Difference between Properties and Fields..

To understand how a property is different from a field, imagine you have derived a class from a Label control, to add it the capability to change the foreground color of the shown text. Your code will be similar to the following:

public class ColoredLabel : Label
{
   private Color _Foreground = Color.Black;

   ...

   public Color Foreground
   {
      get
      {
         return _Foreground;
      }
      set
      {
         _Foreground = value;
         Invalidate();
      }
   }

   ...

}

As you can see, using a property allow you to do more than simply get and/or set the value of a field.

About the error that you are getting, see the documentation of the Int32.Parse Method (String)[^]: as it could fails and throws an exception in some cases, you have to add exception-handling to your code to make it robust:

try
{
   c1.A = int.Parse(tb1.Text);
}
catch (FormatException e)
{
   // The text in the TextBox doesn't match an int
   ...
}
catch (OverflowException e)
{
   // The number in the TextBox is out of the admitted range for an int
   string text = tb1.Text;
   text.Trim();
   if (text.StartsWith("-"))
      c1.A = int.MinValue;
   else
      c1.A = int.MaxValue;
}
catch (Exception e)
{
   // Something else goes wrong
   ...
}