Question

In this part of the C# tutorial, we will work with string data in more detail. Strings are very important in computer languages. That is why we dedicate a whole chapter to working with strings in C#.

A string is a sequence of characters. In C#, a string is a sequence of Unicode characters. It is a data type which stores a sequence of data values, usually bytes, in which elements usually stand for characters according to a character encoding. When a string appears literally in the source code, it is known as a string literal.

Strings are objects. There are two basic classes for working with strings:

• System.String
• System.Text.StringBuilder

The String is an immutable sequence of characters. The StringBuilder is a mutable sequence of characters.

In C#, string is an alias for System.String . The string is a language keyword and the System.String is a .NET type.

Initializing strings

There are multiple ways of creating strings, both immutable and mutable. We will show a few of them.

using System;
using System.Text;

public class Initialization
{
  static void Main() 
  {
    char[] cdb = {'M', 'y', 'S', 'q', 'l'};

    string lang = "C#";
    String ide = "NetBeans";
    string db = new string(cdb);

    Console.WriteLine(lang);
    Console.WriteLine(ide);
    Console.WriteLine(db);
    
    StringBuilder sb1 = new StringBuilder(lang);
    StringBuilder sb2 = new StringBuilder();
    sb2.Append("Fields");
    sb2.Append(" of ");
    sb2.Append("glory");
    
    Console.WriteLine(sb1);
    Console.WriteLine(sb2);
  }
}

The example shows a few ways of creating System.String and System.Text.StringBuilder objects.

using System.Text;

This statement enables to use the System.Text.StringBuilder type without qualification.

string lang = "C#";
String ide = "NetBeans";

The most common way is to create a string object from a string literal.

string db = new string(cdb);

Here we create a string object from an array of characters. The string is an alias for the System.String .

StringBuilder sb1 = new StringBuilder(lang);

A StringBuilder object is created from a String .

StringBuilder sb2 = new StringBuilder();
sb2.Append("Fields");
sb2.Append(" of ");
sb2.Append("glory");

We create an empty StringBuilder object. We append three strings into the object.

$ ./initialization.exe 
C#
NetBeans
MySql
C#
Fields of glory

Running the example gives this result.

Strings are objects

Strings are objects. They are reference types. Strings are instances of the System.String or System.Text.StringBuilder class. Since they are objects, they have multiple methods available for doing various work.

using System;

public class CSharpApp
{
  static void Main() 
  {
    string lang = "Java";
         
    string bclass = lang.GetType().Name;
    Console.WriteLine(bclass);

    string parclass = lang.GetType().BaseType.Name;
    Console.WriteLine(parclass);

    if (lang.Equals(String.Empty)) 
    {
      
      Console.WriteLine("The string is empty");
    } else 
    {
      
      Console.WriteLine("The string is not empty");
    }
    
    int l = lang.Length;    
    Console.WriteLine("The string has " + l + " characters");
  }
}

In this program, we demonstrate that strings are objects. Objects must have a class name, a parent class and they must also have some methods that we can call or properties to access.

string lang = "Java";

An object of System.String type is created.

string bclass = lang.GetType().Name;
Console.WriteLine(bclass);

We determine the class name of the object to which the lang variable refers.

string parclass = lang.GetType().BaseType.Name;
Console.WriteLine(parclass);

A parent class of our object is received. All objects have at least one parent — the Object .

if (lang.Equals(String.Empty)) 
{  
  Console.WriteLine("The string is empty");
} else 
{  
  Console.WriteLine("The string is not empty");
}

Objects have various methods. With the Equals() method we check if the string is empty.

int l = lang.Length;   

The Length method returns the size of the string.

$ ./stringobjects.exe 
String
Object
The string is not empty
The string has 4 characters

This is the output of the stringobjects.exe program.

Mutable & immutable strings

The String is a sequence of immutable characters, while the StringBuilder is a sequence of mutable characters. The next example will show the difference.

using System;
using System.Text;

public class CSharpApp
{
  static void Main() 
  {
    string name = "Jane";
    string name2 = name.Replace('J', 'K');
    string name3 = name2.Replace('n', 't');
    
    Console.WriteLine(name);
    Console.WriteLine(name3);
    
    StringBuilder sb = new StringBuilder("Jane");
    Console.WriteLine(sb);
    
    sb.Replace('J', 'K', 0, 1);
    sb.Replace('n', 't', 2, 1);
    
    Console.WriteLine(sb);
  }
}

Both objects have methods for replacing characters in a string.

string name = "Jane";
string name2 = name.Replace('J', 'K');
string name3 = name2.Replace('n', 't');

Calling a Replace() method on a String results in returning a new modified string. The original string is not changed.

sb.Replace('J', 'K', 0, 1);
sb.Replace('n', 't', 2, 1);

The Replace() method of a StringBuilder will replace a character at the given index with a new character. The original string is modified.

$ ./mutableimmutable.exe 
Jane
Kate
Jane
Kate

This is the output of the mutableimmutable.exe program.

Concatenating strings

Immutable strings can be added using the + operator or the Concat() method. They will form a new string which is a chain of all concatenated strings. Mutable strings have the Append() method which builds a string from any number of other strings.

using System;
using System.Text;

public class Concatenate
{
  static void Main() 
  {
    Console.WriteLine("Return" + " of " + "the king.");

    Console.WriteLine(string.Concat(string.Concat("Return", " of "), 
      "the king."));
    
    StringBuilder sb = new StringBuilder();
    sb.Append("Return");
    sb.Append(" of ");
    sb.Append("the king.");
    
    Console.WriteLine(sb);
  }
}

The example creates three sentences by adding strings.

Console.WriteLine("Return" + " of " + "the king.");

A new string is formed by using the + operator.

Console.WriteLine(string.Concat(string.Concat("Return", " of "), 
  "the king."));

The Concat() method concatenates two strings. The method is a static method of the System.String class.

StringBuilder sb = new StringBuilder();
sb.Append("Return");
sb.Append(" of ");
sb.Append("the king.");

A mutable object of the StringBuilder type is created by calling the Append() method three times.

$ ./concatenate.exe 
Return of the king.
Return of the king.
Return of the king.

This is the example output.

Using quotes

What if we wanted to display quotes, for example in a direct speech? In such a case, inner quotes must be escaped.

using System;

public class Quotes
{
  static void Main() 
  {
    Console.WriteLine("There are many stars.");
    Console.WriteLine("He said, \"Which one is your favourite?\"");
  }
}

We use the \ character to escape additional quotes.

$ ./quotes.exe 
There are many stars.
He said, "Which one is your favourite?"

This is the output of the quotes.exe program.

Multiline strings

It is possible to create a multiline string in C#.

using System;

public class Multiline
{
  static void Main() 
  {
    string multiString = @"I cheated myself
like I knew I would
I told ya, I was trouble
you know that I'm no good";
     
    Console.WriteLine(multiString);
  }
}

A multiline string can be created easily by using the @ character at the beginning of the string.

$ ./multiline.exe 
I cheated myself
like I knew I would
I told ya, I was trouble
you know that I'm no good

Comparing strings

We can compare two strings with the == operator.

using System;

public class CSharpApp
{
  static void Main() 
  {
    Console.WriteLine("12" == "12"); 
    Console.WriteLine("17" == "9");
    Console.WriteLine("aa" == "ab");
  }
}

In the example program, we compare strings.

$ ./compare.exe 
True
False
False

This is the output of the program.

There is a string.Compare() method, which compares two specified strings and returns an integer that indicates their relative position in the sort order. If the returned value is less than zero, the first string is less than the second. If it returns zero, both strings are equal. Finally, if the returned value is greater than zero, the first string is greater than the second.

using System;

public class CSharpApp
{
  static void Main() 
  {
    string str1 = "ZetCode";
    string str2 = "zetcode";
     
    Console.WriteLine(string.Compare(str1, str2, true));
    Console.WriteLine(string.Compare(str1, str2, false));
  }
}

There is an optional third ignoreCase argument. It determines if the case should be honored or not.

Console.WriteLine(string.Compare(str1, str2, true));

Compare two strings and ignore the case. This line prints 0 to the console.

String elements

A string is a sequence of characters. A character is a basic element of a string.

using System;

public class Elements
{
  static void Main() 
  {
    char[] crs = {'Z', 'e', 't', 'C', 'o', 'd', 'e' };
    String s = new String(crs);
    
    char c1 = s[0];
    char c2 = s[(s.Length-1)];
    
    Console.WriteLine(c1);
    Console.WriteLine(c2);
    
    int i1 = s.IndexOf('e');
    int i2 = s.LastIndexOf('e');
    
    Console.WriteLine("The first index of character e is " + i1);
    Console.WriteLine("The last index of character e is " + i2);
    
    Console.WriteLine(s.Contains("t"));
    Console.WriteLine(s.Contains("f"));
    
    char[] elements = s.ToCharArray();
    
    foreach (char el in elements) 
    {      
      Console.WriteLine(el);
    }    
  }
}

In the first example, we will work with an immutable string.

char[] crs = {'Z', 'e', 't', 'C', 'o', 'd', 'e' };
String s = new String(crs);

A new immutable string is formed from an array of characters.

char c1 = s[0];
char c2 = s[(s.Length-1)];

Using the array access notation, we we get the first and the last char value of the string.

int i1 = s.IndexOf('e');
int i2 = s.LastIndexOf('e');

With the above methods, we get the first and the last occurrence of the character 'e'.

Console.WriteLine(s.Contains("t"));
Console.WriteLine(s.Contains("f"));

With the Contains() method we check if the string contains the 't' character. The method returns a boolean value.

char[] elements = s.ToCharArray();

foreach (char el in elements) 
{  
  Console.WriteLine(el);
} 

The ToCharArray() method creates a character array from the string. We go through the array and print each of the characters.

$ ./stringelemetns.exe 
Z
e
The first index of character e is 1
The last index of character e is 6
True
False
Z
e
t
C
o
d
e

This is the example output.

In the second example, we will work with the elements of a mutable string.

using System;
using System.Text;

public class StringBuilderElements
{
  static void Main() 
  {
    StringBuilder sb = new StringBuilder("Misty mountains");
    Console.WriteLine(sb);
    
    sb.Remove(sb.Length-1, 1);
    Console.WriteLine(sb);
    
    sb.Append('s');
    Console.WriteLine(sb);
    
    sb.Insert(0, 'T');
    sb.Insert(1, 'h');
    sb.Insert(2, 'e');
    sb.Insert(3, ' ');
    Console.WriteLine(sb);
    
    sb.Replace('M', 'm', 4, 1);
    Console.WriteLine(sb); 
  }
}

A mutable string is formed. We modify the contents of the string by deleting, appending, inserting, and replacing characters.

sb.Remove(sb.Length-1, 1);

This line deletes the last character.

sb.Append('s');

The deleted character is appended back to the string.

sb.Insert(0, 'T');
sb.Insert(1, 'h');
sb.Insert(2, 'e');
sb.Insert(3, ' ');

We insert four characters at the beginning of the string.

sb.Replace('M', 'm', 4, 1);

Finally, we replace a character at index 4.

$ ./stringbuilderelements.exe 
Misty mountains
Misty mountain
Misty mountains
The Misty mountains
The misty mountains

From the output we can see how the mutable string is changing.

String methods

C# has useful built-in methods that can be used for working with strings.

The Join() and Split() are very handy methods.

using System;

public class JoinSplit
{
  static void Main() 
  {
    string[] items = new string[] {"C#", "Visual Basic", 
      "Java", "Perl"};

    string langs = string.Join(",", items);
    Console.WriteLine(langs);

    string[] ls = langs.Split(',');

    foreach (string lang in ls) 
    {
      Console.WriteLine(lang);
    }
  }
}

In our program, we will join and split strings.

string[] items = new string[] {"C#", "Visual Basic", 
  "Java", "Perl"};

This is an array of strings. These strings are going to be joined.

string langs = string.Join(",", items);

All words from the array are joined. We build one string from them. There will be a comma character between each two words.

string[] ls = langs.Split(',');

As a reverse operation, we split the langs string. The Split() method returns an array of words, delimited by a character. In our case it is a comma character.

foreach (string lang in ls) 
{
  Console.WriteLine(lang);
}

We go through the array and print its elements.

$ ./joinsplit.exe 
C#,Visual Basic,Java,Perl
C#
Visual Basic
Java
Perl

This is the output of the example.

using System;

public class StringMethods
{
  static void Main() 
  {
    string str = "Determination";

    Console.WriteLine(str.Contains("e"));
    Console.WriteLine(str.IndexOf("e"));
    Console.WriteLine(str.LastIndexOf("i"));

    Console.WriteLine(str.ToUpper());
    Console.WriteLine(str.ToLower());
  }
}

We introduce five string methods in the above example.

Console.WriteLine(str.Contains("e"));

The Contains() method returns True if the string contains a specific character.

Console.WriteLine(str.IndexOf("e"));

The IndexOf() returns the first index of a letter in the string.

Console.WriteLine(str.LastIndexOf("i"));

The LastIndexOf() methods returns the last index of a letter in a string.

Console.WriteLine(str.ToUpper());
Console.WriteLine(str.ToLower());

Letters of the string are converted to uppercase with the ToUpper() method and to lowercase with the ToLower() method.

$ ./strmethods.exe 
True
1
10
DETERMINATION
determination

Running the program.

Copy vs Clone

We will describe a difference between two methods: Copy() and Clone() . The Copy() method creates a new instance of string with the same value as a specified string. The Clone() method returns a reference to the string which is being cloned. It is not an independent copy of the string on the Heap. It is another reference on the same string.

using System;

public class CopyClone
{
  static void Main() 
  {
    string str = "ZetCode";
    
    string cloned = (string) str.Clone();
    string copied = string.Copy(str);

    Console.WriteLine(str.Equals(cloned)); // prints true
    Console.WriteLine(str.Equals(copied)); // prints true

    Console.WriteLine(ReferenceEquals(str, cloned)); // prints true
    Console.WriteLine(ReferenceEquals(str, copied)); // prints false
  }
}

Our example demonstrates the difference between the two methods.

string cloned = (string) str.Clone();
string copied = string.Copy(str);

The string value is cloned and copied.

Console.WriteLine(str.Equals(cloned)); // prints true
Console.WriteLine(str.Equals(copied)); // prints true

The Equals() method determines whether two string objects have the same value. The contents of all three strings are the same.

Console.WriteLine(ReferenceEquals(str, cloned)); // prints true
Console.WriteLine(ReferenceEquals(str, copied)); // prints false

The ReferenceEquals() method compares the two reference objects. Therefore comparing a copied string to the original string returns false. Because they are two distinct objects.

Formatting strings

In the next examples, we will format strings. The .NET Framework has a feature called composite formatting. It is supported by Format() and WriteLine() methods. A method takes a list of objects and a composite format string as input. The format string consists of fixed string and some format items. These format items are indexed placeholders which correspond to the objects in the list.

The format item has the following syntax:

{index[,length][:formatString]}

The index component is mandatory. It is a number starting from 0 that refers to an item from the list of objects. Multiple items can refer to the same element of the list of objects. An object is ignored if it is not referenced by a format item. If we refer outside the bounds of the list of objects, a runtime exception is thrown.

The length component is optional. It is the minimum number of characters in the string representation of the parameter. If positive, the parameter is right-aligned; if negative, it is left-aligned. If it is specified, there must by a colon separating the index and the length.

The formatString is optional. It is a string that formats a value is a specific way. It can be used to format dates, times, numbers or enumerations.

Here we show, how to work with length component of the format items. We print three columns of numbers to the terminal. Left, middle and right aligned.

using System;

public class Format1
{
  static void Main() 
  {
    int oranges = 2; 
    int apples  = 4;
    int bananas = 3;
  
    string str1 = "There are {0} oranges, {1} apples and " + 
      "{2} bananas";

    string str2 = "There are {1} oranges, {2} bananas and " + 
      "{0} apples";
      
    Console.WriteLine(str1, oranges, apples, bananas);
    Console.WriteLine(str2, apples, oranges, bananas);
  }
}

We print a simple message to the console. We use only index component of the format item.

string str1 = "There are {0} oranges, {1} apples and " + 
  "{2} bananas";

The {0} , {1} , and {2} are format items. We specify the index component. Other components are optional.

Console.WriteLine(str1, oranges, apples, bananas);

Now we put together the composite formatting. We have the string and the list of objects (oranges, apples, bananas). The {0} format item refers to the oranges. The WriteLine() method replaces the {0} format item with the contents of the oranges variable.

string str2 = "There are {1} oranges, {2} bananas and " + 
  "{0} apples";

The order of the format items referring to the objects is notation important.

$ ./format1.exe 
There are 2 oranges, 4 apples and 3 bananas
There are 2 oranges, 3 bananas and 4 apples

We can see the outcome of the format1.exe program.

The next example will format numeric data.

using System;

public class Format2
{
  static void Main() 
  {
    Console.WriteLine("{0}  {1, 12}", 
      "Decimal", "Hexadecimal");

    Console.WriteLine("{0:D}  {1,8:X}",  
      502, 546);
    Console.WriteLine("{0:D}  {1,8:X}", 
      345, 765);
    Console.WriteLine("{0:D}  {1,8:X}", 
      320, 654);
    Console.WriteLine("{0:D}  {1,8:X}", 
      120, 834);
    Console.WriteLine("{0:D}  {1,8:X}", 
      620, 454);
  }
}

We print numbers in a decimal and hexadecimal format. We also align the numbers using the length component.

Console.WriteLine("{0:D}  {1,8:X}",  
  502, 546);

The {0:D} format item specifies, the first item from the list of supplied objects will be taken and formatted in the decimal format. The {1,8:X} format item takes the second item. Formats it in the hexadecimal format :X . And the string length will be 8 characters 8 . Because the number has only three characters, it is right aligned and padded with empty strings.

$ ./format2.exe 
Decimal   Hexadecimal
502     222
345     2FD
320     28E
120     342
620     1C6

Running the example we get this outcome.

The last two examples will format numeric and date data.

using System;

public class Format3
{
  static void Main() 
  {
    Console.WriteLine(string.Format("Number: {0:N}", 126));
    Console.WriteLine(string.Format("Scientific: {0:E}", 126));
    Console.WriteLine(string.Format("Currency: {0:C}", 126));
    Console.WriteLine(string.Format("Percent: {0:P}", 126));
    Console.WriteLine(string.Format("Hexadecimal: {0:X}", 126));
  }
}

The example demonstrates the standard formatting specifiers for numbers. Number 126 is printed in five different formats: normal, scientific, currency, percent and hexadecimal.

$ ./format3.exe 
Number: 126.00
Scientific: 1.260000E+002
Currency: $126.00
Percent: 12,600.00 %
Hexadecimal: 7E

This is the output of the format3.exe program.

Finally, we will format date and time data.

using System;

public class Format4
{
  static void Main() 
  {
    DateTime today = DateTime.Now;

    Console.WriteLine(string.Format("Short date: {0:d}", today));
    Console.WriteLine(string.Format("Long date: {0:D}", today));
    Console.WriteLine(string.Format("Short time: {0:t}", today));
    Console.WriteLine(string.Format("Long time: {0:T}", today));
    Console.WriteLine(string.Format("Month: {0:M}", today));
    Console.WriteLine(string.Format("Year: {0:Y}", today));
  }
}

The preceding example demonstrates the standard formatting specifiers for dates.

$ ./format4.exe 
Short date: 4/11/2011
Long date: Monday, April 11, 2011
Short time: 11:14 AM
Long time: 11:14:54 AM
Month: April 11
Year: April, 2011

This is the output of the example.

This part of the C# tutorial covered strings.