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var list = new HashSet<object>() { };
var result = new StringBuilder();

foreach (var item in list)
{
    if (result.Length > 0)
        result.Remove(result.Length - 3, 3);
    result.Append(item);
    result.Append(",or ");
}

return result.ToString();

Quote:

C#

else if(0==c) {
    result.Append(list[0]);
}

public static string JoinList<T>(this IEnumerable<T> list, string delimiter = ", ", string finalDelimiter = "or ")
{
    int count = list.Count();
    switch (count)
    {
        case 0:
        {
            return string.Empty;
        }
        case 1:
        {
            return Convert.ToString(list.First());
        }
        default:
        {
            int index = count;
            var result = new StringBuilder();
            foreach (var item in list)
            {
                if (result.Length != 0) result.Append(delimiter);
                if (index == 1) result.Append(finalDelimiter);
                result.Append(item);
                --index;
            }
            
            return result.ToString();
        }
    }
}

var result = new StringBuilder();
list.ForEach(item => result.Append(item).Append(item == list.Last() ? "" : item == list.Take(list.Count - 1).Last() ? ", or " : ", "));

var result = new StringBuilder();
list.ForEach(item => result.Append(item).Append(item == list.Last() ? "" : item == list.Take(list.Count - 1).Last() ? " or " : ", "));