I have a C++ struct
struct UnmanagedStruct
{
char* s;
// Other members
};
and a C# struct
struct ManagedStruct {
[MarshalAs(UnmanagedType.LPStr)]
string s;
// Other members
}
the C++ library exposes
extern "C" UnmanagedStruct __declspec(dllexport) foo( char* input );
And it is imported like
[DllImport("SomeDLL.dll", CharSet = CharSet.Ansi)]
static extern ManagedStruct foo( string input );
However when I call this function I get
MarshalDirectiveException was unhandled
Method's type signature is not PInvoke compatible.
The thing is, this function call works if I remove the char* s and the string s from the structs.
For this type of scenario, do not use a String directly. Instead switch the type to be an IntPtr value and use Marshal.PtrToStringAuto/Ansi/Uni as appropriate. In this case, since your native code uses char*, PtrToStringAnsi is the best choice.
struct ManagedStruct {
IntPtr s;
public string sAsString { get { return Marshal.PtrToStringAnsi(s); } }
}
Related
I have these two functions exposed from Rust
extern crate libc;
use std::mem;
use std::ffi::{CString, CStr};
use libc::c_char;
pub static FFI_LIB_VERSION: &'static str = env!("CARGO_PKG_VERSION"); // '
#[no_mangle]
pub extern "C" fn rustffi_get_version() -> *const c_char {
let s = CString::new(FFI_LIB_VERSION).unwrap();
let p = s.as_ptr();
mem::forget(s);
p as *const _
}
#[no_mangle]
pub extern "C" fn rustffi_get_version_free(s: *mut c_char) {
unsafe {
if s.is_null() {
return;
}
let c_str: &CStr = CStr::from_ptr(s);
let bytes_len: usize = c_str.to_bytes_with_nul().len();
let temp_vec: Vec<c_char> = Vec::from_raw_parts(s, bytes_len, bytes_len);
}
}
fn main() {}
They are imported by C# as below
namespace rustFfiLibrary
{
public class RustFfiApi
{
[DllImport("rustffilib.dll", EntryPoint = "rustffi_get_version")]
public static extern string rustffi_get_version();
[DllImport("rustffilib.dll", EntryPoint = "rustffi_get_version_free")]
public static extern void rustffi_get_version_free(string s);
}
}
The memory of the string returned from rustffi_get_version is not managed by Rust anymore as mem::forget has been called. In C#, I want to call the get version function, get the string, and then pass it back to Rust for memory deallocation like below.
public class RustService
{
public static string GetVersion()
{
string temp = RustFfiApi.rustffi_get_version();
string ver = (string)temp.Clone();
RustFfiApi.rustffi_get_version_free(temp);
return ver ;
}
}
But the C# program crashes when it runs rustffi_get_version_free(temp). How to free the forgotten string memory in C#? What should be passed back to Rust for deallocation?
Instead of defining string as the argument in the C# extern, I changed it to pointer.
[DllImport("rustffilib.dll", EntryPoint = "rustffi_get_version")]
public static extern System.IntPtr rustffi_get_version();
[DllImport("rustffilib.dll", EntryPoint = "rustffi_get_version_free")]
public static extern void rustffi_get_version_free(System.IntPtr s);
public static string GetVersion()
{
System.IntPtr tempPointer = RustFfiApi.rustffi_get_version();
string tempString = Marshal.PtrToStringAnsi(tempPointer);
string ver = (string)tempString.Clone();
RustFfiApi.rustffi_get_version_free(tempPointer);
return ver ;
}
The IntPtr from rustffi_get_version can be successfully converted to a C# managed string type. tempString and ver are good.
When rustffi_get_version_free(tempPointer) runs, it throws an exception saying stack unbalanced:
A call to PInvoke function 'rustFfiLibrary!rustFfiLibrary.RustFfiApi::rustffi_get_version_free' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.
sizeof(IntPtr) and sizeof(char *) are both 4 on my system. Plus, IntPtr works for return value; why doesn't it work as an input parameter?
extern "C" fn in Rust means the function uses the C calling convention.
C# expects P/Invoke functions to use the stdcall calling convention by default.
You can tell C# to use the C calling convention:
[DllImport("rustffilib.dll", CallingConvention = CallingConvention.Cdecl)]
Alternatively, you could use extern "stdcall" fn on the Rust side.
I'm trying to build a wrapper to use cpp code inside c# code, and I want to return custom struct (Class) as output for method2, and std::string for method1, and this is my code in cpp
extern "C" __declspec(dllexport) std::string method1()
{
std::string s;
//Some Code/////////
return s;
}
and this is the method that should return custom struct (or class)
extern "C" __declspec(dllexport) MyStruct method2()
{
MyStruct s;
//Some Code//////
return s;
}
and I tried to write c# code for both of these methods and here is my c# code
[DllImport("<dllPath>", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string method1(); //Exception
[DllImport("<DllPath>")]
public static extern MyStruct method2(); //Compile Error
Now when I'm trying to run the c# code I get MarshalDirectiveException for method1, and compiling error for method2?
In the first PInvoke, the C++ method should return const char* (s.c_str()). You should remove "[return [...]]" and replace string by IntPtr. Then, you can convert the IntPtr to string with Marshal.PtrToStringAnsi(ptr). http://msdn.microsoft.com/en-US/library/s9ts558h(v=vs.110).aspx can help you.
In the second PInvoke, you should define in C# the MyStruct (but I can't be more precise because I have no information about MyStruct). This link can help you : http://www.codeproject.com/Articles/66243/Marshaling-with-C-Chapter-Marshaling-Compound-Ty
EDIT : Thanks to hvd's comment, it is better sometimes to use BSTR, because the use of char* can be dangerous !
I am trying to access and make changes to elements in a struct that are in a DLL. I have followed this example on how to use struct from a DLL: http://nawatt.com/index.php/corporate/blog/78-using-c-dlls-in-c
I have been successful in making changes to non-array variables, but whenever I try to make changes to an array I get a Runtime Error.
This is an example of my C DLL code:
//lib_qrs.dll
#ifdef __cplusplus
extern "C" {
#endif
typedef struct TEST_STRUCT
{
unsigned short check[5];
} test_struct;
__declspec(dllexport) void __stdcall test(test_struct *test, unsigned short val){
// This is an example of what DOES NOT WORK
test->check[0]=val+1;
test->check[1]=val+2;
test->check[2]=val+3;
test->check[3]=val+4;
test->check[4]=val+5;
}
#ifdef __cplusplus
}
#endif
This is an example of my C# code:
[StructLayout(LayoutKind.Sequential)]
public struct TEST_STRUCT
{
public UInt16[] check;
}
public class Program
{
[DllImport("lib_qrs.dll", EntryPoint="test", CallingConvention = CallingConvention.StdCall)]
public static extern void test(ref TEST_STRUCT test, int val);
public TEST_STRUCT testStruct = new TEST_STRUCT();
static void Main(string[] args)
{
testStruct.check=new UInt16[5];
// WHERE ERROR OCCURS
test(ref testStruct, 5);
}
}
The error that I get is:
*"An unhandled exception of type 'System.AccessViolationException' occurred in Test.exe
Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt."
I understand that I have to be very careful with memory allocation when I replicate my structure in C#, but I don't know what I am doing wrong or how I can fix this array issue.
Does anyone have an idea of how I could get around this?
The default marshaling for that array is unsigned short*, not unsigned short[]. You'll need to apply the [MarshalAs] attribute to tell the pinvoke marshaller about it. Fix:
[StructLayout(LayoutKind.Sequential)]
public struct TEST_STRUCT {
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)]
public UInt16[] check;
}
And since you are returning values in the array, you also have to tell the pinvoke marshaller that it needs to copy the array values back. That requires the [Out] attribute:
[DllImport("lib_qrs.dll")]
public static extern void test([Out] ref TEST_STRUCT test, int val);
Do note that neither is necessary at all if the you just declare the argument as ushort[], assuming that the structure doesn't contain any additional fields.
I have DLL, written in C++. There is structure in this DLL.
typedef struct TransmitData
{
wchar_t szPath[MAX_PATH];
DWORD dwResult;
} *lpTransmitData;
And I have one function to fill this struct
extern "C" __declspec(dllexport) int GetData(struct TransmitData &data)
{
//I HAVE ONE THE SAME STRUCT IN THIS DLL
memcpy(&data, &transmitData, sizeof(struct TransmitData));
return ret_value;
}
In C# program I use functions with simple data types well, but for structure it doesn't work. There is code on C#:
public struct TransmitData
{
[MarshalAs(UnmanagedType.LPWStr, SizeConst = 260)] //260 = MAX_PATH
public string szPath;
public uint dwResult;
}
//...
[DllImport("MyDLL")]
public static extern int GetData(ref TransmitData data);
What am I doing wrong?
Thanks!
I suggest you to replace
[DllImport("MyDLL")]
with
[DllImport("MyDLL", CallingConvention = CallingConvention.Cdecl)]
and
extert "C"
with
extern "C"
Moreover, as Joe suggested, add
[StructLayout(LayoutKind.Sequential)]
before the declaration of the struct in C# code
I am attempting to call a C++ function in a DLL from C# via interop. The DLL was written by someone else. The function signature is as follows:
AXI_DLL_EXPORT int __stdcall GetFileType(StringParam *stringParam, int *fileType)
StringParam is a struct defined as follows:
struct StringParam
{
int size; // 4 byte integer for the size of the string buffer
LPWSTR buff; // Pointer to a wide char buffer
}
This struct is used for passing strings back and forth. It can be used as both in and out parameters. The purpose of the size field in the struct is primarily for returning strings to the caller and to indicate the size of the buffer required. If the size of the buffer (provided by the caller) is too small to hold the string the caller can be informed of this and can then provide a larger buffer in a successive call to the function.
In C# I created a corresponding struct as follows:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
internal struct StringParam
{
public int Size;
[MarshalAs(UnmanagedType.LPWStr)]
public string Text;
public StringParam(string text)
{
this.Text = text;
this.Size = text.Length;
}
}
I declared the DllImport function call as follows:
[DllImport("Custom.dll", CharSet = CharSet.Unicode)]
public static extern int GetFileType(StringParam stringParam, out int fileType);
The call fails with the following error:
A call to PInvoke function '... GetFileType' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.
How should I declare the struct and call the native function from C#?
public static extern int GetFileType(ref StringParam stringParam, out int fileType);
Have you tried :
[DllImport("Custom.dll", CharSet = CharSet.Unicode)]
public static extern int GetFileType(StringParam stringParam, ref int fileType);
And I think you have to pin the ref int before you pInvoke.