248: Accessing Outside the Bounds of an Array, Throwing an Exception in Visual C#.NET

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Listing 248: Accessing Outside the Bounds of an Array, Throwing an Exception
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string languages = new string[9]; // RUNTIME ERROR: index out of bounds should // be 8 for the last element languages[4] = languages[9];
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The Length member returns the number of items in the array, not the highest index The Length member for the languages variable is 9, but the highest index for the languages variable is 8, because that is how far it is from the start
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Language Contrast: C++ Buffer Overflow Bugs
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Unmanaged C++ does not always check whether you overstep the bounds on an array Not only can this be difficult to debug, but making this mistake can also result in a potential security error called a buffer overrun In contrast, the Common Language Runtime protects all C# (and Managed C++) code from overstepping array bounds, virtually eliminating the possibility of a buffer overrun issue in managed code
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It is a good practice to use Length in place of the hardcoded array size To use Length as an index, for example, it is necessary to subtract 1 to avoid an out-of-bounds error (see Listing 249)
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Listing 249: Using Length - 1 in the Array Index
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string languages = new string[9]; languages[4] = languages[languagesLength - 1];
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To avoid overstepping the bounds on an array use a length check to verify it has a length greater than 0 as well as using Length 1 in place of a hardcoded value when accessing the last item in the array (see Listing 249) Length returns the total number of elements in an array Therefore, if you had a multidimensional array such as bool cells[,,] of size 2 3 3, Length would return the total number of elements, 18 For a jagged array, Length returns the number of elements in the first array a jagged array is an array of arrays, so Length evaluates only the outside, containing array and returns its element count, regardless of what is inside the internal arrays
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Arrays include additional methods for manipulating the elements within the array These include Sort(), BinarySearch(), Reverse(), and Clear() (see Listing 250)
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Listing 250: Additional Array Methods
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class ProgrammingLanguages { static void Main() { string[] languages = new string[]{ "C#", "COBOL", "Java", "C++", "Visual Basic", "Pascal", "Fortran", "Lisp", "J#"}; SystemArraySort(languages); searchString = "COBOL";
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index = SystemArrayBinarySearch( languages, searchString); SystemConsoleWriteLine( "The wave of the future, {0}, is at index {1}", searchString, index); SystemConsoleWriteLine(); SystemConsoleWriteLine("{0,-20}{1,-20}", "First Element", "Last Element"); SystemConsoleWriteLine("{0,-20}{1,-20}", "-------------", "------------"); SystemConsoleWriteLine("{0,-20}{1,-20}", languages[0], languages[languagesLength-1]); SystemArrayReverse(languages); SystemConsoleWriteLine("{0,-20}{1,-20}", languages[0], languages[languagesLength-1]); // Note this does not remove all items from the array // Rather it sets each item to the type s default value SystemArrayClear(languages, 0, languagesLength); SystemConsoleWriteLine("{0,-20}{1,-20}", languages[0], languages[languagesLength-1]); SystemConsoleWriteLine( "After clearing, the array size is: {0}", languagesLength); } }
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The results of Listing 250 are shown in Output 220
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OUTPUT 220:
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The wave of the future, COBOL, is at index 1 First Element ------------C# Visual Basic Last Element -----------Visual Basic C#
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After clearing, the array size is: 9
Access to these methods is on the SystemArray class For the most part, using these methods is self-explanatory, except for two noteworthy items
Arrays Before using the BinarySearch() method, it is important to sort the
array If values are not sorted in increasing order, then the incorrect index may be returned If the search element does not exist, then the value returned is negative (Using the complement operator, ~index, returns the first index, if any, that is larger than the searched value)
The Clear() method does not remove elements of the array and does
not set the length to zero The array size is fixed and cannot be modified Therefore, the Clear() method sets each element in the array to its default value (false, 0, or null) This explains why ConsoleWriteLine() creates a blank line when writing out the array after Clear() is called
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