Стек
Stack<string> numbers = new Stack<string>();
numbers.Push("one");
numbers.Push("two");
numbers.Push("three");
// Peek()
// Pop()
using System.Collections;
Stack myStack = new Stack();
myStack.Push(textBox1.Text);
if (myStack.Count > 0)
{
richTextBox1.Text = myStack.Pop().ToString();
}