How to Stop getting System.TypeInitializationException everytime I use My stack class

Question

I got a coding exercise in an online C# course where I have to Design my own stack class. The problem I keep encountering with this code is that it keeps giving me a System.TypeInitializationException for some reason. I have tried changing modifiers but nothing has worked. Any solutions? Here is the following code:

public class Stack
{
    public static List<object> StackStorage = new List<object>();
    public static object LastElement = StackStorage[StackStorage.Count - 1];
    public static object FirstElement = StackStorage[0];


    public void Push(object obj)
    {
        if (StackStorage.Count - 1 >= 8)
        {
            StackStorage.Remove(LastElement);
            StackStorage.Insert(0, obj);
        }
        else
        {
            StackStorage.Add(obj);

        }

           //Add Logic in here to check whether or not the List count
           //Is Longer than 8. Remember that .Count gives you a value NOT
           //Zero based. If you want the index of a value you would have
           //To do ExampleList.Count - 1.


    }
    public object Pop()
    {
        StackStorage.Remove(LastElement);
        return LastElement;

    }

    public void Clear()
    {
        StackStorage.Clear();


    }

    public int PrintCount()
    {

        return StackStorage.Count();


    }




}
   And the code I have in the main method:
Stack MyStack = new Stack();
        MyStack.Push(1);
        MyStack.Push(2);
        MyStack.Push(3);
        Console.WriteLine(MyStack.PrintCount());
        Console.ReadKey();

Answer 1

public static List<object> StackStorage = new List<object>();
public static object LastElement = StackStorage[StackStorage.Count - 1];
public static object FirstElement = StackStorage[0];

are computed once on application start and since StackStorage.Count - 1 is -1 on application start it gives you that exception.

What you really need is to calculate them every time you need access them. For example:

public object GetLastElement(){
  if (StackStorage.Count > 0)
      return StackStorage[StackStorage.Count - 1];
  throw new Exception("Collection is empty");
}
//and so on

But I would not recommend to go this road, better approach would be:

• get rid of static (as you want stack underlying collection to belong to instance of class not to instace of running process)
• use indexes
• use generics

In that case your class would look like:

    public class Stack<T>
    {
        private const int MAX_SIZE = 8;

        private List<T> StackStorage = new List<T>();

        private void RemoveLast()
        {
            StackStorage.RemoveAt(StackStorage.Count - 1);
        }

        public void Push(T obj)
        {
            StackStorage.Insert(0, obj);

            if (StackStorage.Count > MAX_SIZE)
                RemoveLast();
        }

        public T Pop()
        {
            if (StackStorage.Count == 0)
                throw new Exception("Stack is empty");

            T result = StackStorage[StackStorage.Count - 1];
            RemoveLast();

            return result;
        }

        public void Clear()
        {
            StackStorage.Clear();
        }

        public int Count => StackStorage.Count;
    }

And you can use it like:

    Stack<int> MyStack = new Stack<int>();
    MyStack.Push(1);
    MyStack.Push(2);
    MyStack.Push(3);
    Console.WriteLine(MyStack.Count);
    Console.ReadKey();

Answer 2

When trying to initialize LastElement, you're trying to set it to the final element of StackStorage, which, at the time, has zero elements, thus no elements, and no last element.

 StackStorage.Count - 1 = 0 - 1 = -1 // invalid array index

Instead of using FirstElement and LastElement, just use the index you want when pushing and popping.

FirstElement and LastElement won't be the actual first and last elements anyway, since you're trying to initialize them at the start, instead of keeping them up to date as the StackStorage changes.