Add scheme to URL if needed

To create a Uri from a string you can do this:

Uri u = new Uri("example.com");

But the problem is if the string (like the one above) doesn't contain the protocol you will get an exception: "Invalid URI: The format of the URI could not be determined."

To avoid the exception you should secure the string includes a protocol, like below:

Uri u = new Uri("http://example.com");

But if you take the url as input, how can you add the protocol if it's missing?
I mean apart from some IndexOf/Substring manipulation?

Something elegant and fast?

You could also use UriBuilder:

public static Uri GetUri(this string s)
{
    return new UriBuilder(s).Uri;
}

Remarks from MSDN:

This constructor initializes a new instance of the UriBuilder class with the Fragment, Host, Path, Port, Query, Scheme, and Uri properties set as specified in uri.

If uri does not specify a scheme, the scheme defaults to "http:".

If you just want to add the scheme, without validating the URL, the fastest/easiest way is to use string lookups, eg:

string url = "mydomain.com";
if (!url.StartsWith("http://", StringComparison.OrdinalIgnoreCase)) url = "http://" + url;

A better approach would be to use Uri to also validate the URL using the TryCreate method:

string url = "mydomain.com";
Uri uri;
if ((Uri.TryCreate(url, UriKind.Absolute, out uri) || Uri.TryCreate("http://" + url, UriKind.Absolute, out uri)) &&
    (uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps))
{
    // Use validated URI here
}

As @JanDavidNarkiewicz pointed out in the comments, validating the Scheme is necessary to guard against invalid schemes when a port is specified without scheme, e.g. mydomain.com:80.

My solution was for protocall-less urls to make sure they have protocal was regex :

Regex.Replace(s, @"^\/\/", "http://");