Get And Set Property

Definition:
         public class MyClass

{
    private int _age;

    public int Age
    {
          get{ return _age;  }
          set{ _age = value; }
    }
}

Uses:

You need to more than just a simple get/set - in this case, you should just avoid using automatic properties for this member.
You want to avoid the performace hit of going through the get or set and just use the member directly - in this case, I'd be suprised if there really was a performace hit. The simple get/set members are very very easy to inline, and in my (admitly limited) testing I haven't found a difference to using the automatic properties vs accessing the member directly.

You only want to have public read access (i.e. just a 'get') and the class write to the member diectly - in this case, you can use a private set in your automatic property. i.e.
public class MyClass
{
    public int Age {get; private set;} }



This usually covers most the reasons for wanting to directly get to the backing field used by the automatic properties.
Example Program : 

using System;

class Example
{
    int number;
    public int Number
    {
 get
 {
     return number;
 }
 set
 {
     number = value;
 }
    }
}

class Program
{
    static void Main()
    {
 Example example = new Example();
 example.Number = 5; // set { }
 Console.WriteLine(example.Number); // get { }
    }
}
OutPut:
       5