.net barcode generator Chars, Strings, and Working with Text in Visual Studio .NET

Maker Denso QR Bar Code in Visual Studio .NET Chars, Strings, and Working with Text

14 Chars, Strings, and Working with Text
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As does the Intern method, the IsInterned method takes a String and looks it up in the internal hash table . If a matching string is in the hash table, IsInterned returns a reference to the interned string object . If a matching string isn t in the hash table, however, IsInterned returns null; it doesn t add the string to the hash table . By default, when an assembly is loaded, the CLR interns all of the literal strings described in the assembly s metadata . Microsoft learned that this hurts performance significantly due to the additional hash table lookups, so it is now possible to turn this feature off . If an assembly is marked with a System.Runtime.CompilerServices.CompilationRelaxationsAttribute specifying the
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System.Runtime.CompilerServices.CompilationRelaxations.NoStringInterning
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flag value, the CLR may, according to the ECMA specification, choose not to intern all of the strings defined in that assembly s metadata . Note that, in an attempt to improve your application s performance, the C# compiler always specifies this attribute/flag whenever you compile an assembly . Even if an assembly has this attribute/flag specified, the CLR may choose to intern the strings, but you should not count on this . In fact, you really should never write code that relies on strings being interned unless you have written code that explicitly calls the String s Intern method yourself . The following code demonstrates string interning:
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String s1 = "Hello"; String s2 = "Hello"; Console.WriteLine(Object.ReferenceEquals(s1, s2)); s1 = String.Intern(s1); s2 = String.Intern(s2); Console.WriteLine(Object.ReferenceEquals(s1, s2));
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// Should be 'False'
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// 'True'
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In the first call to the ReferenceEquals method, s1 refers to a "Hello" string object in the heap, and s2 refers to a different "Hello" string object in the heap . Since the references are different, False should be displayed . However, if you run this on version 4 .0 of the CLR, you ll see that True is displayed . The reason is because this version of the CLR chooses to ignore the attribute/flag emitted by the C# compiler, and the CLR interns the literal "Hello" string when the assembly is loaded into the AppDomain . This means that s1 and s2 refer to the single "Hello" string in the heap . However, as mentioned previously, you should never write code that relies on this behavior because a future version of the CLR might honor the attribute/flag and not intern the "Hello" string . In fact, version 4 .0 of the CLR does honor the attribute/flag when this assembly s code has been compiled using the NGen .exe utility . Before the second call to the ReferenceEquals method, the "Hello" string has been explicitly interned, and s1 now refers to an interned "Hello" . Then by calling Intern again, s2 is set to refer to the same "Hello" string as s1 . Now, when ReferenceEquals is called the second time, we are guaranteed to get a result of True regardless of whether the assembly was compiled with the attribute/flag .
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So now, let s look at an example to see how you can use string interning to improve performance and reduce memory usage . The NumTimesWordAppearsEquals method below takes two arguments: a word and an array of strings in which each array element refers to a single word . This method then determines how many times the specified word appears in the wordlist and returns this count:
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private static Int32 NumTimesWordAppearsEquals(String word, String[] wordlist) { Int32 count = 0; for (Int32 wordnum = 0; wordnum < wordlist.Length; wordnum++) { if (word.Equals(wordlist[wordnum], StringComparison.Ordinal)) count++; } return count; }
As you can see, this method calls String s Equals method, which internally compares the strings individual characters and checks to ensure that all characters match . This comparison can be slow . In addition, the wordlist array might have multiple entries that refer to multiple String objects containing the same set of characters . This means that multiple identical strings might exist in the heap and are surviving ongoing garbage collections . Now, let s look at a version of this method that was written to take advantage of string interning:
private static Int32 NumTimesWordAppearsIntern(String word, String[] wordlist) { // This method assumes that all entries in wordlist refer to interned strings. word = String.Intern(word); Int32 count = 0; for (Int32 wordnum = 0; wordnum < wordlist.Length; wordnum++) { if (Object.ReferenceEquals(word, wordlist[wordnum])) count++; } return count; }
This method interns the word and assumes that the wordlist contains references to interned strings . First, this version might be saving memory if a word appears in the wordlist multiple times because, in this version, wordlist would now contain multiple references to the same single String object in the heap . Second, this version will be faster because determining if the specified word is in the array is simply a matter of comparing pointers . Although the NumTimesWordAppearsIntern method is faster than the NumTimesWordAppearsEquals method, the overall performance of the application might be slower when using the NumTimesWordAppearsIntern method because of the time it takes to intern all of the strings when they were added to the wordlist array (code not shown) . The NumTimesWordAppearsIntern method will really show its performance and memory improvement if the application needs to call the method multiple times using the same wordlist . The point of this discussion is to make it clear that string interning is useful, but it
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