When to use get; set; in c#

Basically, in that case, there is no difference, of the (many) advantages of using a property is the ability to add Events to your property, like so:

  public delegate void ChangedEventHandler(object sender, EventArgs e);

  int m_i = 0;
  public int i 
  {
      get { return m_i; }
      set { m_i = value; iChanged(self, null); }
  }

  public ChangedEventHandler iChanged;

This allows for code to know when I has been changed (there might be some syntax errors, I haven't focused on C# in a while, but the idea is similar). This is extremely important in winforms, as this is the major way of knowing when a button (or similar) has been clicked.

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Also, this allows for additional functionality in the setter of a property, e.g. checking if it is in a certain range, like this:

  int m_i = 0;
  public int i {

  get { return m_i; }
  set { if (value > 10) throw new Exception("I cannot be greater than 10!"); m_i = value; }
  }

If you think you might need a more complicated getter or setter later, the automatic property syntax lets you upgrade without recompiling all callers. Moving from a field (member variable) to a property is a breaking change, however.

Eric Lippert addresses the topic. Twice.

Lets forget properties for a second... The real question you're asking (and you might not even know it) is why do you need properties (or getters and setters in some other languages) in the first place?

It's to promote encapsulation.

Properties just offer a nicer syntax for getters and setters, aka accessors (and indeed, a property just wraps set() and get() methods under the hood).

In c# 3 the c# team came up with auto properties, because the vast number of properties don't do anything with the variables (no additional logic), so auto properties are short hand for that scenario.