming my descendants after me make this unsuitable as a URN.
URNs are different from URLs in this rigid uniqueness constraint, even though they both share the syntax of URIs.
Best Answer
You can just make a System.Uri object, and use IsFile to verify it's a file, then Uri.LocalPath to extract the filename.
This is much safer, as it provides you a means to check the validity of the URI as well.
Edit in response to comment:
To get just the full filename, I'd use:
This does all of the error checking for you, and is platform-neutral. All of the special cases get handled for you quickly and easily.