Generic series: use generic to create a read-only set

4.9 create a read-only set Using Generics

Problem

You want the information in a set in the class to be accessed by the outside world, but you do not want the user to change the set.

Solution

Using ReadOnlyCollection <T> to package, you can easily implement read-only collection classes. For example, the Lottery class contains a winning number, which can be accessed but cannot be changed:

Public class Lottery

{

// Create a list.

List <int> _ numbers = null;

Public Lottery ()

{

// Initialize the internal list

_ Numbers = new List <int> (5 );

// Add Value

_ Numbers. Add (17 );

_ Numbers. Add (21 );

_ Numbers. Add (32 );

_ Numbers. Add (44 );

_ Numbers. Add (58 );

}

Public ReadOnlyCollection <int> Results

{

// Return the result of a package.

Get {return new ReadOnlyCollection <int> (_ numbers );}

}

}

Lottery has an internal List <int>, which contains the winning number entered in the constructor. The interesting part is that it has a public attribute called Results. The returned ReadOnlyCollection <int> type shows the winning number, so that users can use it through the returned instance.

If you try to set a value in the Set, a compilation error is thrown:

Lottery tryYourLuck = new Lottery ();

// Print the result.

For (int I = 0; I <tryYourLuck. Results. Count; I ++)

{

Console. WriteLine ("Lottery Number" + I + "is" + tryYourLuck. Results [I]);

}

// Change the winning number!

TryYourLuck. Results [0] = 29;

// The Last Row triggers an Error: // Error 26 // Property or indexer

// 'System. Collections. ObjectModel. ReadOnlyCollection <int>. this [int]'

// Cannot be assigned to -- it is read only

Discussion

The main advantage of ReadOnlyCollection is its flexibility. It can be used as an interface in any collection that supports IList or IList <T>. ReadOnlyCollection can also Wrap general arrays like this:

Int [] items = new int [3];

Items [0] = 0;

Items [1] = 1;

Items [2] = 2;

New ReadOnlyCollection <int> (items );

This provides a way to standardize the read-only attributes of a class, and makes the class library user accustomed to this simple read-only attribute return type.