CollectionPrettyString

public static class CollectionPrettyString
{
    public static string ToPrettyString(this IEnumerable collection)
    {
        if (collection == null) return "null";

        var builder = new StringBuilder();

        var index = 0;
        builder.Append("[ ");
        foreach (var item in collection)
        {
            if (index > 0) builder.Append(", ");

            if (IsValidCollection(item))
                builder.Append(ToPrettyString(item as IEnumerable));
            else
                builder.Append(item.ToString());

            index++;
        }
        builder.Append(" ]");

        return builder.ToString();
    }

    public static bool IsValidCollection(object instance)
    {
        if (instance.GetType() == typeof(string)) return false;

        return instance is IEnumerable;
    }
}