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} public static IntPtr AllocExecutionBlock(int size) { return AllocExecutionBlock( size, GetCurrentProcessHandle()); } }
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This enables developers to provide the custom error checking that each API uses while still reporting the error in a standard manner Listing 201 and Listing 203 declared the P/Invoke methods as internal or private Except for the simplest of APIs, wrapping methods in public wrappers that reduce the complexity of the P/Invoke API calls is a good guideline that increases API usability and moves toward object-oriented type structure The AllocExecutionBlock() declaration in Listing 206 provides a good example of this
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Using SafeHandle Frequently, P/Invoke involves a resource, such as a window handle, that code needs to clean up after using it Instead of requiring developers to remember this and manually code it each time, it is helpful to provide a class that implements IDisposable and a finalizer In Listing 207, for example, the address returned after VirtualAllocEx() and VirtualProtectEx() requires a follow-up call to VirtualFreeEx() To provide built-in support for this, you define a VirtualMemoryPtr class that derives from SystemRuntimeInteropServicesSafeHandle (this is new in NET 20)
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Listing 207: Managed Resources Using SafeHandle
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public class VirtualMemoryPtr : SystemRuntimeInteropServicesSafeHandle { public VirtualMemoryPtr(int memorySize) : base(IntPtrZero, true) { ProcessHandle = VirtualMemoryManagerGetCurrentProcessHandle(); MemorySize = (IntPtr)memorySize; AllocatedPointer = VirtualMemoryManagerAllocExecutionBlock( memorySize, ProcessHandle);
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Disposed = false; } public readonly IntPtr AllocatedPointer; readonly IntPtr ProcessHandle; readonly IntPtr MemorySize; bool Disposed; public static implicit operator IntPtr( VirtualMemoryPtr virtualMemoryPointer) { return virtualMemoryPointerAllocatedPointer; } // SafeHandle abstract member public override bool IsInvalid { get { return Disposed; } } // SafeHandle abstract member protected override bool ReleaseHandle() { if (!Disposed) { Disposed = true; GCSuppressFinalize(this); VirtualMemoryManagerVirtualFreeEx(ProcessHandle, AllocatedPointer, MemorySize); } return true; } }
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SystemRuntimeInteropServicesSafeHandle includes the abstract
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members IsInvalid and ReleaseHandle() In the latter, you place your cleanup code; the former indicates whether the cleanup code has executed yet With VirtualMemoryPtr, you can allocate memory simply by instantiating the type and specifying the needed memory allocation
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ADVANCED TOPIC Using IDisposable Explicitly in Place of SafeHandle In C# 10, SystemRuntimeInteropServicesSafeHandle is not available Instead, a custom implementation of IDisposable, as shown in Listing 208, is necessary
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Listing 208: Managed Resources without SafeHandle but Using IDisposable
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public struct VirtualMemoryPtr : IDisposable { public VirtualMemoryPtr(int memorySize) { ProcessHandle = VirtualMemoryManagerGetCurrentProcessHandle(); MemorySize = (IntPtr)memorySize; AllocatedPointer = VirtualMemoryManagerAllocExecutionBlock( memorySize, ProcessHandle); Disposed = false; } public readonly IntPtr AllocatedPointer; readonly IntPtr ProcessHandle; readonly IntPtr MemorySize; bool Disposed; public static implicit operator IntPtr( VirtualMemoryPtr virtualMemoryPointer) { return virtualMemoryPointerAllocatedPointer; } #region IDisposable Members public void Dispose() { if (!Disposed) { Disposed = true; GCSuppressFinalize(this); VirtualMemoryManagerVirtualFreeEx(ProcessHandle, AllocatedPointer, MemorySize); } } #endregion }
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In order for VirtualMemoryPtr to behave with value type semantics, you need to implement it as a struct However, the consequence of this is that there can be no finalizer, since the garbage collector does not manage value types This means the developer using the type must remember to clean up the code There is no fallback mechanism if he doesn t The second restriction is not to pass or copy the instance outside the method This is a common guideline of IDisposable implementing types Their scope should be left within a using statement and they should not be passed as parameters to other methods that could potentially save them beyond the life of the using scope
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Calling External Functions Once you declare the P/Invoke functions, you invoke them just as you would any other class member The key, however, is that the imported DLL must be in the path, including the executable directory, so it can be successfully loaded Listing 206 and Listing 207 provide demonstrations of this However, they rely on some constants Since flAllocationType and flProtect are flags, it is a good practice to provide constants or enums for each Instead of expecting the caller to define these, encapsulation suggests you provide them as part of the API declaration, as shown in Listing 209
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