Container boundary check on C#?

Unity Engine
1

Hello. what is a common way to check the boundary of C# array or list<> officially?
It seems to be not supported official MS documents. Thanks.
I expected like that
int [ ] arr = new int[105];
bool iterate(int idx)
{
if (false == arr.RangedIn(idx))
return false;
…
}

2

myArr.Length will tell you the length of an array.
myList.Count will tell you the number of elements in a list.

int myIndex;
bool indexIsInList = myIndex > 0 && myIndex < myList.Count;
bool indexIsInArray = myIndex > 0 && myIndex < myArray.Length;