Basic implementation of Stack.

Stack.cs

using System;
namespace DataStructures
{
	public class Stack<T>
	{
		#region Fields

		private int _count;
		private T[] _values;

		#endregion

		public Stack()
		{
			_values = new T[4];
		}

		public Stack(int capacity)
		{
			if (capacity < 0)
				throw new ArgumentOutOfRangeException(nameof(capacity));

			_values = new T[capacity];
		}

		#region Properties

		public int Count { get { return _count; } }

		#endregion

		public void Push(T data)
		{
			if (_values.Length <= _count)
				Grow();

			_values[_count] = data;
			_count++;
		}

		public T Pop()
		{
			if (_count <= 0)
				throw new ArgumentOutOfRangeException();

			--_count;

			if ((_count * 4) <= _values.Length)
				Shrink();

			T retValue = _values[_count];
			_values[_count] = default(T);

			return retValue;
		}

		private void Grow()
		{
			T[] temp = new T[_values.Length * 2];

			for (int i = 0; i < _values.Length; i++)
			{
				temp[i] = _values[i];
			}

			_values = temp;
		}

		private void Shrink()
		{
			T[] temp = new T[_values.Length / 4];

			for (int i = 0; i < temp.Length; i++)
			{
				temp[i] = _values[i];
			}

			_values = temp;
		}
	}
}