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

You can use the LINQ Concat and ToList methods:

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

Note that there are more efficient ways to do this - the above will basically loop through all the entries, creating a dynamically sized buffer. As you can predict the size to start with, you don't need this dynamic sizing... so you could use:

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

(AddRange is special-cased for ICollection<T> for efficiency.)

I wouldn't take this approach unless you really have to though.

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.

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.