[c#] Merge two (or more) lists into one, in C# .NET

Is it possible to convert two or more lists into one single list, in .NET using C#?

For example,

public static List<Product> GetAllProducts(int categoryId){ .... }
.
.
.
var productCollection1 = GetAllProducts(CategoryId1);
var productCollection2 = GetAllProducts(CategoryId2);
var productCollection3 = GetAllProducts(CategoryId3);

You could use the Concat extension method:

var result = productCollection1
    .Concat(productCollection2)
    .Concat(productCollection3)
    .ToList();

I know this is an old question I thought I might just add my 2 cents.

If you have a List<Something>[] you can join them using Aggregate

public List<TType> Concat<TType>(params List<TType>[] lists)
{
    var result = lists.Aggregate(new List<TType>(), (x, y) => x.Concat(y).ToList());

    return result;
}

Hope this helps.

// I would make it a little bit more simple

 var products = new List<List<product>> {item1, item2, item3 }.SelectMany(id => id).ToList();

This way it is a multi dimensional List and the .SelectMany() will flatten it into a IEnumerable of product then I use the .ToList() method after.

When you got few list but you don't know how many exactly, use this:

listsOfProducts contains few lists filled with objects.

List<Product> productListMerged = new List<Product>();

listsOfProducts.ForEach(q => q.ForEach(e => productListMerged.Add(e)));

Assuming you want a list containing all of the products for the specified category-Ids, you can treat your query as a projection followed by a flattening operation. There's a LINQ operator that does that: SelectMany.

// implicitly List<Product>
var products = new[] { CategoryId1, CategoryId2, CategoryId3 }
                     .SelectMany(id => GetAllProducts(id))
                     .ToList();

In C# 4, you can shorten the SelectMany to: .SelectMany(GetAllProducts)

If you already have lists representing the products for each Id, then what you need is a concatenation, as others point out.

To merge or Combine to Lists into a One list.

• There is one thing that must be true: the type of both list will be equal.

• For Example: if we have list of string so we can add add another list to the existing list which have list of type string otherwise we can't.

Example:

class Program
{
   static void Main(string[] args)
   {
      List<string> CustomerList_One = new List<string> 
      {
         "James",
         "Scott",
         "Mark",
         "John",
         "Sara",
         "Mary",
         "William",
         "Broad",
         "Ben",
         "Rich",
         "Hack",
         "Bob"
      };

      List<string> CustomerList_Two = new List<string> 
      {
         "Perter",
         "Parker",
         "Bond",
         "been",
         "Bilbo",
         "Cooper"
      };

      // Adding all contents of CustomerList_Two to CustomerList_One.
      CustomerList_One.AddRange(CustomerList_Two);

      // Creating another Listlist and assigning all Contents of CustomerList_One.
      List<string> AllCustomers = new List<string>();

      foreach (var item in CustomerList_One)
      {
         AllCustomers.Add(item);
      }

      // Removing CustomerList_One & CustomerList_Two.
      CustomerList_One = null;

      CustomerList_Two = null;
      // CustomerList_One & CustomerList_Two -- (Garbage Collected)
      GC.Collect();

      Console.WriteLine("Total No. of Customers : " +  AllCustomers.Count());
      Console.WriteLine("-------------------------------------------------");
      foreach (var customer in AllCustomers)
      {
         Console.WriteLine("Customer : " + customer);
      }
      Console.WriteLine("-------------------------------------------------");

   }
}

Have a look at List.AddRange to merge Lists

list4 = list1.Concat(list2).Concat(list3).ToList();

In the special case: "All elements of List1 goes to a new List2": (e.g. a string list)

List<string> list2 = new List<string>(list1);

In this case, list2 is generated with all elements from list1.

You need to use Concat operation

you can combine them using LINQ:

  list = list1.Concat(list2).Concat(list3).ToList();

the more traditional approach of using List.AddRange() might be more efficient though.

I've already commented it but I still think is a valid option, just test if in your environment is better one solution or the other. In my particular case, using source.ForEach(p => dest.Add(p)) performs better than the classic AddRange but I've not investigated why at the low level.

You can see an example code here: https://gist.github.com/mcliment/4690433

So the option would be:

var allProducts = new List<Product>(productCollection1.Count +
                                    productCollection2.Count +
                                    productCollection3.Count);

productCollection1.ForEach(p => allProducts.Add(p));
productCollection2.ForEach(p => allProducts.Add(p));
productCollection3.ForEach(p => allProducts.Add(p));

Test it to see if it works for you.

Disclaimer: I'm not advocating for this solution, I find Concat the most clear one. I just stated -in my discussion with Jon- that in my machine this case performs better than AddRange, but he says, with far more knowledge than I, that this does not make sense. There's the gist if you want to compare.