String Basics in Visual Basic
The String data type represents a series of characters (each representing in turn an instance of the Char data type). This topic introduces the basic concepts of strings in Visual Basic.
String Variables
An instance of a string can be assigned a literal value that represents a series of characters. For example:
Dim MyString As String
MyString = "This is an example of the String data type"
A String variable can also accept any expression that evaluates to a string. Examples are shown below:
Dim OneString As String
Dim TwoString As String
OneString = "one, two, three, four, five"
' Evaluates to "two".
TwoString = OneString.Substring(5, 3)
OneString = "1"
' Evaluates to "11".
TwoString = OneString & "1"
Any literal that is assigned to a String variable must be enclosed in quotation marks (""). This means that a quotation mark within a string cannot be represented by a quotation mark. For example, the following code causes a compiler error:
Dim myString As String
' This line would cause an error.
' myString = "He said, "Look at this example!""
This code causes an error because the compiler terminates the string after the second quotation mark, and the remainder of the string is interpreted as code. To solve this problem, Visual Basic interprets two quotation marks in a string literal as one quotation mark in the string. The following example demonstrates the correct way to include a quotation mark in a string:
' The value of myString is: He said, "Look at this example!"
myString = "He said, ""Look at this example!"""
In the preceding example, the two quotation marks preceding the word Look become one quotation mark in the string. The three quotation marks at the end of the line represent one quotation mark in the string and the string termination character.
String literals can contain multiple lines:
Dim x = "hello
world"
The resulting string contains newline sequences that you used in your string literal (vbcr, vbcrlf, etc.). You no longer need to use the old workaround:
Dim x = <xml><![CDATA[Hello
World]]></xml>.Value
Characters in Strings
A string can be thought of as a series of Char values, and the String type has built-in functions that allow you to perform many manipulations on a string that resemble the manipulations allowed by arrays. Like all array in .NET Framework, these are zero-based arrays. You may refer to a specific character in a string through the Chars property, which provides a way to access a character by the position in which it appears in the string. For example:
Dim myString As String = "ABCDE"
Dim myChar As Char
' The value of myChar is "D".
myChar = myString.Chars(3)
In the above example, the Chars property of the string returns the fourth character in the string, which is D, and assigns it to myChar. You can also get the length of a particular string through the Length property. If you need to perform multiple array-type manipulations on a string, you can convert it to an array of Char instances using the ToCharArray function of the string. For example:
Dim myString As String = "abcdefghijklmnop"
Dim myArray As Char() = myString.ToCharArray
The variable myArray now contains an array of Char values, each representing a character from myString.
The Immutability of Strings
A string is immutable, which means its value cannot be changed once it has been created. However, this does not prevent you from assigning more than one value to a string variable. Consider the following example:
Dim myString As String = "This string is immutable"
myString = "Or is it?"
Here, a string variable is created, given a value, and then its value is changed.
More specifically, in the first line, an instance of type String is created and given the value This string is immutable. In the second line of the example, a new instance is created and given the value Or is it?, and the string variable discards its reference to the first instance and stores a reference to the new instance.
Unlike other intrinsic data types, String is a reference type. When a variable of reference type is passed as an argument to a function or subroutine, a reference to the memory address where the data is stored is passed instead of the actual value of the string. So in the previous example, the name of the variable remains the same, but it points to a new and different instance of the String class, which holds the new value.
