C#

public abstract class Document
   {
   public Document Next = null;
   }
public class PdfDocument : Document
   {
   }
public class WordDocument : Document
   {
   }

Document d = new Document();

Document d = new PdfDocument();
d.Next = new WordDocument();

class PrintQueue
{
    private Document head;

    public void Push(Object data)
    {
        Document toAdd = new Document();
        toAdd.data = data;

        Document current = head;
        while (current.next != null)
        {
            current = current.next;
        }
        current.next = toAdd;
    }
}


public abstract class Document
{
    public Document next;
    public Object data;

    public string Subject { get; set; }

    public abstract string type();
}

public class WordDocument : Document
{
    public override string type()
    {
        return "docx";
    }
}

public class PdfDocument : Document
{
    public override string type()
    {
        return "pdf";
    }
}

Document toAdd = new Document();

class PrintQueue
    {
        private Document head;

        private int size;
        public int Count
        {
            get
            {
                return size;
            }
        }

        public void Push(Object data)
        {
            Document toAdd = new Document();
            toAdd.data = data;

            Document current = head;
            while (current.Next != null)
            {
                current = current.Next;
            }
            current.Next = toAdd;
            size++;
        }

        public bool Pop()
        {
            Document tempNode = head;

            Document lastNode = null;
            int count = 0;

            if (size > 0)
            {
                while (tempNode != null)
                {
                    if (count == size - 1)
                    {
                        lastNode.Next = tempNode.Next;
                        return true;
                    }
                    count++;

                    lastNode = tempNode;
                    tempNode = tempNode.Next;
                }
            }
            return false;
        }
    }


    public abstract class Document
    {
        public Document Next;
        public Object data;

        public string Subject { get; set; }

        public abstract string type();
    }

    public class WordDocument : Document
    {
        public override string type()
        {
            return "docx";
        }
    }

    public class PdfDocument : Document
    {
        public override string type()
        {
            return "pdf";
        }
    }

Document toAdd = new Document();

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PrintQueueDemo
{
    class PrintQueue
    {
        Node head;
        Node tail;

        /// <summary>
        /// add a document to the print queue
        /// </summary>
        /// <param name="strType">document type</param>
        /// <param name="strName">docunent name</param>
        public void Push(string strType, string strName)
        {
            if (head == null)
            {
                head = new Node(strType, strName);
                tail = head;
            }
            else
            {
                Node node = new Node(strType, strName);
                tail.setNext(node);
                tail = node;
            }
        }

        /// <summary>
        /// pop a document from the queue
        /// </summary>
        /// <returns>null if printqueue is empty, else document</returns>
        public Node.Document Pop()
        {
            if (head == null)
                return null;

            Node.Document doc = new Node.Document(head.document.Type, head.document.Name);
            head = head.nextnode;
            if (head == null)
                tail = null;
            return doc;
        }

        /// <summary>
        /// get the current content of the queue
        /// </summary>
        /// <returns>string with current content (line per entry)</returns>
        public string DisplayContents()
        {
            string content = "";
            Node node = head;
            if (node == null)
                return "PrintQueue is empty";

            content = node.document.Name + ": " + node.document.Type;
            while (node.nextnode != null)
            {
                node = node.nextnode;
                content += "\r\n" + node.document.Name + ": " + node.document.Type;
            }
            return content;
        }

        public class Node
        {
            public Document document { get; private set; }
            public Node nextnode { get; private set; }

            public Node(string strType, string strName)
            {
                document = new Document(strType, strName);
            }

            public void setNext(Node node)
            {
                nextnode = node;
            }
            
            public class Document
            {
                public string Type { get; private set; }
                public string Name { get; private set; }

                public Document(string strType, string strName)
                {
                    Name = strName;
                    Type = strType;
                }
            }
        }
    }
}

PrintQueue pq = new PrintQueue();
pq.Push("cpp", "main.cpp");
pq.Push("c#", "main.cs");
pq.Push("c", "main.c");
pq.Push("h", "myinclude.h");

Console.WriteLine(pq.DisplayContents());
Console.WriteLine("===");

PrintQueue.Node.Document doc;
doc = pq.Pop();
Console.WriteLine("{0}: {1}", doc.Name, doc.Type);
doc = pq.Pop();
Console.WriteLine("{0}: {1}", doc.Name, doc.Type);
doc = pq.Pop();
Console.WriteLine("{0}: {1}", doc.Name, doc.Type);
doc = pq.Pop();
Console.WriteLine("{0}: {1}", doc.Name, doc.Type);
Console.WriteLine("===");

Console.WriteLine(pq.DisplayContents());
Console.WriteLine("===");

pq.Push("xls", "workbook.xls");
Console.WriteLine(pq.DisplayContents());

content = node.document.Name + ": " + node.document.Type;
while (node.nextnode != null)
{
    node = node.nextnode;
    content += "\r\n" + node.document.Name + ": " + node.document.Type;
}

while (node.nextnode != null)
{
    content += node.document.Name + ": " + node.document.Type + "\r\n";
    node = node.nextnode;
}

public string DisplayContents()
{
    StringBuilder content = new StringBuilder();
    Node node = head;
    while (node != null)
    {
        content.AppendFormat("{0} : {1}\n", node.document.Name, node.document.Type);
        node = node.nextnode;
    }
    return content.ToString();
}

var forma = new FormA();
txtQtyB.Text = forma.panelA.txtQtaA.Text;

var forma = new FormA();
txtQtyB.Text = forma.txtQtaA.Text;