Code within the unsafe block can include unsafe constructs such as pointers in Visual C#

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Code within the unsafe block can include unsafe constructs such as pointers
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It is important to note that it is necessary to explicitly indicate to the compiler that unsafe code is supported
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From the command line, this requires the /unsafe switch For example, to compile the previous code, you need to use the command shown in Output 171
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Output 171
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cscexe /unsafe Programcs
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You need to use the /unsafe switch because unsafe code opens up the possibility of buffer overflows and similar possibilities that expose the potential for security holes The /unsafe switch includes the ability to directly manipulate memory and execute instructions that are unmanaged Requiring /unsafe, therefore, makes the choice of potential exposure explicit
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Pointer Declaration
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Now that you have marked a code block as unsafe, it is time to look at how to write unsafe code First, unsafe code allows the declaration of a pointer Consider the following example byte* pData;
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Assuming pData is not null, its value points to a location that contains one or more sequential bytes; the value of pData represents the memory address of the bytes The type specified before the * is the referent type, or the type located where the value of the pointer refers In this example, pData is the pointer and byte is the referent type, as shown in Figure 171
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Figure 171 Pointers Contain the Address of the Data
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Because pointers (which are just byte values) are not subject to garbage collection, C# does not allow referent types other than unmanaged types, which are types that are not reference types, are not generics, and do not contain reference types Therefore, the following is not valid: string* pMessage
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Neither is this: ServiceStatus* pStatus
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where ServiceStatus is defined as shown in Listing 1713; the problem again is that ServiceStatus includes a string field
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Listing 1713 Invalid Referent Type Example
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struct ServiceStatus { int State; string Description; // Description is a reference type }
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Language Contrast: C/C++Pointer Declaration
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In C/C++, multiple pointers within the same declaration are declared as follows: int *p1, *p2;
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Notice the * on p2; this makes p2 an int* rather than an int In contrast, C# always places the * with the data type: int* p1, p2;
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The result is two variables of type int* The syntax matches that of declaring multiple arrays in a single statement: int[] array1, array2;
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Pointers are an entirely new category of type Unlike structs, enums, and classes, pointers don't ultimately derive from SystemObject
In addition to custom structs that contain only unmanaged types, valid referent types include enums, predefined value types ( sbyte, byte, short, ushort, int , uint, long, ulong, char, float, double, decimal, and bool), and pointer types (such as byte**) Lastly, valid syntax includes void* pointers, which represent pointers to an unknown type
Assigning a Pointer
Once code defines a pointer, it needs to assign a value before accessing it Just like other reference types, pointers can hold the value null; this is their default value The value stored by the pointer is the address of a location Therefore, in order to assign it, you must first retrieve the address of the data You could explicitly cast an integer or a long into a pointer, but this rarely occurs without a means of determining the address of a particular data value at execution time Instead, you need to use the address operator ( &) to retrieve the address of the value type: byte* pData = &bytes[0]; // Compile error
The problem is that in a managed environment, data can move, thereby invalidating the address The error message is "You can only take the address of [an] unfixed expression inside of a fixed statement initializer" In this case, the byte referenced appears within an array and an array is a reference type (a moveable type) Reference types appear on the heap and are subject to garbage collection or relocation A similar problem occurs when referring to a value type field on a moveable type: int* a = &"message"Length;
Either way, to complete the assignment, the data needs to be a value type, fixed, or explicitly allocated on the call stack