The C# Tutorial

by Joe Mayo, 02/10/01

Lesson 10: Properties

This lesson teaches C# Properties. Our objectives are as follows:

	Understand What Properties Are For.
	Implement a Property.
	Create a Read-Only Property.
	Create a Write-Only Property.

Properties are a new language feature introduced with C#. They provide the opportunity to protect a field in a class by reading and writing to it through the property. In other languages, this is accomplished by programs implementing specialized getter and setter methods. C# properties enable this type of protection while also letting you access the property just like it was a field. To get an appreciation for what properties accomplish, let's take a look at how to provide field encapsulation by traditional methods.

Listing 10-1. An Example of Traditional Class Field Access: Accessors.cs

using System;

public class PropertyHolder
{
private int someProperty = 0;

public int getSomeProperty()
{
return someProperty;
}

public void setSomeProperty(int propValue)
{
someProperty = propValue;
}

}

public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();

propHold.setSomeProperty(5);

Console.WriteLine("Property Value: {0}", propHold.getSomeProperty());

return 0;
}
}

Listing 10-1 shows the traditional method of accessing class fields. The PropertyHolder class has the field we're interested in accessing. It has two methods, getSomeProperty and setSomeProperty. The getSomeProperty method returns the value of the someProperty field. The setSomeProperty method sets the value of the someProperty field.

The PropertyTester class uses the methods of the PropertyHolder class to get the value of the someProperty field in the PropertyHolder class. The Main method instantiates a new PropertyHolder object, propHold. Next it sets the someMethod of propHold to the value 5 by using the setSomeProperty method. Then the program prints out the property value with a Console.WriteLine method call. The argument used to obtain the value of the property is a call to the getSomeProperty method of the propHold object. It prints out "Property Value: 5" to the console.

This method of accessing information in a field has been good because it supports the object-oriented concept of encapsulation. If the implementation of someProperty changed from an int type to a byte type, this would still work. Now the same thing can be accomplished much smoother with properties.

Listing 10-2. Accessing Class Fields With Properties: Properties.cs

using System;

public class PropertyHolder
{
private int someProperty = 0;

public int SomeProperty
{
get
{
return someProperty;
}
set
{
someProperty = value;
}
}
}

public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();

propHold.SomeProperty = 5;

Console.WriteLine("Property Value: {0}", propHold.SomeProperty);

return 0;
}
}

Listing 10-2 shows how to create and use a property. The PropertyHolder class has the "SomeProperty" property implementation. Notice that the first letter of the first word is capitalized. That's the only difference between the names of the property "SomeProperty" and the field "someProperty". The property has two accessors, get and set. The get accessor returns the value of the someProperty field. The set accessor sets the value of the someProperty field with the contents of "value". The "value" shown in the set accessor is a C# reserved word. It's normally an error to use the "value" keyword in any other context.

The PropertyTester class uses the SomeProperty property in the PropertyHolder class. The first line of the Main method creates a PropertyHolder object named propHold. Next the value of the someProperty field of the propHold object is set to 5 by using the SomeProperty property. It's that simple -- just assign the value to the property as if it were a field.

After that, the Console.WriteLine method prints the value of the someProperty field of the propHold object. It does this by using the SomeProperty property of the propHold object. Again, it's that simple -- just use the property as if it were a field itself.

Properties can be made read-only. This is accomplished by having only a get accessor in the property implementation.

Listing 10-3. Read-Only Property: ReadOnlyProperty.cs

using System;

public class PropertyHolder
{
private int someProperty = 0;

public PropertyHolder(int propVal)
{
someProperty = propVal;
}

public int SomeProperty

{
get
{
return someProperty;
}
}
}

public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder(5);

Console.WriteLine("Property Value: {0}", propHold.SomeProperty);

return 0;
}
}

Listing 10-3 shows how to implement a read-only property. The PropertyHolder class has a SomeProperty property that only implements a get accessor. It leaves out the set accessor. This particular PropertyHolder class has a constructor which accepts an integer parameter.

The Main method of the PropertyTester class creates a new PropertyHolder object named propHold. The instantiation of the propHold object uses the constructor of the PropertyHolder that takes an int parameter. In this case, it's set to 5. This initializes the someProperty field of the propHold object to 5.

Since the SomeProperty property of the PropertyHolder class is read-only, there is no other way to set the value of the someProperty field. If you inserted "propHold.SomeProperty = 7" into the listing, the program would not compile, because SomeProperty is read-only. When the SomeProperty property is used in the Console.WriteLine method, it works fine. This is because it's a read operation which only invokes the get accessor of the SomeProperty property.

Listing 10-4. Write-Only Property: WriteOnlyProperty.cs

using System;

public class PropertyHolder
{
private int someProperty = 0;

public int SomeProperty
{
set
{
someProperty = value;

Console.WriteLine("someProperty is equal to {0}", someProperty);
}
}
}

public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();

propHold.SomeProperty = 5;

return 0;
}
}

Listing 10-4 shows how to create and use a write-only property. This time the get accessor is removed from the SomeProperty property of the PropertyHolder class. The set accessor has been added, with a bit more logic. It prints out the value of the someProperty field after it's been modified.

The Main method of the PropertyTester class instantiates the PropertyTester class with a default constructor. Then it uses the SomeProperty property of the propHold object to set the someProperty field of the propHold object to 5. This invokes the set accessor of the propHold object, which sets the value of it's someProperty field to 5 and then prints "someProperty is equal to 5" to the console.

In summary, you now know what properties are for and how they're used. You have a feeling for the difference between using properties and traditional techniques using class methods. Properties can be made read-only or write-only and you know how to implement each type.

I invite you to return for Lesson 11: Indexers.

Your feedback is very important and I appreciate any constructive contributions you have. Please feel free to contact me for any questions or comments you may have about this lesson