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To retrieve the address of a moveable data item, it is necessary to fix, or pin, the data, as demonstrated in Listing 1714
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Listing 1714 Fixed Statement
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byte[] bytes = new byte[24]; fixed (byte* pData = &bytes[0]) // pData = bytes also allowed { // }
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Within the code block of a fixed statement, the assigned data will not move In this example, bytes will remain at the same address, at least until the end of the fixed statement The fixed statement requires the declaration of the pointer variable within its scope This avoids accessing the variable outside of the fixed statement, when the data is no longer fixed However, it is the programmer's responsibility to ensure that he doesn't assign the pointer to another variable that survives beyond the scope of the fixed statementpossibly in an API call, for example Similarly, using ref or out parameters will be problematic for data that will not survive beyond the method call Since a string is an invalid referent type, it would appear invalid to define pointers to strings However, as in C++, internally a string is a pointer to the first character of an array of characters, and it is possible to declare pointers to characters using char* Therefore, C# allows declaring a pointer of type char* and assigning it to a string within a fixed statement The fixed statement prevents the movement of the string during the life of the pointer Similarly, it allows any moveable type that supports an implicit conversion to a pointer of another type, given a fixed statement You can replace the verbose assignment of &bytes[0] with the abbreviated bytes, as shown in Listing 1715
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Listing 1715 Fixed Statement without Address or Array Indexer
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byte[] bytes = new byte[24]; fixed (byte* pData = bytes) { // }
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Depending on the frequency and time to execute, fixed statements have the potential to cause fragmentation in the heap because the garbage collector cannot compact fixed objects To reduce this problem, the best practice is to pin blocks early in the execution and to pin fewer large blocks rather than many small blocks NET 20 reduces the NET Framework problem as well, due to some additional fragmentation-aware code
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You should use the fixed statement on an array to prevent the garbage collector from moving the data However, an alternative is to allocate the array on the call stack Stack allocated data is not subject to garbage collection or to the finalizer patterns that accompany it Like referent types, the requirement is that the stackalloc data is an array of unmanaged types For example, instead of
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allocating an array of bytes on the heap, you can place it onto the call stack, as shown in Listing 1716
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Listing 1716 Allocating Data on the Call Stack
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byte* bytes = stackalloc byte[42];
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Because the data type is an array of unmanaged types, it is possible for the runtime to allocate a fixed buffer size for the array and then to restore that buffer once the pointer goes out of scope Specifically, it allocates sizeof(T) * E , where E is the array size and T is the referent type Given the requirement of using stackalloc only on an array of unmanaged types, the runtime restores the buffer back to the system simply by unwinding the stack, eliminating the complexities of iterating over the f-reachable queue and compacting reachable data Therefore, there is no way to explicitly free stackalloc data
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Accessing the value of a type referred to by a pointer requires that you dereference the pointer, placing the indirection operator prior to the pointer type byte data = *pData;, for example, dereferences the location of the byte referred to by pData and returns the single byte at that location Using this principle in unsafe code allows the unorthodox behavior of modifying the "immutable" string, as shown in Listing 1717 In no way is this recommended, but it does expose the potential of low-level memory manipulation
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