As of now, a program can plant any value in a Forest field. For example, if we had an area field of type int, we could set it to 0, 40, or -1249. Can we have a forest of -1249 area? We need a way to define what values are valid and disallow those that are not. C# provides a tool for that: properties.

Properties are another type of class member. Each property is like a spokesperson for a field: it controls the access (getting and setting) to that field. We can use this to validate values before they are set to a field. A property is made up of two methods:

• a get() method, or getter: called when the property is accessed
• a set() method, or setter: called when the property is assigned a value

This shows a basic Area property without validation:

public int area;
public int Area
{
  get { return area; }
  set { area = value; }
}

The Area property is associated with the area field. It’s common to name a property with the title-cased version of its field’s name, e.g. age and Age, name and Name.

The set() method above uses the keyword value, which represents the value we assign to the property. Back in Program.cs, when we access the Area property, the get() and set() methods are called:

Forest f = new Forest();
f.Area = -1; // set() is called
Console.WriteLine(f.Area); // get() is called; prints -1

In the above example, when set() is called, the value variable is -1, so area is set to -1.

Here’s the same property with validation in the set() method. If we try to set Area to a negative value, it will be changed to 0.

public int Area
{
  get { return area; }
  set 
  { 
    if (value < 0) { area = 0; }
    else { area = value; }
  }
}

In Program.cs:

Forest f = new Forest();
// set() is called
f.Area = -1; 
// get() is called; prints 0
Console.WriteLine(f.Area);