using System;
 
public class Test
{
	public static void Main()
	{
		Console.WriteLine("www.example.com > " + Parse("www.example.com"));
		Console.WriteLine("://www.example.com > " + Parse("://www.example.com"));
		Console.WriteLine("http://w...content-available-to-author-only...e.com > " + Parse("http://w...content-available-to-author-only...e.com"));
		Console.WriteLine("https://w...content-available-to-author-only...e.com > " + Parse("https://w...content-available-to-author-only...e.com"));
		Console.WriteLine("ftp://w...content-available-to-author-only...e.com > " + Parse("ftp://w...content-available-to-author-only...e.com"));
		Console.WriteLine("rubbish://www.example.com > " + Parse("rubbish://www.example.com"));
		Console.WriteLine("https://rubbish://www.example.com > " + Parse("https://rubbish://www.example.com"));
		Console.WriteLine("rubbish://morerubbish://www.example.com > " + Parse("rubbish://morerubbish://www.example.com"));
	}
 
	static string Parse(string downloadURL)
	{
		string[] approvedSchemes = new string[] { "http", "https", "ftp" };
	    string userScheme = "";
 
	    if(downloadURL.Contains("://"))
	    {
	        // Get the first scheme defined, we will use this if it is in the approved list.
	        userScheme = downloadURL.Substring(0, downloadURL.IndexOf("://"));
	        // To cater for multiple :// remove all of them
	        downloadURL = downloadURL.Substring(downloadURL.LastIndexOf("://") + 3);
	    }
 
	    // Check if the user defined scheme is in the approved list, if not then set to http.
	    if(Array.IndexOf(approvedSchemes, userScheme.ToLowerInvariant()) > -1)
	        downloadURL = userScheme + "://" + downloadURL;
	    else
	        downloadURL = "http://" + downloadURL;
 
	    return downloadURL;
	}
}

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