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String Manipulation

Visual Studio .NET 2003

The String class of the .NET framework provides many built-in methods to facilitate the comparison and manipulation of strings. It is now a trivial matter to get data about a string, or to create new strings by manipulating current strings. The Visual Basic .NET language also has inherent methods that duplicate many of these functionalities.

Types of String Manipulation Methods

In this section you will read about several different ways to analyze and manipulate your strings. Some of the methods are a part of the Visual Basic language, and others are inherent in the String class.

Visual Basic .NET methods are used as inherent functions of the language. They may be used without qualification in your code. The following example shows typical use of a Visual Basic .NET string-manipulation command:

Dim aString As String = "SomeString"
Dim bString As String
bString = Mid(aString, 3, 3)

In this example, the Mid function performs a direct operation on aString and assigns the value to bString.

You can also manipulate strings with the methods of the String class. There are two types of methods in String: shared methods and instance methods.

A shared method is a method that stems from the String class itself and does not require an instance of that class to work. These methods can be qualified with the name of the class (String) rather than with an instance of the String class. For example:

Dim aString As String
bString = String.Copy("A literal string")

In the preceding example, the String.Copy method is a static method, which acts upon an expression it is given and assigns the resulting value to bString.

Instance methods, by contrast, stem from a particular instance of String and must be qualified with the instance name. For example:

Dim aString As String = "A String"
Dim bString As String
bString = aString.SubString(2,6) ' bString = "String"

In this example, the SubString method is a method of the instance of String (that is, aString). It performs an operation on aString and assigns that value to bString.

Nothing and Strings

The Visual Basic runtime and the .NET Framework evaluate Nothing differently when it comes to strings. Consider the following example:

Dim MyString As String = "This is my string"
Dim stringLength As Integer
' Explicitly set the string to Nothing.
MyString = Nothing
' stringLength = 0
stringLength = Len(MyString)
' This line, however, causes an exception to be thrown.
stringLength = MyString.Length

The Visual Basic .NET runtime evaluates Nothing as an empty string; that is, "". The .NET Framework, however, does not, and will throw an exception whenever an attempt is made to perform a string operation on Nothing.

Comparing Strings

You can compare two strings by using the String.Compare method. This is a static, overloaded method of the base string class. In its most common form, this method can be used to directly compare two strings based on their alphabetical sort order. This is similar to the Visual Basic StrComp Function function. The following example illustrates how this method is used:

Dim myString As String = "Alphabetical"
Dim secondString As String = "Order"
Dim result As Integer
result = String.Compare (myString, secondString)

This method returns an integer that indicates the relationship between the two compared strings based on the sorting order. A positive value for the result indicates that the first string is greater than the second string. A negative result indicates the first string is smaller, and zero indicates equality between the strings. Any string, including an empty string, evaluates to greater than a null reference.

Additional overloads of the String.Compare method allow you to indicate whether or not to take case or culture formatting into account, and to compare substrings within the supplied strings. For more information on how to compare strings, see String.Compare Method. Related methods include String.CompareOrdinal Method and String.CompareTo Method.

Searching for Strings Within Your Strings

There are times when it is useful to have data about the characters in your string and the positions of those characters within your string. A string can be thought of as an array of characters (Char instances); you can retrieve a particular character by referencing the index of that character through the Chars property. For example:

Dim myString As String = "ABCDE"
Dim myChar As Char
myChar = myString.Chars(3) ' myChar = "D"

You can use the String.IndexOf method to return the index where a particular character is encountered, as in the following example:

Dim myString As String = "ABCDE"
Dim myInteger As Integer
myInteger = myString.IndexOf("D")  ' myInteger = 3

In the previous example, the IndexOf method of myString was used to return the index corresponding to the first instance of the character "C" in the string. IndexOf is an overloaded method, and the other overloads provide methods to search for any of a set of characters, or to search for a string within your string, among others. The Visual Basic .NET command InStr also allows you to perform similar functions. For more information of these methods, see String.IndexOf Method and InStr Function. You can also use the String.LastIndexOf Method to search for the last occurrence of a character in your string.

Creating New Strings from Old

When using strings, you may want to modify your strings and create new ones. You may want to do something as simple as convert the entire string to uppercase, or trim off trailing spaces; or you may want to do something more complex, such as extracting a substring from your string. The System.String class provides a wide range of options for modifying, manipulating, and making new strings out of your old ones.

To combine multiple strings, you can use the concatenation operators (& or +). You can also use the String.Concat Method to concatenate a series of strings or strings contained in objects. An example of the String.Concat method follows:

Dim aString As String = "A"
Dim bString As String = "B"
Dim cString As String = "C"
Dim dString As String = "D"
Dim myString As String
' myString = "ABCD"
myString = String.Concat(aString, bString, cString, dString) 

You can convert your strings to all uppercase or all lowercase strings using either the Visual Basic .NET functions UCase Function and LCase Function or the String.ToUpper Method and String.ToLower Method methods. An example is shown below:

Dim myString As String = "UpPeR oR LoWeR cAsE"
Dim newString As String
' newString = "UPPER OR LOWER CASE"
newString = UCase(myString)
' newString = "upper or lower case"
newString = LCase(myString)
' newString = "UPPER OR LOWER CASE"
newString = myString.ToUpper
' newString = "upper or lower case"
newString = myString.ToLower

The String.Format method and the Visual Basic .NET Format command can generate a new string by applying formatting to a given string. For information on these commands, see Format Function or String.Format Method.

You may at times need to remove trailing or leading spaces from your string. For instance, you might be parsing a string that had spaces inserted for the purposes of alignment. You can remove these spaces using the String.Trim Method function, or the Visual Basic .NET Trim function. An example is shown:

Dim spaceString As String = _
"        This string will have the spaces removed        "
Dim oneString As String
Dim twoString As String
' This removes all trailing and leading spaces.
oneString = spaceString.Trim
' This also removes all trailing and leading spaces.
twoString = Trim(spaceString)

If you only want to remove trailing spaces, you can use the String.TrimEnd Method or the RTrim function, and for leading spaces you can use the String.TrimStart Method or the LTrim function. For more details, see LTrim, RTrim, and Trim Functions functions.

The String.Trim functions and related functions also allow you to remove instances of a specific character from the ends of your string. The following example trims all leading and trailing instances of the "#" character:

Dim myString As String = "#####Remove those!######"
Dim oneString As String
OneString = myString.Trim("#")

You can also add leading or trailing characters by using the String.PadLeft Method or the String.PadRight Method.

If you have excess characters within the body of your string, you can excise them by using the String.Remove Method, or you can replace them with another character using the String.Replace Method. For example:

Dim aString As String = "This is My Str@o@o@ing"
Dim myString As String
Dim anotherString As String
' myString = "This is My String"
myString = aString.Remove(14, 5)
' anotherString = "This is Another String"
anotherString = myString.Replace("My", "Another")

You can use the String.Replace method to replace either individual characters or strings of characters. The Visual Basic .NET Mid Statement can also be used to replace an interior string with another string.

You can also use the String.Insert Method to insert a string within another string, as in the following example:

Dim aString As String = "This is My Stng"
Dim myString As String
' Results in a value of "This is My String".
myString = aString.Insert(13, "ri")

The first parameter that the String.Insert method takes is the index of the character the string is to be inserted after, and the second parameter is the string to be inserted.

You can concatenate an array of strings together with a separator string by using the String.Join Method. Here is an example:

Dim shoppingItem(2) As String
Dim shoppingList As String
shoppingItem(0) = "Milk"
shoppingItem(1) = "Eggs"
shoppingItem(2) = "Bread"
shoppingList = String.Join(",", shoppingItem)

The value of shoppingList after running this code is "Milk,Eggs,Bread". Note that if your array has empty members, the method still adds a separator string between all the empty instances in your array.

You can also create an array of strings from a single string by using the String.Split Method. The following example demonstrates the reverse of the previous example: it takes a shopping list and turns it into an array of shopping items. The separator in this case is an instance of the Char data type; thus it is appended with the literal type character c.

Dim shoppingList As String = "Milk,Eggs,Bread"
Dim shoppingItem(2) As String
shoppingItem = shoppingList.Split(","c)

The Visual Basic .NET Mid Function can be used to generate substrings of your string. The following example shows this functions in use:

Dim aString As String = "Left Center Right"
Dim rString, lString, mString As String
' rString = "Right"
rString = Mid(aString, 13)
' lString = "Left"
lString = Mid(aString, 1, 4)
' mString = "Center"
mString = Mid(aString, 6,6)

Substrings of your string can also be generated using the String.Substring Method. This method takes two arguments: the character index where the substring is to start, and the length of the substring. The String.Substring method operates much like the Mid function. An example is shown below:

Dim aString As String = "Left Center Right"
Dim subString As String
' subString = "Center"
subString = aString.SubString(5,6)

There is one very important difference between the String.SubString method and the Mid function. The Mid function takes an argument that indicates the character position for the substring to start, starting with position 1. The String.SubString method takes an index of the character in the string at which the substring is to start, starting with position 0. Thus, if you have a string "ABCDE", the individual characters are numbered 1,2,3,4,5 for use with the Mid function, but 0,1,2,3,4 for use with the System.String function.

See Also

Strings | String Class | Mid Function | Mid Statement | Nothing | StrComp Function | InStr Function | UCase Function | LCase Function | Format Function | LTrim, RTrim, and Trim Functions

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