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.NET Forum / .NET Framework / Interop / August 2006String array in c++ dll to vb.net
Hello, I have an unmanaged c++ dll which exports the function: long GetAllFormationNames(long LithSchemeID, long LevelID, int & numData, long ** Ids, char *** Names); it's the final argument that I am having trouble converting/calling from VB.NET - everything I have tried has resulted in either an array containing rubbish or an array with 1 element containing only the first element that *should* be returned I am rather new to the .Net thing - so be gentle! Jeremy No problem, but first a few questions: Do you know how many names will be returned? Are you supposed to allocate buffers for the data to be populated does the dll allocate the memory for the Strings (and you must have to free it later)? Tom > Hello, > [quoted text clipped - 11 lines] > > Jeremy Hi, 1. I don't know in advance how many names will be returned - the returned parameter numData is the number of names returned.. 2.. the dll allocates the memory for the strings. hope that makes it clearer! Regards, Jeremy >No problem, but first a few questions: > [quoted text clipped - 19 lines] >> >> Jeremy It certainly does. Here is what I recommend. Pass a ByRef IntPtr to the Names parameter. This will get you the address of the array. Of course, this array is really a continuous block of memory that has consecutive pointers to your strings (char arrays). You can use Marshal.ReadIntPtr(ptr, offset) to walk the array and get the IntPtrs to the actual data. >From those IntPtrs you can use Marshal.PtrToStringAnsi (or one of the other Marshal.PtrToStringXXX methods) to get a .NET String. Don't forget to call whatever method the dll provides to free the memory is allocated for this call. Tom > Hi, > [quoted text clipped - 32 lines] > >> > >> Jeremy OK - I tried this: >Dim formations As String() >Dim r As Long [quoted text clipped - 7 lines] > >formations(0) = Marshal.PtrToStringAuto(ip) the resulting string looks like rubbish though (I tried the other PtrToStringxxx functions too) ... I know the dll function works btw as I can call it from C++ hmm.. any more ideas? Cheers, Jeremy >It certainly does. > [quoted text clipped - 51 lines] >> >> >> >> Jeremy >>r = GetAllFormationNames(1, 2, numdata, ids, ip) >> >>ReDim formations(numdata) >> >>formations(0) = Marshal.PtrToStringAuto(ip) You missed one level of indirection: ip does not point to string data, it points to the array of char pointers. Try Marshal.PtrToStringAuto(Marshal.ReadIntPtr(ip,0)). Jeremy, Christian is correct (though you'd eventually put it in a for loop to get all the strings, not just the first one). By the way, a C++ longs is an VB Integer (or Int32 or UInt32) in .NET so you have another slight error. Tom > >>r = GetAllFormationNames(1, 2, numdata, ids, ip) > >> [quoted text clipped - 5 lines] > to string data, it points to the array of char pointers. > Try Marshal.PtrToStringAuto(Marshal.ReadIntPtr(ip,0)). Aha - now we are getting there.. I can now get a whole array of strings back like this: >r = GetAllFormationNames(1, 2, numdatap, idsp, ipp) > [quoted text clipped - 14 lines] > Next > End If however at (seemingly) random points in the formationnames array I get short (1 or 2 chars) strings of characters with ascii values over 128 - I originally tried incrementing offset by the value of the last string found instead of by 1, but then I found that a lot of the elements of formationnames were empty.. ..I can write a check to ensure that the strings returned are alphanumeric, but that's a rather ugly solution! anyway - thanks for the help in getting to this point! Jeremy >>>r = GetAllFormationNames(1, 2, numdata, ids, ip) >>> [quoted text clipped - 5 lines] >to string data, it points to the array of char pointers. >Try Marshal.PtrToStringAuto(Marshal.ReadIntPtr(ip,0)). I think offset is a byte-offset as opposed to an IntPtr-width offset. In other words, since an IntPtr is 4 bytes, increment the offset by 4 each time. Tom > Aha - now we are getting there.. > [quoted text clipped - 41 lines] > >to string data, it points to the array of char pointers. > >Try Marshal.PtrToStringAuto(Marshal.ReadIntPtr(ip,0)). Just noticed your while...I don't know why that is there. It should look more like this: For ndx as Integer = 0 to numOfNames -1 formationnames(ndx) = Marshal.PtrToStringAnsi(Marshal.ReadIntPtr(ptr, offset)) offset += 4 Next ndx Tom Jeremy, Be sure to let us know how you made out. That way everyone can learn what works and what doesn't. Tom > Just noticed your while...I don't know why that is there. It should > look more like this: [quoted text clipped - 8 lines] > > Tom
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