I am writing a C# wrapper around a third party C++ library and with a new version a method parameter changed from uint to ulong.
I still need to support the old version preferably without changing the wrapper interface.
Now I would need to know if I can call the new or the old method. I already tested that I can call the new method with the old library without a C# exception but of course the results will be wrong.
Is there a way to check if a native method with specific parameter types exists? Or maybe another way around it?
This would one example of a changed method (notice the value parameter):
[DllImport("Foo.dll")]
public extern static uint Bar(IntPtr ptr, uint value, out IntPtr optr);
[DllImport("Foo.dll")]
public extern static uint Bar(IntPtr ptr, ulong value, out IntPtr optr);
I have an unmanaged function call that is throwing this exception when I try to pass it the path to a file name.
I've read that this is likely caused by the DLL itself but I don't think that is the case since the DLL is used in another application, so the problem is likely in my method calling the function.
The specification:
libvlc_media_new_path (libvlc_instance_t *p_instance, const char *path)
Description:
p_instance the instance
path local filesystem path
And my method:
[DllImport("libvlc", EntryPoint = "libvlc_media_new_path")]
public static extern IntPtr NewMedia(IntPtr instance,
[MarshalAs(UnmanagedType.LPStr)] string path);
I think I'm missing the convention call but what would that likely be? Or would it be something else causing this exception?
EDIT: Based on some comments I did some poking around and found... well, nothing. The struct for the instance is opaque, which means I have no idea in Laymans terms. My guess is that it means you don't need to reconstruct it in the application that is using it?
In a blind guess based on this, I replaced the return value that I had been using with the function responsible for setting the *p_instance value to a long instead of an IntPtr since when it was an IntPtr it was returning 0, and with a long I was seeing a value. Again, what an IntPtr is I don't really know. I was pretty happy to see something not 0 in the instance variable but when I ran it past that, it errored out again.
EDIT: I've expanded the question to here.
Based on the exception you're seeing and the declaration you've provided for the native function,
libvlc_media_new_path (libvlc_instance_t *p_instance, const char *path)
your p/invoke declaration is incorrect. You've mismatched the calling conventions. The default for the .NET p/invoke system is stdcall (to match the Windows API functions), but the default for C and C++ code is cdecl. You have to tell .NET explicitly that your function uses the cdecl calling convention.
So change it to look like this:
[DllImport("libvlc", EntryPoint = "libvlc_media_new_path", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr NewMedia(IntPtr instance,
[MarshalAs(UnmanagedType.LPStr)] string path);
Of course, I'm guessing that you're right about the return value being a pointer. The native function declaration you've shown is missing the return type.
As for your question about the instance parameter, and whether you are correctly using the IntPtr type: The parameter is a pointer to a libvlc_instance_t, so you have two basic ways of making that work using p/invoke. First is to declare the parameter as an IntPtr, which gets it marshalled like a raw pointer value. This is not particularly useful for cases where the pointer needs to be anything other than opaque (i.e. retrieved from one native function, stored, and then passed to another native function). Second is to declare a managed structure that mirrors the native structure, and then write the p/invoke declaration to use this structure so that the marshaller will handle things automatically. This is most useful if you actually need to interact with the values stored in the structure pointed to by the pointer.
In this case, after a Google search, it looks like you're using one of the VLC APIs. Specifically this one. That also tells us what an libvlc_instance_t is: it is an opaque structure that represents a libvlc instance. So declaring a managed structure is not an option here, because the structure is treated as opaque even by the native code. All you really need is the pointer, passed back and forth; a perfect case for the first method I talked about above. So the declaration shown above is your winner.
The only battle now is obtaining a valid pointer to a libvlc instance that you can pass to the function whenever you call it. Chances are good that will come from a prior call to a function like libvlc_new, which is documented as creating and intializing a new libvlc instance. Its return value is exactly what you need here. So unless you've already created a libvlc instance (in which case, use that pointer), you will also need to call this function and store its result.
If the documentation is correct about the required values for the libvlc_new function's parameters, you can declare it very simply:
[DllImport("libvlc", EntryPoint = "libvlc_new", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr NewCore(int argc, IntPtr argv);
And call it thus:
IntPtr pLibVlc = NewCore(0, IntPtr.Zero);
// now pLibVlc should be non-zero
And of course, I know nothing about the VLC APIs, but my general knowledge of API design tells me that you will probably need to call the libvlc_release function with that same pointer to an instance once you're finished with it.
Try without the [MarshalAs(UnmanagedType.LPStr)], usually works for me.
I have a main application written in C# that runs as a x64 bit application, it communicates through dll import with a standard native unmanaged C/C++ dll of which I have also the header.
I need help for setting out the correct dataTypes.
So i expose one of the methods I have to call and the data types defined in the dll header.
typedef int DLL_IMP_EXP (* INJECTDLL)(HANDLE, DWORD, LPCTSTR, DWORD, HINSTANCE *);
HANDLE is defined as void*
DWORD is defined as unsigned long
LPCTSTR is defined as __nullterminated CONST CHAR*
HINSTANCE gives me this line for definition: DECLARE_HANDLE(HINSTANCE); ?!?
Using Unicode declaration of the function:
LPCWSTR is defined as __nullterminated CONST WCHAR*
Please help me writing the correct declaration of the:
[DllImport ("Inject.dll", VariousParameters)]
public static extern int InjectDll(CorrectDataTypes);
Compiling VariousParameters if needed, and obviously CorrectDataTypes.
And IntPtr is used for a pointer or a handle - it will be 32 bits on a 32 bit system and 64 on a 64 bit system. So if you have anything that is a raw pointer or a handle use an IntPtr and it will work correctly on both systems. However your last parameter is a pointer to a handle - use a ref to handle the pointer. So in this case since it's a pointer to a handle, the parameter will be a ref to an IntPtr.
For standard numeric types, those will map directly to the .NET data types - you can get more details at MSDN.
Null terminated strings are handled correctly, although you'll need to specify whether it uses ANSI or Unicode strings.
Finally P/Invoke assumes a StdCall calling convention (which is what the Windows API uses). If you're not using that, which the function prototype would include STDCALL or __stdcall in it, the standard C calling convention is Cdecl. Although you'll have to find out what DLL_IMP_EXP expands to.
So your P/Invoke declaration should be:
[DllImport ("Inject.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern int InjectDll(IntPtr handle, uint dword, string str, uint dword2, ref IntPtr hInstance);
I want to call the following function from my managed code:
short LS_LoadConfig(LS_ID SensorID,LPVARIANT varConfigPathFile,BOOL bInit)
Here is how I declare the extern function in my C# class:
[DllImport("LineSensor.dll", EntryPoint = "#16")]
private static extern Int16 LS_LoadConfig(
Int16 deviceId,
IntPtr variantFilePath,
int init);
And here is how I create the VARIANT instance and I obtain a pointer to it. Then I call the C# function:
string filepath = #"C:\Windows\ ...";
IntPtr variantFilePath = Marshal.AllocCoTaskMem(200);
Marshal.GetNativeVariantForObject(filepath, variantFilePath);
LS_LoadConfig(device.Id, variantFilePath, initLineSensor);
The problem is that I keep receiving error messages such as "calling the LS_LoadConfig function has unbalanced the stack, check that parameters match the unmanaged signature".
It seems that the problem is caused by the second argument "variantFilePath", like it is not properly marshaled and its size on the unmanaged heap doesn't correspond to the one of an address (32-bit in my case). I tried to change the type in the C# function signature from IntPtr to int as follows:
[DllImport("LineSensor.dll", EntryPoint = "#16")]
private static extern Int16 LS_LoadConfig(
Int16 deviceId,
int variantFilePath,
int init);
I tried to call the function passing a random number and it got slightly better, I have just received an error "memory access violation". Obviously because the random number wasn't a valid address.
Does anybody knows the solution to this problem?
Thank you for any helpful information.
The access violation you created is not better. It also prevents the MDA warning from being generated. Short from the argument types being wrong, the int16 looks pretty weird, the most likely trouble is caused by the CallingConvention. Try StdCall.
And declare the 2nd argument as "object", its default marshaling is to a VARIANT. Declare it with the "ref" keyword to get an LPVARIANT.
"calling the LS_LoadConfig function has unbalanced the stack, check that parameters match the unmanaged signature".
This usually means that you're using conflicting calling conventions between your native and managed code. C# by default uses stdcall, and c/c++ uses cdecl. Try specifying CallingConvention = CallingConvention.Cdecl.
Probably a noob question but interop isn't one of my strong points yet.
Aside from limiting the number of overloads is there any reason I should declare my DllImports like:
[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hWnd, int msg, int wParam, IntPtr lParam);
And use them like this:
IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(formatrange));
Marshal.StructureToPtr(formatrange, lParam, false);
int returnValue = User32.SendMessage(_RichTextBox.Handle, ApiConstants.EM_FORMATRANGE, wParam, lParam);
Marshal.FreeCoTaskMem(lParam);
Rather than creating a targeted overload:
[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hWnd, int msg, int wParam, ref FORMATRANGE lParam);
And using it like:
FORMATRANGE lParam = new FORMATRANGE();
int returnValue = User32.SendMessage(_RichTextBox.Handle, ApiConstants.EM_FORMATRANGE, wParam, ref lParam);
The by ref overload ends up being easier to use but I'm wondering if there is a drawback that I'm not aware of.
Edit:
Lots of great info so far guys.
#P Daddy: Do you have an example of basing the struct class off an abstract (or any) class? I changed my signature to:
[DllImport("user32.dll", SetLastError = true)]
public static extern int SendMessage(IntPtr hWnd, int msg, int wParam, [In, Out, MarshalAs(UnmanagedType.LPStruct)] CHARFORMAT2 lParam);
Without the In, Out, and MarshalAs the SendMessage (EM_GETCHARFORMAT in my test) fail. The above example works well but if I change it to:
[DllImport("user32.dll", SetLastError = true)]
public static extern int SendMessage(IntPtr hWnd, int msg, int wParam, [In, Out, MarshalAs(UnmanagedType.LPStruct)] NativeStruct lParam);
I get a System.TypeLoadException that says the CHARFORMAT2 format is not valid (I'll try and capture it for here).
The exception:
Could not load type 'CC.Utilities.WindowsApi.CHARFORMAT2' from assembly 'CC.Utilities, Version=1.0.9.1212, Culture=neutral, PublicKeyToken=111aac7a42f7965e' because the format is invalid.
The NativeStruct class:
public class NativeStruct
{
}
I've tried abstract, adding the StructLayout attribute, etc. and I get the same exception.
[StructLayout(LayoutKind.Sequential)]
public class CHARFORMAT2: NativeStruct
{
...
}
Edit:
I didn't follow the FAQ and I asked a question that can be discussed but not positively answered. Aside from that there has been lot's of insightful information in this thread. So I'll leave it up to the readers to vote up an answer. First one to over 10 up-votes will be the answer. If no answer meets this in two days (12/17 PST) I'll add my own answer that summarizes all the yummy knowledge in the thread :-)
Edit Again:
I lied, accepting P Daddy's answer because he is the man and has been a great help (he has a cute little monkey too :-P)
If the struct is marshalable without custom processing, I greatly prefer the latter approach, where you declare the p/invoke function as taking a ref (pointer to) your type. Alternatively, you can declare your types as classes instead of structs, and then you can pass null, as well.
[StructLayout(LayoutKind.Sequential)]
struct NativeType{
...
}
[DllImport("...")]
static extern bool NativeFunction(ref NativeType foo);
// can't pass null to NativeFunction
// unless you also include an overload that takes IntPtr
[DllImport("...")]
static extern bool NativeFunction(IntPtr foo);
// but declaring NativeType as a class works, too
[StructLayout(LayoutKind.Sequential)]
class NativeType2{
...
}
[DllImport("...")]
static extern bool NativeFunction(NativeType2 foo);
// and now you can pass null
<pedantry>
By the way, in your example passing a pointer as an IntPtr, you've used the wrong Alloc. SendMessage is not a COM function, so you shouldn't be using the COM allocator. Use Marshal.AllocHGlobal and Marshal.FreeHGlobal. They're poorly named; the names only make sense if you've done Windows API programming, and maybe not even then. AllocHGlobal calls GlobalAlloc in kernel32.dll, which returns an HGLOBAL. This used to be different from an HLOCAL, returned by LocalAlloc back in the 16-bit days, but in 32-bit Windows they are the same.
The use of the term HGLOBAL to refer to a block of (native) user-space memory just kind of stuck, I guess, and the people designing the Marshal class must not have taken the time to think about how unintuitive that would be for most .NET developers. On the other hand, most .NET developers don't need to allocate unmanaged memory, so....
</pedantry>
Edit
You mention you're getting a TypeLoadException when using a class instead of a struct, and ask for a sample. I did up a quick test using CHARFORMAT2, since it looks like that's what you're trying to use.
First the ABC1:
[StructLayout(LayoutKind.Sequential)]
abstract class NativeStruct{} // simple enough
The StructLayout attribute is required, or you will get a TypeLoadException.
Now the CHARFORMAT2 class:
[StructLayout(LayoutKind.Sequential, Pack=4, CharSet=CharSet.Auto)]
class CHARFORMAT2 : NativeStruct{
public DWORD cbSize = (DWORD)Marshal.SizeOf(typeof(CHARFORMAT2));
public CFM dwMask;
public CFE dwEffects;
public int yHeight;
public int yOffset;
public COLORREF crTextColor;
public byte bCharSet;
public byte bPitchAndFamily;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst=32)]
public string szFaceName;
public WORD wWeight;
public short sSpacing;
public COLORREF crBackColor;
public LCID lcid;
public DWORD dwReserved;
public short sStyle;
public WORD wKerning;
public byte bUnderlineType;
public byte bAnimation;
public byte bRevAuthor;
public byte bReserved1;
}
I've used using statements to alias System.UInt32 as DWORD, LCID, and COLORREF, and alias System.UInt16 as WORD. I try to keep my P/Invoke definitions as true to SDK spec as I can. CFM and CFE are enums that contain the flag values for these fields. I've left their definitions out for brevity, but can add them in if needed.
I've declared SendMessage as:
[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern IntPtr SendMessage(
HWND hWnd, MSG msg, WPARAM wParam, [In, Out] NativeStruct lParam);
HWND is an alias for System.IntPtr, MSG is System.UInt32, and WPARAM is System.UIntPtr.
[In, Out] attribute on lParam is required for this to work, otherwise, it doesn't seem to get marshaled both directions (before and after call to native code).
I call it with:
CHARFORMAT2 cf = new CHARFORMAT2();
SendMessage(rtfControl.Handle, (MSG)EM.GETCHARFORMAT, (WPARAM)SCF.DEFAULT, cf);
EM and SCF are enums I've, again, left out for (relative) brevity.
I check success with:
Console.WriteLine(cf.szFaceName);
and I get:
Microsoft Sans Serif
Works like a charm!
Um, or not, depending on how much sleep you've had and how many things you're trying to do at once, I suppose.
This would work if CHARFORMAT2 were a blittable type. (A blittable type is a type that has the same representation in managed memory as in unmanaged memory.) For instance, the MINMAXINFO type does work as described.
[StructLayout(LayoutKind.Sequential)]
class MINMAXINFO : NativeStruct{
public Point ptReserved;
public Point ptMaxSize;
public Point ptMaxPosition;
public Point ptMinTrackSize;
public Point ptMaxTrackSize;
}
This is because blittable types are not really marshaled. They're just pinned in memory—this keeps the GC from moving them—and the address of their location in managed memory is passed to the native function.
Non-blittable types have to be marshaled. The CLR allocates unmanaged memory and copies the data between the managed object and its unmanaged representation, making the necessary conversions between formats as it goes.
The CHARFORMAT2 structure is non-blittable because of the string member. The CLR can't just pass a pointer to a .NET string object where a fixed-length character array is expected to be. So the CHARFORMAT2 structure must be marshaled.
As it would appear, for correct marshaling to occur, the interop function must be declared with the type to be marshaled. In other words, given the above definition, the CLR must be making some sort of determination based on the static type of NativeStruct. I would guess that it's correctly detecting that the object needs to be marshaled, but then only "marshaling" a zero-byte object, the size of NativeStruct itself.
So in order to get your code working for CHARFORMAT2 (and any other non-blittable types you might use), you'll have to go back to declaring SendMessage as taking a CHARFORMAT2 object. Sorry I led you astray on this one.
Captcha for the previous edit:
the whippet
Yeah, whip it good!
Cory,
This is off topic, but I notice a potential problem for you in the app it looks like you're making.
The rich textbox control uses standard GDI text-measuring and text-drawing functions. Why is this a problem? Because, despite claims that a TrueType font looks the same on screen as on paper, GDI does not accurately place characters. The problem is rounding.
GDI uses all-integer routines to measure text and place characters. The width of each character (and height of each line, for that matter) is rounded to the nearest whole number of pixels, with no error correction.
The error can easily be seen in your test app. Set the font to Courier New at 12 points. This fixed-width font should space characters exactly 10 per inch, or 0.1 inches per character. This should mean that, given your starting line width of 5.5 inches, you should be able to fit 55 characters on the first line before wrap occurs.
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123
But if you try, you'll see that wrap occurs after only 54 characters. What's more the 54th character and part of the 53rd overhang the apparent margin shown on the ruler bar.
This assumes you have your settings at standard 96 DPI (normal fonts). If you use 120 DPI (large fonts), you won't see this problem, although it appears that you size your control incorrectly in this case. You also won't likely see this on the printed page.
What's going on here? The problem is that 0.1 inches (the width of one character) is 9.6 pixels (again, using 96 DPI). GDI doesn't space characters using floating point numbers, so it rounds this up to 10 pixels. So 55 characters takes up 55 * 10 = 550 pixels / 96 DPI = 5.7291666... inches, whereas what we were expecting was 5.5 inches.
While this will probably be less noticeable in the normal use case for a word processor program, there is a likelihood of instances where word wrap occurs at different places on screen versus on page, or that things don't line up the same once printed as they did on screen. This could turn out to be a problem for you if this is a commercial application you're working on.
Unfortunately, the fix for this problem is not easy. It means you'll have to dispense with the rich textbox control, which means a huge hassle of implementing yourself everything it does for you, which is quite a lot. It also means that the text drawing code you'll have to implement becomes fairly complicated. I've got code that does it, but it's too complex to post here. You might, however, find this example or this one helpful.
Good luck!
1 Abstract Base Class
I've had some fun cases where a parameter is something like ref Guid parent and the corresponding documentation says:
"Pointer to a GUID specifying the parent. Pass a null pointer to use [insert some system-defined item]."
If null (or IntPtr.Zero for IntPtr parameters) really is an invalid parameter, then you're fine using a ref parameter - maybe even better off since it's extra clear exactly what you need to pass.
If null is a valid parameter, you can pass ClassType instead of ref StructType. Objects of a reference type (class) are passed as a pointer, and they allow null.
No, you cannot overload SendMessage and make the wparam argument an int. That will make your program fail on a 64-bit version of the operating system. It has to be a pointer, either IntPtr, a blittable reference or an out or ref value type. Overloading the out/ref type is otherwise fine.
EDIT: As the OP pointed out, this is not actually a problem. The 64-bit function calling convention passes the first 4 arguments through registers, not the stack. There is thus no danger of stack mis-alignment for the wparam and lparam arguments.
I don't see any drawbacks.
By-ref is often enough for simple type and simple structure.
IntPtr should be favored if the structure has a variable-size or if you want to do custom processing.
Using ref is simpler and less error-prone than manipulating pointers manually, so I see no good reason for not using it... Another benefit of using ref is that you don't have to worry about freeing unmanaged allocated memory