Q:

How to append unique numbers to a list of strings

I have this function, which works and gives the correct result:
  <System.Runtime.CompilerServices.Extension()>
  Public Function Unique(List As List(Of String)) As List(Of String)
    Return List.Select(Function(x, index) x & System.Text.RegularExpressions.Regex.Replace((List.GetRange(0, index).Where(Function(y) y = x).Count + 1).ToString, "\b1\b", String.Empty)).ToList
  End Function

This function appends a "2", "3", etc as needed, to those items that are not unique, to make them unique. 
How can I remove the regex while a) staying in the same linq statement (the same line of code), b) without introducing a loop or c) evaluating the expression twice, as would be needed in an IIF statement?
This is not a duplicate of Getting index of duplicate items in a list in c#, because a) my list does not change during the function and b) that question was not answered with a ready to apply code example, and here I'm looking for a specific fix to a specific line of code. Those answers will not fix my issue; they do not apply here.

A:

You could do it using GroupBy and if you want to preserve the original order you can create an anonymous type to include it, then group, then re-sort by the original order.        
    string[] input = new[]{ "Apple", "Orange", "Apple", "Pear", "Banana", 
                            "Apple", "Apple", "Orange" };

    var result = input
        // Remember the initial order
        .Select((name, i) => new {name, i})
        .GroupBy(x => x.name)
        // Now label the entries in each group
        .SelectMany(g => g.Select((item, j) => 
            new {i = item.i, name = (j == 0 ? item.name : item.name + (j+1))}))
        // Now reorder them by their original order
        .OrderBy(x => x.i)
        // Remove the order value to get back to just the name
        .Select(x => x.name)
        .ToList();

    foreach (var r in result)
        Console.WriteLine(r);

Result
Apple
Orange
Apple2
Pear
Banana
Apple3
Apple4
Orange2

