Private and Public variables in C#

Declaring class level variables (or 'fields') as public is generally frowned upon by experienced developers.

It is considered better to declare the field to be private and then controlling access to it using a public property. That way you can make sure that the user doesn't assign an invalid value to it. 

For example:

class Person
{
    int _age;

   public int Age
   {
       get { return _age; }

       set
       {
           if (value < 0 || value > 125)
               throw new ArgumentException ("Age must be be between 0 and 125");
           else
              _age = value;
       } 
   }
}

Even when the argument is not checked for correctness, it's still considered better to use properties in case checking is introduced at a later stage. If you have to change from a field to a property, this changes the public interface of your class.

At one time, this used to be quite tedious and lead to code bloat but since the introduction of automatic properties it's no longer a problem:

public string Name { get; set; }