204: Using ref and out Rather Than Pointers in C#.NET

Encoding PDF-417 2d barcode in C#.NET 204: Using ref and out Rather Than Pointers
Listing 204: Using ref and out Rather Than Pointers
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class VirtualMemoryManager { // [DllImport("kernel32dll", SetLastError = true)] static extern bool VirtualProtectEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize, uint flNewProtect, ref uint lpflOldProtect); }
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In spite of the fact that lpflOldProtect is documented as [out], the description goes on to mention that the parameter must point to a valid
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20: Platform Interoperability and Unsafe Code
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variable and not NULL The inconsistency is confusing, but common The guideline is to use ref rather than out for P/Invoke type parameters since the callee can always ignore the data passed with ref, but the converse will not necessarily succeed The other parameters are virtually the same as VirtualAllocEx(), except that the lpAddress is the address returned from VirtualAllocEx() In addition, flNewProtect specifies the exact type of memory protection: page execute, page read-only, and so on
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Using StructLayoutAttribute for Sequential Layout Some APIs involve types that have no corresponding managed type To call these requires redeclaration of the type in managed code You declare the unmanaged COLORREF struct, for example, in managed code (see Listing 205)
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Listing 205: Declaring Types from Unmanaged Structs
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[StructLayout(LayoutKindSequential)] struct ColorRef { public byte Red; public byte Green; public byte Blue; // Turn off warning about not accessing Unused #pragma warning disable 414 private byte Unused; #pragma warning restore 414 public ColorRef(byte red, byte green, byte blue) { Blue = blue; Green = green; Red = red; Unused = 0; } }
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Various Microsoft Windows color APIs use COLORREF to represent RGB colors (levels of red, green, and blue) The key in this declaration is StructLayoutAttribute By default, managed code can optimize the memory layouts of types, so layouts may not be sequential from one field to the next To force sequential layouts so that
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Platform Invoke
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a type maps directly and can be copied bit for bit (blitted) from managed to unmanaged code and vice versa, you add the StructLayoutAttribute with the LayoutKindSequential enum value (This is also useful when writing data to and from filestreams where a sequential layout may be expected) Since the unmanaged (C++) definition for struct does not map to the C# definition, there is not a direct mapping of unmanaged struct to managed struct Instead, developers should follow the usual C# guidelines about whether the type should behave like a value or a reference type, and whether the size is small (approximately less than 16 bytes)
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Error Handling One inconvenient characteristic of Win32 API programming is that it frequently reports errors in inconsistent ways For example, some APIs return a value (0, 1, false, and so on) to indicate an error, and others set an out parameter in some way Furthermore, the details of what went wrong require additional calls to the GetLastError() API and then an additional call to FormatMessage() to retrieve an error message corresponding to the error In summary, Win32 error reporting in unmanaged code seldom occurs via exceptions Fortunately, the P/Invoke designers provided a mechanism for handling this To enable this, given the SetLastError named parameter of the DllImport attribute is true, it is possible to instantiate a SystemComponentModelWin32Exception() that is automatically initialized with the Win32 error data immediately following the P/Invoke call (see Listing 206)
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Listing 206: Win32 Error Handling
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class VirtualMemoryManager { [DllImport("kernel32dll", ", SetLastError = true)] private static extern IntPtr VirtualAllocEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize, AllocationType flAllocationType, uint flProtect); //
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[DllImport("kernel32dll", SetLastError = true)] static extern bool VirtualProtectEx( IntPtr hProcess, IntPtr lpAddress, IntPtr dwSize, uint flNewProtect, ref uint lpflOldProtect); [Flags] private enum AllocationType : uint { // } [Flags] private enum ProtectionOptions { // } [Flags] private enum MemoryFreeType { // } public static IntPtr AllocExecutionBlock( int size, IntPtr hProcess) { IntPtr codeBytesPtr; codeBytesPtr = VirtualAllocEx( hProcess, IntPtrZero, (IntPtr)size, AllocationTypeReserve | AllocationTypeCommit, (uint)ProtectionOptionsPageExecuteReadWrite); if (codeBytesPtr == IntPtrZero) { throw new SystemComponentModelWin32Exception();
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uint lpflOldProtect = 0; if (!VirtualProtectEx( hProcess, codeBytesPtr, (IntPtr)size, (uint)ProtectionOptionsPageExecuteReadWrite, ref lpflOldProtect)) { throw new SystemComponentModelWin32Exception();
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return codeBytesPtr;
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