StructureToPtr not copying primitive type fields in native struct to ref struct correctly - c++-cli

Question

I have a native struct, (which is quite large so I have to use new key word to instantiate, below is just to make a MCVE I cant change the struct as it is provided as external dependencies),
struct NativeStruct
{
char BrokerID[11];
char InvestorID[13];
char InstrumentID[31];
char OrderRef[13];
char UserID[16];
char OrderPriceType;
char Direction;
double LimitPrice;
}
I want to convert NativeStruct to managed object, so I defined a ref struct to mirror it, this also used two enums as below,
public enum struct EnumOrderPriceTypeType
{
AnyPrice = (Byte)'1',
LimitPrice = (Byte)'2',
BestPrice = (Byte)'3',
LastPrice = (Byte)'4',
LastPricePlusOneTicks = (Byte)'5',
LastPricePlusTwoTicks = (Byte)'6',
LastPricePlusThreeTicks = (Byte)'7',
AskPrice1 = (Byte)'8',
AskPrice1PlusOneTicks = (Byte)'9',
AskPrice1PlusTwoTicks = (Byte)'A',
AskPrice1PlusThreeTicks = (Byte)'B',
BidPrice1 = (Byte)'C',
BidPrice1PlusOneTicks = (Byte)'D',
BidPrice1PlusTwoTicks = (Byte)'E',
BidPrice1PlusThreeTicks = (Byte)'F'
};
public enum struct EnumDirectionType
{
Buy = (Byte)'0',
Sell = (Byte)'1'
};
[StructLayout(LayoutKind::Sequential)]
public ref struct ManagedStruct
{
[MarshalAs(UnmanagedType::ByValTStr, SizeConst = 11)]
String^ BrokerID;
[MarshalAs(UnmanagedType::ByValTStr, SizeConst = 13)]
String^ InvestorID;
[MarshalAs(UnmanagedType::ByValTStr, SizeConst = 31)]
String^ InstrumentID;
[MarshalAs(UnmanagedType::ByValTStr, SizeConst = 13)]
String^ OrderRef;
[MarshalAs(UnmanagedType::ByValTStr, SizeConst = 16)]
String^ UserID;
EnumOrderPriceTypeType OrderPriceType;
EnumDirectionType Direction;
double LimitPrice;
};
Then I use StructureToPtr to copy the native object to managed object, and use WriteLine to test if the copy is successful,
NativeStruct *native = new NativeStruct();
ManagedStruct^ managed = gcnew ManagedStruct();
managed->LimitPrice = 95.5;
managed->BrokerID = "666666";
Marshal::StructureToPtr(managed, IntPtr(native), false);
int i;
for (i = 0; i < 11; i++)
Console::Write(native->BrokerID[i]);
Console::WriteLine();
Console::WriteLine(native->LimitPrice);
Console::WriteLine(L"Hello ");
Console::ReadLine();
My question is why LimitPrice is not copied successfuly? I have been battling this for a week, any help will be welcomed. Thanks a lot.

Answer 1

Marshal::StructureToPtr() can only work correctly when the managed and the native struct are an exact match. By far the simplest way to verify this is to check the sizes of the structures, they must be identical. So add this code to your program:
auto nlen = sizeof(NativeStruct);
auto mlen = Marshal::SizeOf(ManagedStruct::typeid);
System::Diagnostics::Debug::Assert(nlen == mlen);
Kaboom. The native struct takes 96 bytes and the managed one takes 104. Consequences are dire, you corrupt memory and that has a lot more unpleasant side effects than the LimitPrice member value getting copied to the wrong offset.
Two basic ways to trouble-shoot this. You can simply populate all of the managed struct members with unique values and check the first member of the native struct that has the wrong value. The member before it is wrong. Keep going until the you no longer get the kaboom. Or you can write code that uses offsetof() on the native struct members and compare them with Marshal::OffsetOf().
Just to save you the trouble, the problem are the enum declarations. Their size in the native struct is 1 byte but the managed versions take 4 bytes. Fix:
public enum struct EnumOrderPriceTypeType : Byte
and
public enum struct EnumDirectionType : Byte
Note the added : Byte to force the enum to take 1 byte of storage. It should be noted that copying the members one-by-one instead of using Marshal::StructureToPtr() is quicker and would have saved you a week of trouble.