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Passing a return char* from C++ to C# with DllImport

I am a newbie on both C# WPF and C++.
Recently, I got an external .dll, which returns a char*, and I want to receive the return value in C# by using DllImport. Then, using str.Split(';') to separate the characters. To the purpose, I created a button to show the first character in the string I split on a label when I click the button.
Therefore, I use an IntPtr to receive a char*, which is from C++ .dll, and call Marshal.PtrToStringAnsi() to turn it into a string. However, when I execute the code, it sometimes works but sometimes crushes. Then error code always shows
Unhandled exception at 0x00007FFC06839269 (ntdll.dll) in UITest.exe: 0xC0000374: heap corruption (parameters: 0x00007FFC068A27F0).
I thought my code is reasonable and I couldn't find root causes. Can anyone help me? Thanks!
The below shows the content in .dll and also the code I used in C#.
C++ code in Dlltest.h:
#define DLL_EXPORT extern "C" __declspec(dllexport)
char* getRbtData = nullptr;
DLL_EXPORT char* func_getRbtData();
C++ code in Dlltest.cpp:
char* func_getRbtData()
{
getRbtData = new char(128);
memset(getRbtData, 0, strlen(getRbtData));
char* _getRbtData = "1.0;23.0;55.0;91.0;594.0;";
memcpy(getRbtData, _getRbtData, strlen(_getRbtData));
return getRbtData;
};
C# code in UITest.xaml.cs:
[DllImport("DllTest.dll",EntryPoint = "func_getRbtData", CharSet = CharSet.Ansi)]
public static extern IntPtr func_getRbtData();
string[] words;
private void btn_test_Click(object sender, RoutedEventArgs e)
{
IntPtr intptr = func_getRbtData();
string str = Marshal.PtrToStringAnsi(intptr);
words = str.Split(';');
lb_content.Content = words[1];
}

There are several problems with your code.
On the C++ side, your DLL function is implemented all wrong:
getRbtData = new char(128);
You are allocating a single char whose value is 128, not an array of 128 chars. You need to use new char[128] instead for that.
memset(getRbtData, 0, strlen(getRbtData));
getRbtData is not a pointer to a null-terminated string, so strlen(getRbtData) is undefined behavior. It reads into surrounding memory while calculating the length until it finds a random 0x00 byte in memory.
And then the subsequent memset() into getRbtData will overwrite that surrounding memory. If it doesn't just crash outright.
char* _getRbtData = "1.0;23.0;55.0;91.0;594.0;";
Prior to C++11, this assignment is OK but discouraged. In C++11 and later, this assignment is actually illegal and won't compile.
String literals are read-only data, so you need to use const char instead of char in your pointer type. You should do that even in older compilers.
memcpy(getRbtData, _getRbtData, strlen(_getRbtData));
strlen(_getRbtData) is OK since _getRbtData is a pointer to a null-terminated string.
However, since getRbtData is not allocated with enough memory to receive all of the copied chars, memcpy() into getRbtData is also undefined behavior and will trash memory, if not crash outright.
return getRbtData;
This is OK to pass the pointer to C#.
However, since the memory is being allocated with new (better, new[]), it needs to be freed with delete (delete[]), which you are not doing. So you are leaking the memory.
Marshal.PtrToStringAnsi() on the C# side will not (and cannot) free new'ed memory for you. So your C# code will need to pass the pointer back to the DLL so it can delete the memory properly.
Otherwise, you will need to allocate the memory using the Win32 API LocalAlloc() or CoTaskMemAlloc() function so that the Marshal class can be used on the C# side to free the memory directly without passing it back to the DLL at all.
On the C# side, you are using the wrong calling convention on your DllImport statement. The default is StdCall for compatibility with most Win32 API functions. But your DLL function is not specifying any calling convention at all. Most C/C++ compilers will default to __cdecl unless configured differently.
With that said, try this instead:
Dlltest.h
#define DLL_EXPORT extern "C" __declspec(dllexport)
DLL_EXPORT char* func_getRbtData();
DLL_EXPORT void func_freeRbtData(char*);
Dlltest.cpp
char* func_getRbtData()
{
const char* _getRbtData = "1.0;23.0;55.0;91.0;594.0;";
int len = strlen(_getRbtData);
char *getRbtData = new char[len+1];
// alternatively:
/*
char *getRbtData = (char*) LocalAlloc(LMEM_FIXED, len+1);
if (!getRbtData) return NULL;
*/
memcpy(getRbtData, _getRbtData, len+1);
return getRbtData;
}
void func_freeRbtData(char *p)
{
delete[] p;
// alternatively:
// LocalFree((HLOCAL)p);
}
UITest.xaml.cs
[DllImport("DllTest.dll", EntryPoint = "func_getRbtData", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr func_getRbtData();
[DllImport("DllTest.dll", EntryPoint = "func_freeRbtData", CallingConvention = CallingConvention.Cdecl)]
public static extern void func_freeRbtData(IntPtr p);
string[] words;
private void btn_test_Click(object sender, RoutedEventArgs e)
{
IntPtr intptr = func_getRbtData();
string str = Marshal.PtrToStringAnsi(intptr);
func_freeRbtData(intptr);
// alternatively:
// Marshal.FreeHGlobal(intptr);
words = str.Split(';');
lb_content.Content = words[1];
}

new char(128) returns a pointer to one character, with initial value 128.
I could tell you how to allocate 128 characters, but the chief problem with that is that you can't clean it up so that's not a useful answer. Check the existing questions about returning a string to C#.

C DLL does NOT Work in C# but Does work in C++

I don`t understand this code below, that it is working with my c++ but it just does not want to work with c#.
Can you please help me to understand what is wrong here and i think i have to say that i am absolutely new to C#.
My_Lib.h
extern "C" __declspec(dllexport) void __cdecl get(char** buffer);
My_lib.c
void get(char** buffer)
{
*buffer = (char*)calloc(6, sizeof(char));
assert(*buffer);
buffer[5] = '\0';
*buffer = "Hello";
}
in my C#----->
public static class NativeMethods
{
[DllImport("My_C_Lib.dll", CallingConvention = CallingConvention.Cdecl)]
unsafe public static extern void get(char** buffer);
}
//////////////////// Main()///////
unsafe
{
char* my_buf;
NativeMethods.get(&my_buf);
string s = new string(my_buf);
Console.WriteLine(s);
}
NOTE: Actually my DLL does Work when i call this c function from c++ but as i said above NOT in C#, there is no Errors but string s variable in C# prints some undefined sibols, but DLL "works"
thanks in advance!!!

The code is nearly correct, but...
C DLL does NOT Work in C# but Does work in C++
In C and C++ char is a 8-bit data type. In C# char is a 16-bit data type.
This means that C# expects that the pointer returned by the get() function is expected to be a "wide string", while in C++ expects an "ANSI string".
I simply changed one single line in your program:
*buffer = "H\0e\0l\0l\0o\0\0\0";
... and it works!
You may of course also use the "wide string" functions of the modern C compilers:
void get(wchar_t** buffer)
{
*buffer = L"Hello";
}
By the way
There is another error in your program:
*buffer = (char*)calloc(6, sizeof(char));
...
*buffer = "Hello";
This makes no sense:
The second line (*buffer = "Hello";) will not copy the string to the memory allocated by calloc, but it will write the address of the string "Hello" to the variable buffer and the value (address) returned by calloc is overwritten (and lost).

Your best bet is to change the PInvoke signature of your C function to taking a ref IntPtr:
[DllImport("My_C_Lib.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void get(ref IntPtr buffer);
To call this, you'll need to instantiate an IntPtr and pass it by reference to the C function (this is analagous to declaring a char * in C and passing it by address):
IntPtr ptr;
get(ref ptr);
// ptr is now an unmanaged pointer to the allocated string
Now, you need to convert the pointed-to string to a managed string. To do this you must make a copy of the string.
string str = Marshal.PtrToStringAnsi(ptr);
Be sure you understand the distinctions between ANSI and Unicode strings and make sure you call the appropriate PtrToString...() variant.
Note that the .NET runtime will not manage the allocated ptr for you since it was allocated by unmanaged code. You must free it yourself using whatever the appropriate mechanism is for your DLL (ideally, the DLL should provide a corresponding free function since it is the one who allocated the memory in the first place).
For robustness, add appropriate error / null pointer checking. Make sure get() succeeded and that ptr is not IntPtr.Zero.
To avoid leaks, if there is any possibility of any code throwing an exception between the time get() returns and the time you free the pointer, the try/finally construct is your friend.
(Aside: Note that get is a Contextual Keyword in C# and thus while you can use it as an identifier, you may prefer not to in order to avoid confusion.)

How to get a raw memory pointer to a managed class?

How do I find a raw pointer to a managed class in C#, and, hopefully, it's raw size in memory? Obviously, this is not allowed by CLR - more precisely, strictly prohibited, as unmanaged representation of managed classes should never, ever be worked with for both stability and safe reasons - so I'm looking for a hack. I'm not looking for serializing - I do actually need a dump of managed class as it is represented in raw memory.
More precisely, I'm looking for something like function getObjectPtr in the following example:
IntPtr getObjectPtr(Object managedClass) {...}
void main() {
var test=new TestClass();
IntPtr* ptr_to_test=getObjectPtr(test);
Console.WriteLine(ptr_to_test.ToString());
}
Thanks in advance!
EDIT:
I've finally found a solution by myself, and, when came back to post it as an answer, was totally surprised by the amount of so-quickly already posted answers... Thanks to all of you! This was very quick and totally unexpected.
The closest to mine solution was #thehennyy's one, but I'm not posting it since #Chino proposed far better one (sorry I've mistaken it to be wrong at first, I've just forgot to dereference the pointer again). It does not require a code to be unsafe and a bit more tolerates GC:
class Program
{
// Here is the function in case anyone needs it.
// Note, though, it does not preserve the handle while you work with
// pointer, so it is less reliable than the code in Main():
static IntPtr getPointerToObject(Object unmanagedObject)
{
GCHandle gcHandle = GCHandle.Alloc(unmanagedObject, GCHandleType.WeakTrackResurrection);
IntPtr thePointer = Marshal.ReadIntPtr(GCHandle.ToIntPtr(gcHandle));
gcHandle.Free();
return thePointer;
}
class TestClass
{
uint a = 0xDEADBEEF;
}
static void Main(string[] args)
{
byte[] cls = new byte[16];
var test = new TestClass();
GCHandle gcHandle = GCHandle.Alloc(test, GCHandleType.WeakTrackResurrection);
IntPtr thePointer = Marshal.ReadIntPtr(GCHandle.ToIntPtr(gcHandle));
Marshal.Copy(thePointer, cls, 0, 16); //Dump first 16 bytes...
Console.WriteLine(BitConverter.ToString(BitConverter.GetBytes(thePointer.ToInt32())));
Console.WriteLine(BitConverter.ToString(cls));
Console.ReadLine();
gcHandle.Free();
}
}
/* Example output (yours should be different):
40-23-CA-02
4C-38-04-01-EF-BE-AD-DE-00-00-00-80-B4-21-50-73
That field's value is "EF-BE-AD-DE", 0xDEADBEEF as it is stored in memory. Yay, we found it!
*/
Hovewer, now I'm a bit clueless. According to the this article, first 2 addresses in the class should be pointers to SyncBlock and RTTI structure, and therefore the first field's address must be offset by 2 words [8 bytes in 32-bit systems, 16 bytes in 64-bit systems] from the beginning. Mine is 64-bit; however, as you can see in the output, it is obvious that first field's raw offset from the object's address is only 4 bytes, which doesn't make any sense.
I've asked this as a separate question.
Maybe I should ask this as a separate question, but it is possible that there is an error in my solution.

Hey is this what you want?:
GCHandle gcHandle = GCHandle.Alloc(yourObject,GCHandleType.WeakTrackResurrection);
IntPtr thePointer = GCHandle.ToIntPtr(gcHandle);

You can write a small IL function that leaks an objects address.
var o = new object();
var d = new DynamicMethod("GetPtr", typeof(IntPtr), new Type[] {typeof(object)}, Assembly.GetExecutingAssembly().ManifestModule);
var il = d.GetILGenerator();
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ret);
var address = (IntPtr)d.Invoke(null, new object[] {o});
Console.WriteLine(address);
The source is: IllidanS4 / SharpUtils
/ UnsafeTools.cs

How do I call this c function in c# (unmarshalling return struct)?

I want to use c# interop to call a function from a dll written in c. I have the header files.
Take a look at this:
enum CTMBeginTransactionError {
CTM_BEGIN_TRX_SUCCESS = 0,
CTM_BEGIN_TRX_ERROR_ALREADY_IN_PROGRESS,
CTM_BEGIN_TRX_ERROR_NOT_CONNECTED
};
#pragma pack(push)
#pragma pack(1)
struct CTMBeginTransactionResult {
char * szTransactionID;
enum CTMBeginTransactionError error;
};
struct CTMBeginTransactionResult ctm_begin_customer_transaction(const char * szTransactionID);
How do I call ctm_begin_customer_transaction from c#. The const char * mapps well to string, but despite various attempts (looking at stackoverflow and other sites), I fail to marshal the return structure. If I define the function to return IntPtr it works ok...
Edit
I changed the return type to IntPtr and use:
CTMBeginTransactionResult structure = (CTMBeginTransactionResult)Marshal.PtrToStructure(ptr, typeof(CTMBeginTransactionResult));
but it throws AccessViolationException
I also tried:
IntPtr ptr = Transactions.ctm_begin_customer_transaction("");
int size = 50;
byte[] byteArray = new byte[size];
Marshal.Copy(ptr, byteArray, 0, size);
string stringData = Encoding.ASCII.GetString(byteArray);
stringData == "70e3589b-2de0-4d1e-978d-55e22225be95\0\"\0\0\a\0\0\b\b?" at this point. The "70e3589b-2de0-4d1e-978d-55e22225be95" is the szTransactionID from the struct. Where is the Enum? Is it the next byte?

There's a memory management problem hidden in this struct. Who owns the C string pointer? The pinvoke marshaller will always assume that the caller owns it so it will try to release the string. And passes the pointer to CoTaskMemFree(), same function as the one called by Marshal.FreeCoTaskMem(). These functions use the COM memory allocator, the universal interop memory manager in Windows.
This rarely comes to a good end, C code does not typically use that allocator unless the programmer designed his code with interop in mind. In which case he'd never have used a struct as a return value, interop always works much less trouble-free when the caller supplies buffers.
So you cannot afford to let the marshaller do its normal duty. You must declare the return value type as IntPtr so it doesn't try to release the string. And you must marshal it yourself with Marshal.PtrToStructure().
That however still leaves the question unanswered, who owns the string? There is nothing you can do to release the string buffer, you don't have access to the allocator used in the C code. The only hope you have is that the string wasn't actually allocated on the heap. That's possible, the C program might be using string literals. You need to verify that guess. Call the function a billion times in a test program. If that doesn't explode the program then you're good. If not then only C++/CLI can solve your problem. Given the nature of the string, a "transaction ID" ought to change a lot, I'd say you do have a problem.

I hate to answer my own question, but I found the solution to marshal the resulting struct. The struct is 8 bytes long (4 bytes for the char * and 4 bytes for enum). Marshalling the string does not work automatically, but the following works:
// Native (unmanaged)
public enum CTMBeginTransactionError
{
CTM_BEGIN_TRX_SUCCESS = 0,
CTM_BEGIN_TRX_ERROR_ALREADY_IN_PROGRESS,
CTM_BEGIN_TRX_ERROR_NOT_CONNECTED
};
// Native (unmanaged)
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
internal struct CTMBeginTransactionResult
{
public IntPtr szTransactionID;
public CTMBeginTransactionError error;
};
// Managed wrapper around native struct
public class BeginTransactionResult
{
public string TransactionID;
public CTMBeginTransactionError Error;
internal BeginTransactionResult(CTMBeginTransactionResult nativeStruct)
{
// Manually marshal the string
if (nativeStruct.szTransactionID == IntPtr.Zero) this.TransactionID = "";
else this.TransactionID = Marshal.PtrToStringAnsi(nativeStruct.szTransactionID);
this.Error = nativeStruct.error;
}
}
[DllImport("libctmclient-0.dll")]
internal static extern CTMBeginTransactionResult ctm_begin_customer_transaction(string ptr);
public static BeginTransactionResult BeginCustomerTransaction(string transactionId)
{
CTMBeginTransactionResult nativeResult = Transactions.ctm_begin_customer_transaction(transactionId);
return new BeginTransactionResult(nativeResult);
}
The code works, but I still need to investigate, if calling the unmanaged code results in memory leaks.

pinvoke: How to free a malloc'd string?

In a C dll, I have a function like this:
char* GetSomeText(char* szInputText)
{
char* ptrReturnValue = (char*) malloc(strlen(szInputText) * 1000); // Actually done after parsemarkup with the proper length
init_parser(); // Allocates an internal processing buffer for ParseMarkup result, which I need to copy
sprintf(ptrReturnValue, "%s", ParseMarkup(szInputText) );
terminate_parser(); // Frees the internal processing buffer
return ptrReturnValue;
}
I would like to call it from C# using P/invoke.
[DllImport("MyDll.dll")]
private static extern string GetSomeText(string strInput);
How do I properly release the allocated memory?
I am writing cross-platform code targeting both Windows and Linux.
Edit:
Like this
[DllImport("MyDll.dll")]
private static extern System.IntPtr GetSomeText(string strInput);
[DllImport("MyDll.dll")]
private static extern void FreePointer(System.IntPtr ptrInput);
IntPtr ptr = GetSomeText("SomeText");
string result = Marshal.PtrToStringAuto(ptr);
FreePointer(ptr);

You should marshal returned strings as IntPtr otherwise the CLR may free the memory using the wrong allocator, potentially causing heap corruption and all sorts of problems.
See this almost (but not quite) duplicate question PInvoke for C function that returns char *.
Ideally your C dll should also expose a FreeText function for you to use when you wish to free the string. This ensures that the string is deallocated in the correct way (even if the C dll changes).

Add another function ReturnSomeText that calls free or whatever is needed to release the memory again.

If you return to .net memory allocated with your native malloc, then you also have to export the deallocator. I don't regard that to be a desirable action and instead prefer to export the text as a BSTR. This can be freed by the C# runtime because it knows that the BSTR was allocated by the COM allocator. The C# coding becomes a lot simpler.
The only wrinkle is that a BSTR uses Unicode characters and your C++ code uses ANSI. I would work around that like so:
C++
#include <comutil.h>
BSTR ANSItoBSTR(const char* input)
{
BSTR result = NULL;
int lenA = lstrlenA(input);
int lenW = ::MultiByteToWideChar(CP_ACP, 0, input, lenA, NULL, 0);
if (lenW > 0)
{
result = ::SysAllocStringLen(0, lenW);
::MultiByteToWideChar(CP_ACP, 0, input, lenA, result, lenW);
}
return result;
}
BSTR GetSomeText(char* szInputText)
{
return ANSItoBSTR(szInputText);
}
C#
[DllImport("MyDll.dll", CallingConvention=CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.BStr)]
private static extern string GetSomeText(string strInput);