Returning two lists in C#

There are many ways.

1. Return a collection of the lists. This isn't a nice way of doing it unless you don't know the amount of lists or if it is more than 2-3 lists.

   public static IEnumerable<List<int>> Method2(int[] array, int number)
   {
       return new List<List<int>> { list1, list2 };
   }
   

2. Create an object with properties for the list and return it:

   public class YourType
   {
       public List<int> Prop1 { get; set; }
       public List<int> Prop2 { get; set; }
   }
   
   public static YourType Method2(int[] array, int number)
   {
       return new YourType { Prop1 = list1, Prop2 = list2 };
   }
   

3. Return a tuple of two lists - Especially convenient if working with C# 7.0 tuples

   public static (List<int>list1, List<int> list2) Method2(int[] array, int number) 
   {
       return (new List<int>(), new List<int>());
   }
   
   var (l1, l2) = Method2(arr,num);
   

   Tuples prior to C# 7.0:

   public static Tuple<List<int>, List<int>> Method2(int[] array, int number)
   {
       return Tuple.Create(list1, list2); 
   }
   //usage
   var tuple = Method2(arr,num);
   var firstList = tuple.Item1;
   var secondList = tuple.Item2;
   

I'd go for options 2 or 3 depending on the coding style and where this code fits in the bigger scope. Before C# 7.0 I'd probably recommend on option 2.

If you are using later version of .NET and C# then simply use tuples (you may need to Install-Package "System.ValueTuple")

public static void Method1()
{
    int[] array1 = { };
    int number1 = 1;
    (List<int> listA, List<int> listB) = Method2(array1, number1);
}

public static (List<int>, List<int>) Method2(int[] array, int number)
{
    List<int> list1 = new List<int>();
    List<int> list2 = new List<int>();

    return (list1, list2); //<--This is where i need to return the second list
}

Method 1

public static void Method2(int[] array, out List<int> list1, out List<int> list2, int number)
{
    list1= new List<int>();
    list2= new List<int>();
    ...
}

Method 2

public static Tuple<List<int>, List<int>> Method2(int[] array, int number)
{
    list1= new List<int>();
    list2= new List<int>();
    ...

    return Tuple.Create(list1, list2)
}

Method 3

Create a class that have 2 props list1, list 2, return that class, or just return array of lists

and finally on C# 7 you can just do

public static (List<int> list1, List<int> list2) Method2(int[] array, int number)
{
    ...
    return (list1, list2)
}