Question

There is no AddRange() method for IList<T>.

How can I add a list of items to an IList<T> without iterating through the items and using the Add() method?

Answer 1

AddRange is defined on List<T>, not the interface.

You can declare the variable as List<T> instead of IList<T> or cast it to List<T> in order to gain access to AddRange.

((List<myType>)myIList).AddRange(anotherList);

This is not good practice (see comments below), as an IList<T> might not be a List<T>, but some other type that implemented the interface and may very well not have an AddRange method – in such a case, you will only find out when your code throws an exception at runtime.

So, unless you know for certain that the type is indeed a List<T>, you shouldn’t try to use AddRange.

One way to do so is by testing the type with the is or as operators (since C# 7).

if(myIList is List<T>)
{
   // can cast and AddRange
}
else
{
   // iterate with Add
}