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

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You can easily change the case of a string by using the two methods that are described in the following table.

String.ToUpper and String.ToLower provide an override that accepts a culture.

The String.ToUpper method changes all of the characters in a string to uppercase.

The following code example converts the string "hello world!" from lowercase to uppercase:

string MyString = "hello world!"; Console.WriteLine(MyString.ToUpper());

The preceding example displays HELLO WORLD! to the console.

The String.ToLower method is similar to the String.ToUpper method, but it converts all of the characters in a string to lowercase.

The following code example converts the string "HELLO WORLD!" to lowercase.

string MyString = "HELLO WORLD!"; Console.WriteLine(MyString.ToLower());

The preceding example displays hello world! to the console.

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The .NET Framework provides several methods to compare the values of strings. The following table describes some of the value-comparison methods.

String.Compare Compares the values of two strings. Returns an integer value.

String.StartsWith Determines if a string begins with the string passed. Returns a boolean value.

String.IndexOf Returns the index of the first occurrence of a String, or one or more characters, within this instance.

For more information about value comparison methods and for a complete list of these methods, see “Comparing Strings” in the .NET Framework SDK documentation.

For example, the String.Compare method provides a thorough way to compare the current string object to another string or object. You can use this function to compare two strings or substrings of two strings.

Additionally, overloads are provided that regard or disregard case and cultural variance.

Trim and Pad

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When you want to remove or extend the spaces around strings, the

System.String class provides methods for trimming and padding strings.

Trimming

When you are parsing a sentence into individual words, you might have white spaces on either end of a word. You can use one of the trim methods in the System.String class to remove any number of spaces from the beginning or end of the string.

The following table describes two of the available trim methods.

String.Trim Removes white spaces from the beginning and end of a string.

String.Remove Removes a specified number of characters from a specified index position in a string.

For example, you can easily remove white spaces from both ends of a string by using the String.Trim method, as shown in the following code example.

string MyString = " Big "; Console.WriteLine("Hello{0}World!", MyString ); string TrimString = MyString.Trim(); Console.WriteLine("Hello{0}World!", TrimString );

This code outputs the following lines to the console:

Hello Big World!

HelloBigWorld!

For a complete list of trim methods in the System.String class, see “Trimming and Removing Characters” in the .NET Framework SDK documentation.

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The System.String class provides the Split method to break up strings and the

Join method to concatenate strings.

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The String object is immutable. Therefore, every time you use one of the methods in the System.String class, you create a new string object. When you want to perform repeated modifications to a string, the overhead that is associated with creating a new String object can be costly. As an alternative, you can use the System.Text.StringBuilder class to modify a string without creating a new object.

Creating a StringBuilder Object

You can create a new instance of the StringBuilder object by initializing your variable with one of the overloaded constructor methods, as shown in the following code example:

StringBuilder MyStringBuilder = new StringBuilder("Hello");

Although the StringBuilder object is a dynamic object that allows you to expand the number of characters in the string that it encapsulates, you can specify a value for the maximum number of characters that it can hold. This value is called the capacity of the object and must not be confused with the length of the string that the current StringBuilder object holds. Any attempt to expand the StringBuilder class beyond the maximum range causes an

ArgumentOutOfRangeException to be thrown.

The following code example specifies that the MyStringBuilder object can be expanded to a maximum of 25 spaces.

StringBuilder MyStringBuilder =

new StringBuilder("Hello World!", 25);

StringBuilder Methods

The following table describes the methods that you can use to modify the contents of the StringBuilder object.

C# Specifics

@"c:\Docs\Source\a.txt"

@"c:\Docs\Source\a.txt"

// rather than "c:\\Docs\\Source\\a.txt" // rather than "c:\\Docs\\Source\\a.txt"

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The string type represents a string of Unicode characters; string is an alias for

System.String in the .NET Framework.

Although string is a reference type, the equality (==) operator and the inequality (!=) operator are defined to compare the values of string objects, not the references. Comparing the values of string objects makes testing for string equality more intuitive, as in the following example:

string a = "hello"; string b = "hello";

Console.WriteLine( a == b ); // output: True -- same value

The + operator concatenates strings, as in the following example:

string a = "good " + "morning";

The [ ] operator accesses individual characters of a string, as in the following example:

char x = "test"[2]; // x = 's';

String literals are of type string and can be written in two forms: quoted and @-quoted. Quoted string literals are enclosed in quotation marks ("), as in the following example:

"good morning" // a string literal

Quoted string literals can also contain any character literal, including escape sequences, as in the following example:

string a = "\\\u0066\n"; // backslash, letter f, new line

@-quoted string literals start with @ and are enclosed in quotation marks, as in the following example:

@"good morning" // a string literal

The advantage of using @-quoted string literals is that escape sequences are not processed. This makes it easy to write a fully qualified file name, as in the following example:

@"c:\Docs\Source\a.txt"

// rather than "c:\\Docs\\Source\\a.txt"

To include a quoted phrase in an @-quoted string, use two pairs of double quotation marks, as in the following example:

@"""Ahoy!"" cried the captain." // "Ahoy!" cried the captain.

The following code example uses the C# features that are discussed in this topic:

using System; class test

{

public static void Main( String[] args )

{

string a = "\u0068ello "; string b = "world"; Console.WriteLine( a + b );

Console.WriteLine( a + b == "hello world" );

}

}

The preceding code example displays the following output to the console:

hello world True