You can! If you mean Enumerable.Cast
when you say cast then it is available in .NET 3.5 and Unity supports that. With the example code from MS doc, I was able to compile it.
You just need to include using System.Linq
in order to use it. When in doubt, ,you can search for other Unity Mono Compatibly functions here.
using System.Linq
...
void Start()
{
System.Collections.ArrayList fruits = new System.Collections.ArrayList();
fruits.Add("apple");
fruits.Add("mango");
IEnumerable<string> query =
fruits.Cast<string>().Select(fruit => fruit);
foreach (string fruit in query)
{
Debug.Log(fruit);
}
}