ArgumentOutOfRangeException on initialized List

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You cannot index into a list if that offset doesn’t exist. So, for example, indexing an empty list will always throw an exception. Use a method like Add to append the item to the end of the list, or Insert to place the item in the middle of the list somewhere, etc.

For example:

var list = new List<string>();
list[0] = "foo"; // Runtime error -- the index 0 doesn't exist.

On the other hand:

var list = new List<string>();
list.Add("foo");       // Ok.  The list is now { "foo" }.
list.Insert(0, "bar"); // Ok.  The list is now { "bar", "foo" }.
list[1] = "baz";       // Ok.  The list is now { "bar", "baz" }.
list[2] = "hello";     // Runtime error -- the index 2 doesn't exist.

Note that in your code, this is happening when you write to the Courses list, and not when you read from the Course_ID list.

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