Url.Combine is basically a Path.Combine for URLs, ensuring one and only one separator character between parts:
var url = Url.Combine( "http://foo.com/", "/too/", "/many/", "/slashes/", "too", "few?", "x=1", "y=2" // result: "http://www.foo.com/too/many/slashes/too/few?x=1&y=2"
Get Flurl.Http on NuGet:
PM> Install-Package Flurl.Http
Or get the stand-alone URL builder without the HTTP features:
PM> Install-Package Flurl
Uri has a constructor that should do this for you:
new Uri(Uri baseUri, string relativeUri)
Here's an example:
Uri baseUri = new Uri("http://www.contoso.com"); Uri myUri = new Uri(baseUri, "catalog/shownew.htm");
Note from editor: Beware, this method does not work as expected. It can cut part of baseUri in some cases. See comments and other answers.