Flurl

var url = "http://www.some-api.com"
	.AppendPathSegment("endpoint")
	.SetQueryParams(new {
		api_key = ConfigurationManager.AppSettings["SomeApiKey"],
		max_results = 20,
		q = "Don't worry, I'll get encoded!"
	});

At its core is the Url class, which is designed to work seamlessly with strings, as demonstrated with the extension method above. Create a Url via a string extension is purly optional though; you can create one explicitly if you prefer:

var url = new Url("http://www.some-api.com").AppendPathSegment(...

A Url also converts back to a string implicitly, so you can use it directly in any method that takes a string:

var result = await new HttpClient.GetAsync(url);

Flurl also contains the handy Url.Combine method, which is basically a Path.Combine for URLs, ensuring one and only one separator character between segments:

var url = Url.Combine("http://www.foo.com/", "/too/", "/many/", "/slashes/", "too", "few");
// result: "http://www.foo.com/too/many/slashes/too/few"

###Encoding

Flurl takes care of encoding characters in URLs but takes a different approach with path segments than it does with query string values. The assumption is that query string values are highly variable (such as from user input), whereas path segments tend to be more "fixed" and may already be encoded, in which case you don't want to double-encode. Here are the rule Flurl follows:

Query string values are fully URL-encoded.
For path segments, reserved characters such as /, %, and + are not encoded.
For path segments, illegal characters such as spaces are encoded.
For path segments, the ? character is encoded, since query strings get special treatment.

###Url API

The Url API is small, discoverable, and fairly self-explanatory. For completeness, here are all public methods and properties:

Static method:

Combine(string url, params string[] segments) : string (static)

Instance methods (each with equivalent string extension):

AppendPathSegment(string segment) : Url
AppendPathSegments(params string[] segments) : Url
AppendPathSegments(IEnumerable<string> segments) : Url
SetQueryParam(string name, object value) : Url
SetQueryParams(object values) : Url
SetQueryParams(IDictionary values) : Url
RemoveQueryParam(string name) : Url
RemoveQueryParams(params string[] names) : Url
RemoveQueryParams(IEnumerable<string> names) : Url

Properties:

string Path { get; }
NameValueCollection QueryParams { get; }

###Get it on NuGet

PM> Install-Package Flurl

###Roadmap

The next version of Flurl will add a set of asynchronous HTTP methods to the Url API. Things like GetJsonAsync<T>, DownloadFileAsync, and PostFormDataAsync are in the works. These will be thin wrappers around HttpClient that encapsulate best practices, and they will be fully testable. (The core URL builder will always be available as a stand-alone package.) Please follow this project to stay current with the progress. If you have a feature request or suggestion for improvement, please create an issue.