209: Encapsulating the APIs Together in C#

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Listing 209: Encapsulating the APIs Together
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class VirtualMemoryManager { // /// <summary> /// The type of memory allocation This parameter must /// contain one of the following values /// </summary> [Flags] private enum AllocationType : uint { /// <summary> /// Allocates physical storage in memory or in the /// paging file on disk for the specified reserved /// memory pages The function initializes the memory /// to zero /// </summary>
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Commit = 0x1000, /// <summary> /// Reserves a range of the process's virtual address /// space without allocating any actual physical /// storage in memory or in the paging file on disk /// </summary> Reserve = 0x2000, /// <summary> /// Indicates that data in the memory range specified by /// lpAddress and dwSize is no longer of interest The /// pages should not be read from or written to the /// paging file However, the memory block will be used /// again later, so it should not be decommitted This /// value cannot be used with any other value /// </summary> Reset = 0x80000, /// <summary> /// Allocates physical memory with read-write access /// This value is solely for use with Address Windowing /// Extensions (AWE) memory /// </summary> Physical = 0x400000, /// <summary> /// Allocates memory at the highest possible address /// </summary> TopDown = 0x100000, } /// <summary> /// The memory protection for the region of pages to be /// allocated /// </summary> [Flags] private enum ProtectionOptions : uint { /// <summary> /// Enables execute access to the committed region of /// pages An attempt to read or write to the committed /// region results in an access violation /// </summary> Execute = 0x10, /// <summary> /// Enables execute and read access to the committed /// region of pages An attempt to write to the /// committed region results in an access violation /// </summary> PageExecuteRead = 0x20, /// <summary> /// Enables execute, read, and write access to the /// committed region of pages
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20: Platform Interoperability and Unsafe Code
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/// </summary> PageExecuteReadWrite = 0x40, // } /// <summary> /// The type of free operation /// </summary> [Flags] private enum MemoryFreeType : uint { /// <summary> /// Decommits the specified region of committed pages /// After the operation, the pages are in the reserved /// state /// </summary> Decommit = 0x4000, /// <summary> /// Releases the specified region of pages After this /// operation, the pages are in the free state /// </summary> Release = 0x8000 } // }
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The advantage of enums is that they group together each value Furthermore, they limit the scope to nothing else besides these values
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Simplifying API Calls with Wrappers Whether it is error handling, structs, or constant values, one goal of good API developers is to provide a simplified managed API that wraps the underlying Win32 API For example, Listing 2010 overloads VirtualFreeEx() with public versions that simplify the call
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Listing 2010: Wrapping the Underlying API
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class VirtualMemoryManager { // [DllImport("kernel32dll", SetLastError = true)] static extern bool VirtualFreeEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize, IntPtr dwFreeType);
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public static bool VirtualFreeEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize) { bool result = VirtualFreeEx( hProcess, lpAddress, dwSize, (IntPtr)MemoryFreeTypeDecommit); if (!result) { throw new SystemComponentModelWin32Exception(); } return result; } public static bool VirtualFreeEx( IntPtr lpAddress, IntPtr dwSize) { return VirtualFreeEx( GetCurrentProcessHandle(), lpAddress, dwSize); } [DllImport("kernel32", SetLastError = true)] static extern IntPtr VirtualAllocEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize, AllocationType flAllocationType, uint flProtect); // }
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Function Pointers Map to Delegates One last P/Invoke key is that function pointers in unmanaged code map to delegates in managed code To set up a Microsoft Windows timer, for example, you would provide a function pointer that the timer could call back on, once it had expired Specifically, you would pass a delegate instance that matched the signature of the callback Guidelines Given the idiosyncrasies of P/Invoke, there are several guidelines to aid in the process of writing such code
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Check that no managed classes already expose the APIs Define API external methods as private or, in simple cases, internal
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20: Platform Interoperability and Unsafe Code Provide public wrapper methods around the external methods that
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