generate 2d barcode vb.net Part III Essential Types in C#

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an array . The second line allocates an array of 50 Control references; all of these references are initialized to null . Because Control is a reference type, creating the array creates only a bunch of references; the actual objects aren t created at this time . The address of this memory block is returned and saved in the variable myControls . Figure 16-1 shows how arrays of value types and arrays of reference types look in the managed heap .
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myControls
FIguRE 16-1 Arrays of value and reference types in the managed heap
In the figure, the Controls array shows the result after the following lines have executed:
myControls[1] myControls[2] myControls[3] myControls[46] myControls[48] myControls[49] = = = = = = new Button(); new TextBox(); myControls[2]; // Two elements refer to the same object. new DataGrid(); new ComboBox(); new Button();
Common Language Specification (CLS) compliance requires all arrays to be zero-based . This allows a method written in C# to create an array and pass the array s reference to code written in another language, such as Microsoft Visual Basic .NET . In addition, because zero-based arrays are, by far, the most common arrays, Microsoft has spent a lot of time optimizing their performance . However, the CLR does support non-zero based arrays even though their use is discouraged . For those of you who don t care about a slight performance penalty or crosslanguage portability, I ll demonstrate how to create and use non-zero based arrays later in this chapter . Notice in Figure 16-1 that each array has some additional overhead information associated with it . This information contains the rank of the array (number of dimensions), the lower bounds for each dimension of the array (almost always 0), and the length of each dimension .
16 Arrays
The overhead also contains the array s element type . I ll mention the methods that allow you to query this overhead information later in this chapter . So far, I ve shown examples demonstrating how to create single-dimensional arrays . When possible, you should stick with single-dimensional, zero-based arrays, sometimes referred to as SZ arrays, or vectors . Vectors give the best performance because you can use specific Intermediate Language (IL) instructions such as newarr, ldelem, ldelema, ldlen, and stelem to manipulate them . However, if you prefer to work with multi-dimensional arrays, you can . Here are some examples of multi-dimensional arrays:
// Create a two-dimensional array of Doubles. Double[,] myDoubles = new Double[10, 20]; // Create a three-dimensional array of String references. String[,,] myStrings = new String[5, 3, 10];
The CLR also supports jagged arrays, which are arrays of arrays . Zero-based, singledimensional jagged arrays have the same performance as normal vectors . However, accessing the elements of a jagged array means that two or more array accesses must occur . Here are some examples of how to create an array of polygons with each polygon consisting of an array of Point instances:
// Create a single-dimensional array of Point arrays. Point[][] myPolygons = new Point[3][]; // myPolygons[0] refers to an array of 10 Point instances. myPolygons[0] = new Point[10]; // myPolygons[1] refers to an array of 20 Point instances. myPolygons[1] = new Point[20]; // myPolygons[2] refers to an array of 30 Point instances. myPolygons[2] = new Point[30]; // Display the Points in the first polygon. for (Int32 x = 0; x < myPolygons[0].Length; x++) Console.WriteLine(myPolygons[0][x]);
Note The CLR verifies that an index into an array is valid . In other words, you can t create an
array with 100 elements in it (numbered 0 through 99) and then try to access the element at index 5 or 100 . Doing so will cause a System.IndexOutOfRangeException to be thrown . Allowing access to memory outside the range of an array would be a breach of type safety and a potential security hole, and the CLR doesn t allow verifiable code to do this . Usually, the performance degradation associated with index checking is insubstantial because the just-in-time (JIT) compiler normally checks array bounds once before a loop executes instead of at each loop iteration . However, if you re still concerned about the performance hit of the CLR s index checks, you can use unsafe code in C# to access the array . The Array Access Performance section later in this chapter demonstrates how to do this .
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