Returns a new unique key from an existing IEnumerable<string>. The key is based on a prefix followed by a number. For example, in "Customer_0", "Customer_1", "Customer_4"..., GetUniqueKey will return "Customer_2".

GetUniqueKey.cs

public static class Extensions
{
  public static string GetUniqueKey(this IEnumerable<string> keys, string prefix, StringComparison comparisonType = StringComparison.InvariantCultureIgnoreCase)
  {
      int count = 0;
      while (keys.FirstOrDefault(k => k.StartsWith(prefix + count, comparisonType)) != null)
          count++;
  
      return prefix + count;
  }
}