屬性(Properties)
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.
- Create an auto-implemented property.
Overview of Properties
Properties provide the opportunity to protect a field in a class by reading and writing to it through the property. In other languages, this is often 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.
Another benefit of properties over fields is that you can change their internal implementation over time. With a public field, the underlying data type must always be the same because calling code depends on the field being the same. However, with a property, you can change the implementation. For example, if a customer has an ID that is originally stored as an int, you might have a requirements change that made you perform a validation to ensure that calling code could never set the ID to a negative value. If it was a field, you would never be able to do this, but a property allows you to make such a change without breaking code. Now, lets see how to use properties.
Traditional Encapsulation Without Properties
Languages that don't have properties will use methods (functions or procedures) for encapsulation. The idea is to manage the values inside of the object, state, avoiding corruption and misuse by calling code. Listing 10-1 demonstrates how this traditional method works, encapsulating Customer information via accessor methods.
Listing 10-1. An Example of Traditional Class Field Access
public class Customer1
{
private int m_id = -1;
public int GetID()
{
return m_id;
}
public void SetID(int id)
{
m_id = id;
}
private string m_name = string.Empty;
public string GetName()
{
return m_name;
}
public void SetName(string name)
{
m_name = name;
}
}
private void button1_Click(object sender, EventArgs e)
{
Customer1 cust = new Customer1();
cust.SetID(1);
cust.SetName("vito");
Console.WriteLine("ID:{0} Name:{1}", cust.GetID(), cust.GetName());
}
Listing 10-1 shows the traditional method of accessing class fields. The Customer class has four methods, two for each private field that the class encapsulates: m_id and m_name. As you can see, SetID and SetName assign a new values and GetID and GetName return values.
Observe how Main calls the SetXxx methods, which sets m_id to 1 and m_name to "Amelio Rosales" in the Customer instance, cust. The call to Console.WriteLine demonstrates how to read m_id and m_name from cust, via GetID and GetName method calls, respectively.
This is such a common pattern, that C# has embraced it in the form of a language feature called properties, which you'll see in the next section.
Encapsulating Type State with Properties
The practice of accessing field data via methods was good because it supported the object-oriented concept of encapsulation. For example, if the type of m_id or m_name changed from an int type to byte, calling code would still work. Now the same thing can be accomplished in a much smoother fashion with properties, as shown in Listing 10-2.
Listing 10-2. Accessing Class Fields With Properties
public class Customer2
{
private int m_id = -1;
public int ID
{
get
{
return m_id;
}
set
{
m_id = value;
}
}
private string m_name = string.Empty;
public string Name
{
get
{
return m_name;
}
set
{
m_name = value;
}
}
}
private void button2_Click(object sender, EventArgs e)
{
Customer2 cust = new Customer2();
cust.ID = 1;
cust.Name = "vito";
Console.WriteLine("ID:{0} Name:{1}", cust.ID, cust.Name);
}
Listing 10-2 shows how to create and use a property. The Customer class has the ID and Name property implementations. There are also private fields named m_id and m_name; which ID and Name, respectively, encapsulate. Each property has two accessors, get and set. The get accessor returns the value of a field. The set accessor sets the value of a field with the contents of value, which is the value being assigned by calling code. The value shown in the accessor is a C# reserved word.
When setting a property, just assign a value to the property as if it were a field. The CustomerManagerWithProperties class uses the ID and Name properties in the Customer class. The first line of Main instantiates a Customer object named cust. Next the value of the m_id and m_name fields of cust are set by using the ID and Name properties.
To read from a property, use the property as if it were a field. Console.WriteLine prints the value of the m_id and m_name fields of cust. It does this by calling the ID and Name properties of cust.
This was a read/write property, but you can also create read-only properties, which you'll learn about next.
Creating Read-Only Properties
Properties can be made read-only. This is accomplished by having only a get accessor in the property implementation. Listing 10-3 demonstrates how you can create a read-only property.
Listing 10-3. Read-Only Properties
public class Customer3
{
private int m_id = -1;
private string m_name = string.Empty;
public Customer3(int id, string name)
{
m_id = id;
m_name = name;
}
public int ID
{
get
{
return m_id;
}
}
public string Name
{
get
{
return m_name;
}
}
}
private void button3_Click(object sender, EventArgs e)
{
Customer3 cust = new Customer3(1, "vito");
Console.WriteLine("ID:{0} Name:{1}", cust.ID, cust.Name);
}
The Customer class in Listing 10-3 has two read-only properties, ID and Name. You can tell that each property is read-only because they only have get accessors. At some time, values for the m_id and m_name must be assigned, which is the role of the constructor in this example.
The Main method of the ReadOnlyCustomerManager class instantiates a new Customer object named cust. The instantiation of cust uses the constructor of Customer class, which takes int and string type parameters. In this case, the values are 1 and "Amelio Rosales". This initializes the m_id and m_name fields of cust.
Since the ID and Name properties of the Customer class are read-only, there is no other way to set the value of the m_id and m_name fields. If you inserted cust.ID = 7 into the listing, the program would not compile, because ID is read-only; the same goes for Name. When the ID and Name properties are used in Console.WriteLine, they work fine. This is because these are read operations which only invoke the get accessor of the ID and Name properties.
One question you might have now is "If a property can be read-only, can it also be write-only?" The answer is yes, and explained in the next section.
Creating a Write-Only Property
You can assign values to, but not read from, a write-only property. A write-only property only has a set accessor. Listing 10-4 shows you how to create and use write-only properties.
Listing 10-4. Write-Only Properties
public class Customer4
{
private int m_id = -1;
public int ID
{
set
{
m_id = value;
}
}
private string m_name = string.Empty;
public string Name
{
set
{
m_name = value;
}
}
public string DisplayCustomerData()
{
return string.Format("ID: {0}, Name:{1}", m_id, m_name);
}
}
private void button4_Click(object sender, EventArgs e)
{
Customer4 cust = new Customer4();
cust.ID = 1;
cust.Name = "vito";
Console.WriteLine(cust.DisplayCustomerData());
}
This time, the get accessor is removed from the ID and Name properties of the Customer class, shown in Listing 10-1. The set accessors have been added, assigning value to the backing store fields, m_id and m_name.
The Main method of the WriteOnlyCustomerManager class instantiates the Customer class with a default constructor. Then it uses the ID and Name properties of cust to set the m_id and m_name fields of cust to 1 and "Amelio Rosales", respectively. This invokes the set accessor of ID and Name properties from the cust instance.
When you have a lot of properties in a class or struct, there can also be a lot of code associated with those properties. In the next section, you'll see how to write properties with less code.
Creating Auto-Implemented Properties
The patterns you see here, where a property encapsulates a property with get and set accessors, without any other logic is common. It is more code than we should have to write for such a common scenario. That's why C# 3.0 introduced a new syntax for a property, called an auto-implemented property, which allows you to create properties without get and set accessor implementations. Listing 10-5 shows how to add auto-implemented properties to a class.
Listing 10-5. Auto-Implemented Properties
public class Customer5
{
public int ID { get; set; }
public string Name { get; set; }
}
private void button5_Click(object sender, EventArgs e)
{
Customer5 cust = new Customer5();
cust.ID = 1;
cust.Name = "vito";
Console.WriteLine("ID:{0} Name:{1}", cust.ID, cust.Name);
}
Listing 10-6. Readonly Auto-Implemented Properties
public class Customer6
{
public int ID { private get; set; } //write-only
public string Name { get; private set; } //read-only
public Customer6() // using default constructor to set the defautl values
{
ID = "";
Name = "";
}
public Customer6(int id, string name)
{
ID = id;
Name = name;
}
}
private void button6_Click(object sender, EventArgs e)
{
Customer6 cust = new Customer6(1, "vito");
cust.ID = 2;
//cust.Name = "vito2"; //cust.Name can't write
//Console.WriteLine("ID:{0}", cust.ID); //cust.ID can't read
Console.WriteLine("Name:{0}", cust.Name);
}
Notice how the get and set accessors in Listing 10-5 do not have implementations. In an auto-implemented property, the C# compiler creates the backing store field behind the scenes, giving the same logic that exists with traditional properties, but saving you from having to use all of the syntax of the traditional property. As you can see in the Main method, the usage of an auto-implemented property is exactly the same as traditional properties, which you learned about in previous sections.
Summary
You now know what properties are for and how they're used. Traditional techniques of encapsulation have relied on separate methods. Properties allow you to access objects state with field-like syntax. Properties can be made read-only or write-only. You also learned how to write properties with less code by using auto-implemented properties.
沒有留言:
張貼留言