C# combine strings in x lists in any possible way

Hello Trucha ji
You can use LINQ CONCAT & TOLIST

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.)