Using string's Length Member in Visual C#.NET

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Listing 213 Using string's Length Member
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classPalindromeLength { static void Main() { string palindrome; SystemConsoleWrite("Enter a palindrome: "); palindrome = SystemConsoleReadLine(); SystemConsoleWriteLine( "The palindrome, \"{0}\" is {1} characters", palindrome, palindromeLength); } }
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Output 211
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Enter a palindrome: Never odd or even The palindrome, "Never odd or even" is 17 characters
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The length for a string cannot be set directly; it is calculated from the number of characters in the string Furthermore, the length of a string cannot change because a string is immutable
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Strings Are Immutable
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The key characteristic of the string type is the fact that it is immutable A string variable can be assigned an entirely new value, but for performance reasons, there is no facility for modifying the contents of a string It is not possible, therefore, to convert a string to all uppercase letters It is trivial to create a new string that is composed of an uppercase version of the old string, but the old string is not modified in the process Consider Listing 214 as an example
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Listing 214 Error; string Is Immutable
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classUppercase { static void Main() { string text; SystemConsoleWrite("Enter text: "); text = SystemConsoleReadLine(); // UNEXPECTED: Does not convert text to uppercase textToUpper(); SystemConsoleWrite-Line(text); } }
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Output 212 shows the results of Listing 214
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Output 212
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Enter text: This is a test of the emergency broadcast system This is a test of the emergency broadcast system
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At a glance, it would appear that textToUpper() should convert the characters within text to uppercase However, strings are immutable and, therefore, textToUpper() will make no such modification Instead, textToUpper() returns a new string that needs to be saved into a variable or passed to SystemConsoleWriteLine() directly The corrected code is shown in Listing 215, and its output is shown in Output 213
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Listing 215 Working with Strings
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classUppercase { static void Main() { string text, uppercase; SystemConsoleWrite("Enter text: "); text = SystemConsoleReadLine(); // Return a new string in uppercase uppercase = textToUpper(); SystemConsoleWriteLine(uppercase); } }
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Output 213
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Enter text: This is a test of the emergency broadcast system THIS IS A TEST OF THE EMERGENCY BROADCAST SYSTEM
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If the immutability of a string is ignored, mistakes similar to those shown in Listing 214 can occur with other string methods as well To actually change the value in text, assign the value from ToUpper() back into text, as in the following: text = textToUpper();
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SystemTextStringBuilder
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If considerable string modification is needed, such as when constructing a long string in multiple steps, you should use the data type SystemTextStringBuilder rather than string SystemTextStringBuilder includes methods such as Append() , AppendFormat(), Insert() , Remove() , and Replace(), some of which also appear on string The key difference, however, is that on SystemTextStringBuilder, these methods will modify the data in the variable, and will not simply return a new string
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null and void
Two additional keywords relating to types are null and voidnull is a literal value used to indicate that the data type (specifically, a reference type) is assigned nothing void is used to indicate the absence of a type or the absence of any value altogether
null
null can also be used as a type of string "literal" null indicates that a variable is set to nothing Only reference types can be assigned the value null The only reference type covered so far in this book is string; 5 covers the topic of reference types in detail For now, suffice it to say that a
reference type contains a pointer, an address, or a reference to a location in memory that is different from where the actual data resides Code that sets a variable to null explicitly assigns the reference to point at nothing In fact, it is even possible to check whether a reference type points to nothing Listing 216 demonstrates assigning null to a string variable
Listing 216 Assigning null to a String
static void Main() { string faxNumber; // // Clear the value of faxNumber faxNumber = null;
// }
It is important to note that assigning the value null to a reference type is distinct from not assigning it at all In other words, a variable that has been assigned null has still been set, and a variable with no assignment has not been set and therefore will often cause a compile error if used prior to assignment Assigning the value null to a string is distinctly different from assigning an empty string, "" null indicates that the variable has no value "" indicates that there is a value: an empty string This type of distinction can be quite useful For example, the programming logic could interpret a faxNumber of null to mean that the fax number is unknown, while a faxNumber value of "" could indicate that there is no fax number