int[] arr = new int[5] {1,2,3,4,5};

string => "1,2,3,4,5"

var result = string.Join(",", arr);

public static string Join<T>(string separator, IEnumerable<T> values);

string.Join(",", arr)

string.Join(",", Array.ConvertAll(arr, x => x.ToString()))

public static string ToDelimitedString<T>(this IEnumerable<T> lst, string separator = ", ")
{
    return lst.ToDelimitedString(p => p, separator);
}

public static string ToDelimitedString<S, T>(this IEnumerable<S> lst, Func<S, T> selector,
                                             string separator = ", ")
{
    return string.Join(separator, lst.Select(selector));
}

new int[] { 1, 2, 3, 4, 5 }.ToDelimitedString();

int[] arr = new int[5] {1,2,3,4,5};

String arrTostr = arr.Select(a => a.ToString()).Aggregate((i, j) => i + "," + j);