c# data matrix code Managed Array Initialization in Visual C#

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Managed Array Initialization
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When a managed array is created, the data for all elements is implicitly set to zero values, and the default constructor if available is called. This behavior differs from the initialization of native arrays. To initialize a native array, the default constructor would be called for every single argument. If no constructor is present, the native array s data is not initialized. Initializing a managed array with zero values first and then calling a potential default constructor sounds like an overhead. However, in most cases, there is no default constructor that could be called. None of the public value types from the FCL has a default constructor. To
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CHAPTER 2 MANAGED TYPES, INSTANCES, AND MEMORY
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support fast array initialization, most .NET languages, including C++/CLI and C#, do not allow defining value types with default constructors. However, there are a few .NET languages that support creating value types with default constructors. C++ Managed Extensions (the predecessor of C++/CLI) is one of them. If you instantiate an array of value types that have a default constructor, C++/CLI first instantiates the array normally, which implies zero-initialization, and then calls Array::Initialize on it. This method calls the default constructor for all elements. Most other .NET languages, including C#, do not initialize arrays of value types with custom default constructors correctly! To ensure a correct initialization in these languages, you have to call Array::Initialize manually, after instantiating such an array. If you migrate old C++ Managed Extensions code from .NET 1.1 to .NET 2.0, I strongly recommend making sure that no value types have default constructors.
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Iterating Through an Array
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A C++/CLI programmer can use different alternatives to iterate through a managed array. To obtain an element from an array, the typical array-like syntax can be used. This allows you to iterate through an array with a normal for loop. To determine the number of elements (in all dimensions) of an array, the implicit base class System::Array offers a public member called Length. array<int>^ arr = { 1, 2, 3, 4, 5 }; for (int i = 0; i < arr->Length; ++i) System::Console::WriteLine(arr[i]); C++/CLI also provides a for each loop to iterate through an array. This construct is often more convenient: array<int>^ arr = GetManagedArrayFromSomeWhere(); for each (int value in arr) System::Console::WriteLine(value); Finally, it is possible to access elements of a value array in a pointer-based way. As Figure 2-4 shows, the elements of a one-dimensional managed array are laid out and ordered sequentially. Multidimensional arrays, and some seldom-used arrays with arbitrary bounds, have a different layout, but their elements are laid out and ordered sequentially, too.
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Figure 2-4. Memory layout of a one-dimensional managed array of value types
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However, using native pointers would not be sufficient here. Since managed arrays are instantiated on the managed heap, where they can be relocated to defragment the managed heap, a special kind of pointer is necessary. Like a tracking handle, pointers of this type need to be updated when the managed array is moved. However, the tracking handle would not be sufficient either. As you can see in Figure 2-4, a tracking handle always refers to a managed object s header. The pointer type needed here should refer to an object s data area. For this new pointer concept, a template-like syntax is used. The keyword interior_ptr intends to make clear that this managed pointer concept refers to an object s data, not to an object s header. To differentiate a tracking handle from an interior pointer, a tracking handle is sometimes called whole-object pointer. Figure 2-5 shows the difference between a tracking handle and an interior pointer.
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Figure 2-5. Tracking handles and interior pointers In cases in which the keyword interior_ptr would conflict with an identifier, the pseudonamespace cli can be used again. The following code shows how to use interior pointers to iterate through a one-dimensional array of integers: void WeakEncrypt(array<unsigned char>^ bytes, unsigned char key) { cli::interior_ptr<unsigned char> pb = &(bytes[0]); interior_ptr<unsigned char> pbEnd = pb + bytes->Length; while (pb < pbEnd) { *pb ^= key; pb++; } } The function WeakEncrpyt expects a managed byte array that will be encrypted and a byte that is used as the encryption key. WeakEncrpyt is probably the weakest possible encryption algorithm. When you need to encrypt data in your projects, use types from the System:: Security::Cryptography namespace instead. Nevertheless, this function is sufficient to show how interior pointers can be used.
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