creating data maytrix c# Managed Code and Data in C#

Encoder DataMatrix in C# Managed Code and Data

Managed Code and Data
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So what is managed code, then Managed code is code that provides enough information to allow the common language runtime to perform the following tasks: Given an address inside the code, locate the metadata describing the method Walk the stack Handle exceptions Store and retrieve security information For the runtime to carry out these tasks, the code must pass a verification process unless a network administrator has established a policy that allows code to run without verification. During the verification process, the JIT compiler examines the MSIL code and the metadata to determine whether the code can be classified as type-safe. Typesafe code is code that can be determined to access only memory locations that it owns. This restriction ensures that the code works and plays well with other programs and that the code is safe from causing accidental or malicious corruption. Without type safety, there s no way to reliably enforce security restrictions. Related to managed code is managed data. Managed data is data that is allocated and freed automatically through the runtime using a process called garbage collection. With garbage collection, whenever an allocated item goes out of scope, the runtime cleans it up. One consequence of using garbage collection is that the time and potentially even the order of destruction of objects can t be determined. For instance, consider the following C# code snippet. (I ll introduce C# in 3; but even without knowing anything about C#, you should be able to understand this simple example.) class MainApp { public static void Main() { System.String Hello = "Hello"; System.String World = "World!"; System.Console.WriteLine(Hello); System.Console.WriteLine(World); } } Here two String objects are created, one containing the literal Hello and the other containing the literal World! . Although the literals are declared in that order, there s no assurance that they will be destroyed in any particular order. Furthermore, there s no assurance that they will be destroyed as the strings go out of scope. The order, or timing, in the preceding example is meaningless, but it might make a difference in other examples. The nondeterministic freeing of objects isn t a problem unless the object holds some persistent resources that the runtime doesn t manage for instance, a database connection or a window handle. When an object holds such resources, the solution is to provide a Dispose method and implement the IDisposable interface, which can be explicitly called to free resources. You ll see examples of this technique in subsequent chapters.
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About Unsafe Code
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There are cases in which you can t use managed code. For instance, many native Win32 functions require pointers. One of the problems C/C++ programmers often had with Visual Basic was the lack of pointers. Visual Basic .NET still doesn t support pointers, but it does support a similar mechanism called a reference type. Still, in some cases, pointers can come in handy. You might also need to access unmanaged legacy code. In the best of all worlds, all our programs would magically be converted to managed code because of the advantages it offers, but sometimes this isn t possible. So what about the times when you really need a pointer or need to access legacy code For these situations, C# provides a special keyword: unsafe. A method or a section of code can be declared as unsafe, and when compiled using the /unsafe compiler switch, will generate unsafe (unmanaged) code that isn t verifiable by the runtime. In addition to unsafe, C# provides the fixed keyword. During the process of garbage collection, variables are often moved to make more efficient use of memory. If several smaller blocks of free memory are required for a single, larger allocation, the garbage collector can move the blocks to make the single larger block available. Such rearranging would obviously be disastrous for any program that had stored, within unsafe code, a pointer to one of the variables the garbage collector moved. The fixed keyword was added for just this situation. Within a fixed block, the variables referenced will be pinned and won t be movable. After exiting the fixed block, the variables are once again available for the garbage collector to move as required.
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