C# Strings

C#的string的結尾沒有終止符，因此C# string可以嵌入任意數量的null characters (‘\0’)
string vs. System.String：C#中string是String的別名，因此它們實際上是一樣的。

// Declare without initializing.string message1;// Initialize to null.string message2 = null;// Initialize as an empty string.// Use the Empty constant instead of the literal "".string message3 = System.String.Empty;//Initialize with a regular string literal.string oldPath = "c:\\Program Files\\Microsoft Visual Studio 8.0";// Initialize with a verbatim string literal.string newPath = @"c:\Program Files\Microsoft Visual Studio 9.0";// Use System.String if you prefer.System.String greeting = "Hello World!";// In local variables (i.e. within a method body)// you can use implicit typing.var temp = "I'm still a strongly-typed System.String!";// Use a const string to prevent 'message4' from// being used to store another string value.const string message4 = "You can't get rid of me!";// Use the String constructor only when creating// a string from a char*, char[], or sbyte*. See// System.String documentation for details.char[] letters = { 'A', 'B', 'C' };string alphabet = new string(letters);

除了使用字符數組初始化字符串之外，不需要使用new操作符來創建字符串對象。
Immutability of String Objects
string對象是不可變的，所有對string對象的操作改變都是會返回一個新的string對象。

string s1 = "A string is more ";string s2 = "than the sum of its chars.";// Concatenate s1 and s2. This actually creates a new// string object and stores it in s1, releasing the// reference to the original object.s1 += s2;System.Console.WriteLine(s1);// Output: A string is more than the sum of its chars.

string s1 = "Hello ";string s2 = s1;s1 += "World";System.Console.WriteLine(s2);//Output: Hello

How to: Modify String Contents

using System;using System.Text;using System.Text.RegularExpressions;namespace ReplaceSubstrings{    class ReplaceSubstrings    {        string searchFor;        string replaceWith;        // Custom match method called by Regex.Replace        // using System.Text.RegularExpressions        string ReplaceMatchCase(Match m)        {            // Test whether the match is capitalized            if (Char.IsUpper(m.Value[0]) == true)            {                // Capitalize the replacement string                // using System.Text;                StringBuilder sb = new StringBuilder(replaceWith);                sb[0] = (Char.ToUpper(sb[0]));                return sb.ToString();            }            else            {                return replaceWith;            }        }        static void Main(string[] args)        {            ReplaceSubstrings app = new ReplaceSubstrings();            string s = "The mountains are behind the clouds today.";            Console.WriteLine(s);            // Replace one substring with another with String.Replace.            // Only exact matches are supported.            s = s.Replace("mountains", "peaks");            Console.WriteLine(s);            // Output: The peaks are behind the clouds today.            // Use Regex.Replace for more flexibility.             // Replace "the" or "The" with "many" or "Many".            // using System.Text.RegularExpressions            app.searchFor = "the"; // A very simple regular expression.            app.replaceWith = "many";            s = Regex.Replace(s, app.searchFor, app.ReplaceMatchCase, RegexOptions.IgnoreCase);            Console.WriteLine(s);            // Output: Many peaks are behind many clouds today.            // Replace all occurrences of one char with another.            s = s.Replace(' ', '_');            Console.WriteLine(s);            // Output: Many_peaks_are_behind_many_clouds_today.            // Remove a substring from the middle of the string.            string temp = "many_";            int i = s.IndexOf(temp);            if (i >= 0)            {                s = s.Remove(i, temp.Length);            }            Console.WriteLine(s);            // Output: Many_peaks_are_behind_clouds_today.            // Remove trailing and leading whitespace.            // See also the TrimStart and TrimEnd methods.            string s2 = "    I'm wider than I need to be.      ";            // Store the results in a new string variable.            temp = s2.Trim();            Console.WriteLine(temp);            // Output: I'm wider than I need to be.            // Keep the console window open in debug mode.            Console.WriteLine("Press any key to exit");            Console.ReadKey();        }    }}

ToCharArray Method

class ModifyStrings{    static void Main()    {        string str = "The quick brown fox jumped over the fence";        System.Console.WriteLine(str);        char[] chars = str.ToCharArray();        int animalIndex = str.IndexOf("fox");        if (animalIndex != -1)        {            chars[animalIndex++] = 'c';            chars[animalIndex++] = 'a';            chars[animalIndex] = 't';        }        string str2 = new string(chars);        System.Console.WriteLine(str2);        // Keep the console window open in debug mode        System.Console.WriteLine("Press any key to exit.");        System.Console.ReadKey();    }}/* Output:  The quick brown fox jumped over the fence  The quick brown cat jumped over the fence */

fixed keyword unsafe operations

using System;namespace UnsafeString{    class Program    {        unsafe static void Main(string[] args)        {            // Compiler will store (intern)             // these strings in same location.            string s1 = "Hello";            string s2 = "Hello";            Console.WriteLine(s1); // Hello            Console.WriteLine(s2); // Hello            // Change one string using unsafe code.            fixed (char* p = s1)            {                p[0] = 'C';            }            // Both strings have changed.            Console.WriteLine(s1); // Cello            Console.WriteLine(s2); // Cello            // Keep console window open in debug mode.            Console.WriteLine("Press any key to exit.");            Console.ReadKey();        }    }}

Regular and Verbatim String Literals

string columns = "Column 1\tColumn 2\tColumn 3";//Output: Column 1        Column 2        Column 3string rows = "Row 1\r\nRow 2\r\nRow 3";/* Output:  Row 1  Row 2  Row 3*/string title = "\"The \u00C6olean Harp\", by Samuel Taylor Coleridge";//Output: "The Æolean Harp", by Samuel Taylor Coleridge

string filePath = @"C:\Users\scoleridge\Documents\";//Output: C:\Users\scoleridge\Documents\string text = @"My pensive SARA ! thy soft cheek reclined    Thus on mine arm, most soothing sweet it is    To sit beside our Cot,...";/* Output:My pensive SARA ! thy soft cheek reclined   Thus on mine arm, most soothing sweet it is   To sit beside our Cot,... */string quote = @"Her name was ""Sara.""";//Output: Her name was "Sara."

String Escape Sequences

Escape sequence

Character name

Unicode encoding

\’

Single quote

0x0027

\”

Double quote

0x0022

\\

Backslash

0x005C

\0

Null

0x0000

\a

Alert

0x0007

\b

Backspace

0x0008

\f

Form feed

0x000C

\n

New line

0x000A

\r

Carriage return

0x000D

\t

Horizontal tab

0x0009

\U

Unicode escape sequence for surrogate pairs.

\Unnnnnnnn

\u

Unicode escape sequence

\u0041 = “A”

\v

Vertical tab

0x000B

\x

Unicode escape sequence similar to “\u” except with variable length.

\x0041 = “A”

For example, the verbatim string @"C:\files.txt" will appear in the watch window as "C:\\files.txt".

Format Strings

class FormatString{    static void Main()    {        // Get user input.        System.Console.WriteLine("Enter a number");        string input = System.Console.ReadLine();        // Convert the input string to an int.        int j;        System.Int32.TryParse(input, out j);        // Write a different string each iteration.        string s;        for (int i = 0; i < 10; i++)        {            // A simple format string with no alignment formatting.            s = System.String.Format("{0} times {1} = {2}", i, j, (i * j));            System.Console.WriteLine(s);        }        //Keep the console window open in debug mode.        System.Console.ReadKey();    }}

Formatting Types in the .NET Framework
Substrings

string s3 = "Visual C# Express";System.Console.WriteLine(s3.Substring(7, 2));// Output: "C#"System.Console.WriteLine(s3.Replace("C#", "Basic"));// Output: "Visual Basic Express"// Index values are zero-basedint index = s3.IndexOf("C");// index = 7

How to: Search Strings Using String Methods

class StringSearch{    static void Main()    {        string str = "Extension methods have all the capabilities of regular static methods.";        // Write the string and include the quotation marks.        System.Console.WriteLine("\"{0}\"", str);        // Simple comparisons are always case sensitive!        bool test1 = str.StartsWith("extension");        System.Console.WriteLine("Starts with \"extension\"? {0}", test1);        // For user input and strings that will be displayed to the end user,         // use the StringComparison parameter on methods that have it to specify how to match strings.        bool test2 = str.StartsWith("extension", System.StringComparison.CurrentCultureIgnoreCase);        System.Console.WriteLine("Starts with \"extension\"? {0} (ignoring case)", test2);        bool test3 = str.EndsWith(".", System.StringComparison.CurrentCultureIgnoreCase);        System.Console.WriteLine("Ends with '.'? {0}", test3);        // This search returns the substring between two strings, so         // the first index is moved to the character just after the first string.        int first = str.IndexOf("methods") + "methods".Length;        int last = str.LastIndexOf("methods");        string str2 = str.Substring(first, last - first);        System.Console.WriteLine("Substring between \"methods\" and \"methods\": '{0}'", str2);        // Keep the console window open in debug mode        System.Console.WriteLine("Press any key to exit.");        System.Console.ReadKey();    }}/*Output:"Extension methods have all the capabilities of regular static methods."Starts with "extension"? FalseStarts with "extension"? True (ignoring case)Ends with '.'? TrueSubstring between "methods" and "methods": ' have all the capabilities of regular static 'Press any key to exit.     */

Accessing Individual Characters

string s5 = "Printing backwards";for (int i = 0; i < s5.Length; i++){    System.Console.Write(s5[s5.Length - i - 1]);}// Output: "sdrawkcab gnitnirP"

string question = "hOW DOES mICROSOFT wORD DEAL WITH THE cAPS lOCK KEY?";System.Text.StringBuilder sb = new System.Text.StringBuilder(question);for (int j = 0; j < sb.Length; j++){    if (System.Char.IsLower(sb[j]) == true)        sb[j] = System.Char.ToUpper(sb[j]);    else if (System.Char.IsUpper(sb[j]) == true)        sb[j] = System.Char.ToLower(sb[j]);}// Store the new string.string corrected = sb.ToString();System.Console.WriteLine(corrected);// Output: How does Microsoft Word deal with the Caps Lock key?

Null Strings and Empty Strings

string s = String.Empty;

static void Main(){    string str = "hello";    string nullStr = null;    string emptyStr = String.Empty;    string tempStr = str + nullStr;    // Output of the following line: hello    Console.WriteLine(tempStr);    bool b = (emptyStr == nullStr);    // Output of the following line: False    Console.WriteLine(b);    // The following line creates a new empty string.    string newStr = emptyStr + nullStr;    // Null strings and empty strings behave differently. The following    // two lines display 0.    Console.WriteLine(emptyStr.Length);    Console.WriteLine(newStr.Length);    // The following line raises a NullReferenceException.    //Console.WriteLine(nullStr.Length);    // The null character can be displayed and counted, like other chars.    string s1 = "\x0" + "abc";    string s2 = "abc" + "\x0";    // Output of the following line: * abc*    Console.WriteLine("*" + s1 + "*");    // Output of the following line: *abc *    Console.WriteLine("*" + s2 + "*");    // Output of the following line: 4    Console.WriteLine(s2.Length);}

Using StringBuilder for Fast String Creation

System.Text.StringBuilder sb = new System.Text.StringBuilder("Rat: the ideal pet");sb[0] = 'C';System.Console.WriteLine(sb.ToString());System.Console.ReadLine();//Outputs Cat: the ideal pet

class TestStringBuilder{    static void Main()    {        System.Text.StringBuilder sb = new System.Text.StringBuilder();        // Create a string composed of numbers 0 - 9        for (int i = 0; i < 10; i++)        {            sb.Append(i.ToString());        }        System.Console.WriteLine(sb);  // displays 0123456789        // Copy one character of the string (not possible with a System.String)        sb[0] = sb[9];        System.Console.WriteLine(sb);  // displays 9123456789    }}

相關課目：

Topic

Description

How to: Modify String Contents (C# Programming Guide)

Provides a code example that illustrates how to modify the contents of strings.

How to: Concatenate Multiple Strings (C# Programming Guide)

Illustrates how to use the + operator and the Stringbuilder class to join strings together at compile time and run time.

How to: Compare Strings (C# Programming Guide)

Shows how to perform ordinal comparisons of strings.

How to: Parse Strings Using String.Split (C# Programming Guide)

Contains a code example that illustrates how to use the String.Split method to parse strings.

How to: Search Strings Using String Methods (C# Programming Guide)

Explains how to use specific methods to search strings.

How to: Search Strings Using Regular Expressions (C# Programming Guide)

Explains how to use regular expressions to search strings.

How to: Determine Whether a String Represents a Numeric Value (C# Programming Guide)

Shows how to safely parse a string to see whether it has a valid numeric value.

How to: Convert a String to a DateTime (C# Programming Guide)

Shows how to convert a string such as “01/24/2008” to a System.DateTime object.

Basic String Operations in the .NET Framework

Provides links to topics that use System.String and System.Text.StringBuilder methods to perform basic string operations.

Parsing Strings in the .NET Framework

Describes how to insert characters or empty spaces into a string.

Comparing Strings in the .NET Framework

Includes information about how to compare strings and provides examples in C# and Visual Basic.

Using the StringBuilder Class in the .NET Framework

Describes how to create and modify dynamic string objects by using the StringBuilder class.

LINQ and Strings

Provides information about how to perform various string operations by using LINQ queries.

C# Programming Guide

Provides links to topics that explain programming constructs in C#.