String Basics (C# Programming Guide)

Artikel
02/15/2013

A string is an object of type String whose value is text. Internally, the text is stored as a readonly collection of Char objects, each of which represents one Unicode character encoded in UTF-16. There is no null-terminating character at the end of a C# string (unlike C and C+); therefore a C# string can contain any number of embedded null characters ('\0'). The length of a string represents the number of characters regardless of whether the characters are formed from Unicode surrogate pairs or not. To access the individual Unicode code points in a string, use the StringInfo object.

string vs. System.String

In C#, the string keyword is an alias for String. Therefore, String and string are equivalent, and you can use whichever naming convention you prefer. The String class provides many methods for safely creating, manipulating, and comparing strings. In addition, the C# language overloads some operators to simplify common string operations. For more information about the keyword, see string (C# Reference). For more information about the type and its methods, see String.

Declaring and Initializing Strings

You can declare and initialize strings in various ways, as shown in the following example:

// 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);

Note that you do not use the new operator to create a string object except when initializing the string with an array of chars.

Initialize a string with the Empty constant value to create a new String object whose string is of zero length. The string literal representation of a zero-length string is "". By initializing strings with the Empty value instead of null, you can reduce the chances of a NullReferenceException occurring. Use the static IsNullOrEmpty(String) method to verify the value of a string before you try to access it.

Immutability of String Objects

String objects are immutable: they cannot be changed after they have been created. All of the String methods and C# operators that appear to modify a string actually return the results in a new string object. In the following example, when the contents of s1 and s2 are concatenated to form a single string, the two original strings are unmodified. The += operator creates a new string that contains the combined contents. That new object is assigned to the variable s1, and the original object that was assigned to s1 is released for garbage collection because no other variable holds a reference to it.

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.

Because a string "modification" is actually a new string creation, you must use caution when you create references to strings. If you create a reference to a string, and then "modify" the original string, the reference will continue to point to the original object instead of the new object that was created when the string was modified. The following code illustrates this behavior:

string s1 = "Hello ";
string s2 = s1;
s1 += "World";

System.Console.WriteLine(s2);
//Output: Hello

For more information about how to create new strings that are based on modifications such as search and replace operations on the original string, see How to: Modify String Contents (C# Programming Guide).

Regular and Verbatim String Literals

Use regular string literals when you must embed escape characters provided by C#, as shown in the following example:

string columns = "Column 1\tColumn 2\tColumn 3";
//Output: Column 1        Column 2        Column 3 

string 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

Use verbatim strings for convenience and better readability when the string text contains backslash characters, for example in file paths. Because verbatim strings preserve new line characters as part of the string text, they can be used to initialize multiline strings. Use double quotation marks to embed a quotation mark inside a verbatim string. The following example shows some common uses for verbatim strings:

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"

Note

At compile time, verbatim strings are converted to ordinary strings with all the same escape sequences. Therefore, if you view a verbatim string in the debugger watch window, you will see the escape characters that were added by the compiler, not the verbatim version from your source code. For example, the verbatim string @"C:\files.txt" will appear in the watch window as "C:\\files.txt".

Format Strings

A format string is a string whose contents can be determined dynamically at runtime. You create a format string by using the static Format method and embedding placeholders in braces that will be replaced by other values at runtime. The following example uses a format string to output the result of each iteration of a loop:

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();
    }
}

One overload of the WriteLine method takes a format string as a parameter. Therefore, you can just embed a format string literal without an explicit call to the method. However, if you use the WriteLine method to display debug output in the Visual Studio Output window, you have to explicitly call the Format method because WriteLine only accepts a string, not a format string. For more information about format strings, see Formatting Types.

Substrings

A substring is any sequence of characters that is contained in a string. Use the Substring method to create a new string from a part of the original string. You can search for one or more occurrences of a substring by using the IndexOf method. Use the Replace method to replace all occurrences of a specified substring with a new string. Like the Substring method, Replace actually returns a new string and does not modify the original string. For more information, see How to: Search Strings Using String Methods (C# Programming Guide) and How to: Modify String Contents (C# Programming Guide).

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-based 
int index = s3.IndexOf("C");
// index = 7

Accessing Individual Characters

You can use array notation with an index value to acquire read-only access to individual characters, as in the following example:

string s5 = "Printing backwards";

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

If the String methods do not provide the functionality that you must have to modify individual characters in a string, you can use a StringBuilder object to modify the individual chars "in-place", and then create a new string to store the results by using the StringBuilder methods. In the following example, assume that you must modify the original string in a particular way and then store the results for future use:

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

An empty string is an instance of a System.String object that contains zero characters. Empty strings are used often in various programming scenarios to represent a blank text field. You can call methods on empty strings because they are valid System.String objects. Empty strings are initialized as follows:

string s = String.Empty;

By contrast, a null string does not refer to an instance of a System.String object and any attempt to call a method on a null string causes a NullReferenceException. However, you can use null strings in concatenation and comparison operations with other strings. The following examples illustrate some cases in which a reference to a null string does and does not cause an exception to be thrown:

static void Main()
{
    string str = "hello";
    string nullStr = null;
    string emptyStr = "";

    string tempStr = str + nullStr; // tempStr = "hello" 
    bool b = (emptyStr == nullStr);// b = false; 
    string newStr = emptyStr + nullStr; // creates a new empty string 
    int len = nullStr.Length; // throws NullReferenceException

}

Using StringBuilder for Fast String Creation

String operations in .NET are highly optimized and in most cases do not significantly impact performance. However, in some scenarios such as tight loops that are executing many hundreds or thousands of times, string operations can affect performance. The StringBuilder class creates a string buffer that offers better performance if your program performs many string manipulations. The StringBuilder string also enables you to reassign individual characters, something the built-in string data type does not support. This code, for example, changes the content of a string without creating a new string:

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

In this example, a StringBuilder object is used to create a string from a set of numeric types:

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
    }
}

Strings, Extension Methods and LINQ

Because the String type implements IEnumerable<T>, you can use the extension methods defined in the Enumerable class on strings. To avoid visual clutter, these methods are excluded from IntelliSense for the String type, but they are available nevertheless. You can also use LINQ query expressions on strings. For more information, see LINQ and Strings.

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