string name = "Franco".ToUpper();
string surname = "Di Sorte".ToUpper().Replace(" ","");

List<string> rawAlias = new List<string>();

var range = string.Empty;

if (name.Length > surname.Length) { range = surname; } else { range = name; }

for (int i = 1;
     i < 3;
     i++)
{
    if (i != 1)
    {
        rawAlias.Add($"{name[0..i]}{surname[0..i]}");
        rawAlias.Add($"{name[0..i]}{surname[0]}");
        rawAlias.Add($"{name[0]}{surname[0..i]}");

        continue;
    }

    rawAlias.Add($"{name[0..i]}{surname[0..i]}");
}

rawAlias.Sort();
var alias = rawAlias.OrderBy(x => x.Length);

var existAlias = new List<string>()
{
    {""}
};

foreach (var a in alias)
{
    if(!existAlias.Contains(a))
        Console.WriteLine(a);
}