C# ПІДРУЧНИКИ / c# / MS Press - Msdn Training Programming Net Framework With C#

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Terminology – Collections

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" .NET Framework Arrays

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The System.Array class is the base class of all array types and contains methods for creating, manipulating, searching, and sorting arrays. The Array class is not part of the Collections namespace. However, it is still a collection because it is based on the IList interface.

In an array style of collection, an item in the collection is referred to by the term element. Specific elements are identified by their array index. The low bound or lower bound of an Array is the index of its first element.

To understand the functionality of the various collection classes, you must understand their key interfaces. In this section, you will learn about the interfaces that are used by the methods of System.Array.

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The System.Array class is the base class of all array types and contains methods for creating, manipulating, searching, and sorting arrays. Arrays are always allocated on the garbage-collected heap. Arrays can be single dimensional or multidimensional. You can also create arrays of arrays, called jagged arrays.

For optimum performance, it is highly recommended that each rank of an array be zero-based. Additionally, each rank of an array must be zero-based when passing arrays between programming languages.

Defining an Array Type

You define an array type by specifying the element type of the array, the rank, or number of dimensions, of the array, and the upper and lower bounds of each dimension of the array. All of these details are included in any signature of an array type.

The runtime automatically creates exact array types as they are required. No separate definition of the array type is needed. Arrays of a particular type can only hold elements of that type. If you need to manipulate a set of unlike objects or value types, consider using one of the collection types that are defined in the System.Collections namespace.

The following table describes some of the public instance methods that are available through the System.Array class.

For complete lists of public members of the System.Array class, see “Array Members” in the .NET Framework SDK documentation.

Empty Arrays

Nulls should only be returned by reference properties that refer to another object or component. String and Array properties should never return null, because a programmer typically does not expect null in this context. For example, a programmer typically would assume that the following code works:

public void DoSomething(…) { string[] sa = SomeOtherFunc();

// The following line assumes sa is never null if (sa.Length > 0) {

// do something else

}

}

Generally, null, empty string, and empty (0 item) arrays should be treated in the same way. Therefore, return an Empty array, instead of a null reference.

C# Specifics

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C# arrays are zero-indexed. That means the array indexes start at zero. Arrays in C# and arrays in most other popular languages work similarly, but there are a few differences that you should be aware of.

When declaring an array, the brackets ([]) must come after the type, and not after the identifier. Placing the brackets after the identifier is not legal syntax in C#, as shown in the following example:

int[] table; // not int table[];

In addition, and unlike the C language, the size of the array is not part of its type. This allows you to declare and assign to an array any array of int objects, regardless of the array’s length, as in the following examples.

int[] numbers; // declare numbers as an int array of any size numbers = new int[10]; // numbers is a 10-element array numbers = new int[20]; // now it's a 20-element array

Declaring an Array

C# supports single-dimensional arrays and multidimensional arrays, which are also know as rectangular arrays, and jagged arrays.

The following code examples show how to declare each of these arrays:

int[] numbers; // single-dimensional array

string[,] names; // multidimensional array

byte[][] scores; //Array-of-arrays

Instantiating an Array

Declaring arrays, as shown in the preceding examples, does not actually create the arrays. As discussed later in this topic, arrays in C# are objects and must be instantiated.

The following code examples show the syntax to create an array for each of the arrays already discussed:

int[] numbers = new int[5]; // Single-dimensional array

string[,] names = new string[5,4]; // Multi-dimensional array

byte[][] scores = new byte[5][]; // Array-of-arrays for (int x = 0; x < scores.Length; x++)

{

scores[x] = new byte[4];

}

Single-Dimensional Array

The following examples show different ways to initialize single-dimensional arrays:

int[] numbers = new int[5] {1, 2, 3, 4, 5};

string[] names = new string[3] {"Matt", "Joanne", "Robert"};

You can omit the size of the array, as in the following examples:

int[] numbers = new int[] {1, 2, 3, 4, 5};

string[] names = new string[] {"Matt", "Joanne", "Robert"};

If an initializer is provided, you can also omit the new statement, as in the following examples:

int[] numbers = {1, 2, 3, 4, 5};

string[] names = {"Matt", "Joanne", "Robert"};

Multidimensional Array

The following examples show different ways to initialize multidimensional arrays:

int[,] numbers = new int[3, 2] { {1, 2}, {3, 4}, {5, 6} };

string[,] siblings = new string[2, 2] { {"Mike","Amy"}, {"Mary","Albert"} };

You can omit the size of the array, as in the following examples:

int[,] numbers = new int[,] { {1, 2}, {3, 4}, {5, 6} };

string[,] siblings = new string[,] { {"Mike","Amy"}, {"Mary","Ray"} };

If an initializer is provided, you can also omit the new statement, as in the following examples:

int[,] numbers = { {1, 2}, {3, 4}, {5, 6} };

string[,] siblings = { {"Mike", "Amy"}, {"Mary", "Albert"} };