MSDN Magazine - January 2008 - (Page 23)

SysStringAllocByteLen/SysStringFree, since CoGetMalloc is subject to future change. Let’s look at an example. The GetAnsiStringFromNativeCode takes a char ** argument as [in, out] and returns a char * as [out, retval]. For the char ** argument, it can choose to call CoTaskMemFree to free the memory that is allocated by the CLR, then allocate new memory by using CoTaskMemAlloc and overwrite the pointer with a new memory pointer. Later, the CLR will free the memory and create a copy for the managed string. As for the return value, it only needs to allocate a new piece of memory by using CoTaskMemAlloc and return it to the caller. After return, the newly allocated memory is now owned by the CLR. The CLR will first create a new managed string from it and then call CoTaskMemFree to free it. Let’s consider the first option (see Figure 9). The corresponding C# function declaration is as follows: class Lib { [DllImport(@”MarshalLib.dll”, CharSet= CharSet.Ansi)] public static extern string GetAnsiStringFromNativeCode( ref string inOutString); } Figure 9 Using Pointers MARSHALLIB_API char stdcall GetAnsiStringFromNativeCode(char **arg) { char *szRet = (char *)::CoTaskMemAlloc(255); strcpy(szRet, “Returned String From Native Code”); printf(“Inside GetAnsiStringFromNativeCode: *arg = %s\n”, *arg); printf(“Inside GetAnsiStringFromNativeCode: CoTaskMemFree(*arg); *arg = CoTaskMemAlloc(100); strcpy(*arg, \”Changed\”)\n”); ::CoTaskMemFree(*arg); *arg = (char *)::CoTaskMemAlloc(100); strcpy(*arg, “Changed”); } return szRet; Figure 10 Using a Thunk When the following C# code makes a call to GetAnsiStringFromNativeCode string argStr = “Before”; Console.WriteLine(“Before GetAnsiStringFromNativeCode : argStr = \”” + argStr + “\””); string retStr = Lib.GetAnsiStringFromNativeCode(ref argStr); Console.WriteLine(“AnsiStringFromNativeCode() returns \”” + retStr + “\”” ); Console.WriteLine(“After GetAnsiStringFromNativeCode : argStr = \”” + argStr + “\””); In fact, what’s happening is that the CLR creates a thunk, which forwards the calls from native code to the actual delegate, then to the real function (see Figure 10). Usually, you won’t have to worry about the lifetime of delegates. Whenever you are passing a delegate to unmanaged code, the CLR will make sure the delegate is alive during the call. the output is: Before GetAnsiStringFromNativeCode : argStr = “Before” Inside GetAnsiStringFromNativeCode: *arg = Before Inside GetAnsiStringFromNativeCode: CoTaskMemFree(*arg); *arg = CoTaskMemAlloc(100); strcpy(*arg, “Changed”) AnsiStringFromNativeCode() returns “Returned String From Native Code” After GetAnsiStringFromNativeCode : argStr = “Changed” If the native function you are going to call doesn’t follow this convention, you’ll have to do the marshaling by yourself to avoid memory corruption. This could easily happen because the function for an unmanaged function could return whatever it wants; it can return the same piece of memory every time or return a new block of memory allocated by malloc/new, and so forth, again depending on the contract. Besides memory allocation, the size of the memory being passed in or out is also very important. As discussed in the StringBuilder case, it is very important to change the Capacity property so that the CLR can allocate a piece of memory big enough to hold the results. In addition, marshaling a string as [InAttribute, OutAttribute] (without out or ref and any other attribute) is a bad idea because you don’t know whether the string will be big enough. You can use SizeParamIndex and SizeConst fields in MarshalAsAttribute to specify the size of the buffer. However, these attributes cannot be used when passing by reference. Reverse P/Invoke and Delegate Lifetime The CLR allows you to pass a delegate to the unmanaged world so that it can call the delegate as an unmanaged function pointer. CLR Inside Out january2008 23 http://www.windowscontrols.com http://www.windowscontrols.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2008

MSDN Magazine - January 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Cutting Edge IIS 7.0 - Enhance Your Apps with the Integrated ASP.NET Pipeline World Ready - Around the World with ASP.NET AJAX Applications WCF Syndication - HTTP Programming with WCF and the .NET Framework 3.5 Wicked Code Foundations Extreme ASP.NET {End Bracket}

MSDN Magazine - January 2008

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