Passing string from C# to C++

Harish Iyer

Hello,
I have a C# client in which I make a call to a C++ DLL. The C++ code looks as follows ;
BYTE mmc_write(unsigned long addr, BYTE* Buffer)
{
}

On the C# side, I have declared the function as
[DllImport("somecpp.dll")
internal static extern byte mmc_write(ulong addr, [MarshalAs(UnManagedType.LPStr)]string sendBuffer);
and I use it in a function as below :
int Send(string sData)
{
   byte yRet = mmc_write(0, sData);
   if(yRet > 0)
      retutn sData.Length;
   return 0;
}

Now when I debug, I see that the string received on the C++ side is a bad pointer. I tried wrapping the string in C# to a StringBuilder also, but to no avail.
Am I doing something wrong here ? It would be great if someone can help me in this

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

Harish Iyer17y ago
hey thanks for the reply..i actually found the reason for the problem, but was unable to post it soon :(. The problem was with the first parameter of the function in C#. Since it is unsigned long in C++, I happily used ulong of C# as the parameter in DllImport. There is a very interesting point to note here. I remember Jeff Richter's suggestion in CLR via C# that the .NET developers should not have ever had decorator names for the FCL data types like ulong, int, long and so on and instead used only FCL names like System.UInt64, System.UInt32 etc. This was exactly the problem with my code. In C# the ulong is UInt64 whereas in C++ usigned long is 32 bit. Due to this mismatch, the remaining function parameter that is char* gets corrupted! I just changed the C# data type to UInt32 and it worked :)

i.e. I just used

[DllImport("somecpp.dll")
internal static extern byte mmc_write(UInt32 addr, Intptr ptrsendBuffer);
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Sam Hobbs17y ago
Do you know what the C++ calling convention is? It is probably either cdecl or stdcall. It is probabaly the correct calling convention, but if it is not, then that is a problem that could be very difficult to spot.
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