Encapsulate some code module in the code behind file of c#

 Add a new project to you solution as class library. You can find class library under C# windows section. Name it as per your convenience. after adding, you will get a class file, with the name of you supplied. make this class public. Now you can write you logic in this class file, build it. it will create a dll file. add reference by right clicking on you main application project file select "Add reference", path of dll file. in your application, add the namespace of your class file and then call class and its methods.  
 below is a sample class file  
 using System;  
 using System.Data;  
 namespace myEncapsulatedNameSpace  
 {  
 public class myEncapsulatedClass  
 {  
 //property declaration  
 //properties are used for passing values between application layers  
 private string _myName = string.empty;  
 public string myName  
 {  
 get  
 {  
 return _myName;  
 }  
 set  
 {  
 _myName = value;  
 }  
 public string myMethod()  
 {  
 return myName = "sk";  
 }  
 }  
 }  
 }  
 add namespace like this  
 using System;  
 :using myEncapsulatedNameSpace;  
 public partial class yourAppClass  
 {  
 myEncapsulatedClass objMEClass = new myEncapsulatedClass();  
 //here must be some methods or events  
 //call you class method  
 private void dislayName()  
 {  
 MessageBox.Show(objMEClass .myMethod())  
 }  
 }  
 //also you can add value to that property:  
 pucblic void SetValue()  
 {  
 objMEClass.myName ="sk";  
 displayName();  
 }