UrlString provides a simple wrapper around URLs, both relative and absolute. Many Sitecore UI components receive data from querystring parameters, and UrlString provides convenient  method for constructing URLs with querystring parameters.

Assembling an URL:

UrlString url = new UrlString("/path/to/mypage.aspx");

url["id"] = "{some-id}";
url["db"] = "master";

string result = url.ToString(); // "/path/to/mypage.aspx?id=some-id&db=master"

Tweaking existing url:

UrlString url = new UrlString("/path/to/mypage.aspx?id=some-id&db=master");

url["db"] = "web";
url["mode"] = "new";

string result = url.ToString(); // "/path/to/mypage.aspx?id=some-id&db=web&mode=new"

Some methods in Sitecore API accept UrlString to add more data. ItemUri class can embed itself in a UrlString, so that you can pass information uniquely identifying a Sitecore item in a URL:

UrlString url = new UrlString("page.aspx");

Sitecore.Context.Item.Uri.AddToUrlString(url);

string result = url.ToString(); // "page.aspx?id={id}&la=en&v=1&db=master"

Compared to System.Uri, Sitecore's UrlString is very lax in accepting any kinds of input and giving it back.

Most of the data in Sitecore ends up stored and passed around as a string, hence we have quite a few helper classes to deal with string values. ListString class helps maintaining a list of strings that is serialized as a string itself. Like this:

“Fred|John|Derek”

What you see above is three strings, “Fred”, “John” and “Derek” serialized to a single string, each string being separated by the pipe ‘|’ symbol. Here’s how to use the ListString to create similar list:

ListString list = new ListString();

list.Add("Fred");
list.Add("John");
list.Add("Derek");

string result = list.ToString(); // -> "Fred|John|Derek"

Now you need to parse it somewhere else:

string rawValue = “Fred|John|Derek”;

ListString list = new ListString(rawValue);

int count = list.Count; // -> 3
string first = list[0]; // -> "Fred"

foreach(string s in list)
{
  // supports enumeration too
}

list.Add("Mary");
string result = list.ToString(); // -> "Fred|John|Derek|Mary"

Note that the class doesn’t check and escape incoming strings, so it’s your responsibility to make sure the values you pass do not contain a separator symbol.

Sitecore uses ListString alot internally, but it’s always better to use higher level API designed for each specific case, if such exists. A good example of that is MultilistField. So when else ListString can be handy? Whenever you need to pass multiple values as one, such as passing parameters, implementing a new field type that stores multiple values, etc.