Importing C++ dll to C#

Question

I am new to C#, Please help me with below case.

I am trying to import C++ dll into my C# Code and I am getting the following error.

A call to PInvoke function 'SerialInterface!SerialInterface.Form1::ReadTagData' 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.

Below is the C# code

const int buffSize = 33;
const int addr = 112;
const int readBytes = 8;

[DllImport(@"C:\Visual Studio 2010\Projects\SerialInterface\SerialInterface\bin\Debug\AP4600_SDK.dll")]

public static extern int ReadTagData(string tagID, string tagBuffer, Int32 szTagDataBuf, Int32 TagAddress, Int32 nBytes);

string asciiRead = "";
int s = ReadTagData(TagId, asciiRead, buffSize, addr, readBytes);

The function ReadTagData definition in the AP4600_SDK.dll is

AP4600_SDK_API int ReadTagData(
const char *pTagId,     /* TagId of tag to be read, from Identify */
char *pTagDataBuf,      /* (Output) The data read (ASCII representation of HEX), min size is 2*nBytes+1 (33 for Allegro) */
size_t szTagDataBuf,    /* Size of TagDataBuf, minimum is 2*nBytes+1 (33 for Allegro) */
int TagAddress,         /* Address of first byte to read */
size_t nBytes           /* Number of bytes to read (between 1 and 8, inclusive) */
);                      /* Returns zero for success, non zero failure */

Did you try adding it as a reference into your project? http://stackoverflow.com/questions/12992286/how-to-add-a-dll-reference-to-a-project-in-visual-studio — Hackerman, Oct 21 '16 at 13:44
Possible duplicate of [A call to PInvoke function '\[...\]' has unbalanced the stack](http://stackoverflow.com/questions/2941960/a-call-to-pinvoke-function-has-unbalanced-the-stack) — meJustAndrew, Oct 21 '16 at 13:46
size_t is int on 32 bit systems, long on 64 bit systems, that's why the stack gets unbalanced. — Gusman, Oct 21 '16 at 13:52

score 1 · Answer 1 · answered Oct 21 '16 at 14:16

Here are the mistakes that I can see:

Depending on how the macro AP4600_SDK_API is defined, the calling convention could be wrong. I can't say for sure since I can't see how AP4600_SDK_API evaluates. You may need to specify the calling convention explicitly in your p/invoke. At present it uses the default, CallingConvention.StdCall.
The second argument is a modifiable buffer and so must be StringBuilder rather than `string.
The two size_t arguments should be pointer sized. Use UIntPtr.

So assuming the calling convention is StdCall it would be:

[DllImport(@"...", CallingConvention.StdCall)] // or perhaps CallingConvention.Cdecl
public static extern int ReadTagData(
    string tagID, 
    StringBuilder tagBuffer, 
    UIntPtr szTagDataBuf, 
    int TagAddress, 
    UIntPtr nBytes
);

Call it like this:

StringBuilder tagBuffer = new StringBuilder(nBytes*2+1)
int retval = ReadTagData(tagId, tagBuffer, tagBuffer.Capacity, addr, nBytes);
if (retval != 0)
    // handle error
// output can now be read with tagBuffer.ToString()

You can take a look in to this http://www.codeproject.com/Articles/27298/Dynamic-Invoke-C-DLL-function-in-C — Meena, Oct 21 '16 at 14:32

Russell Trahan · Answer 2 · 2016-10-21T14:05:31.327

-1

The problem is explicitly stated in the error message that you received.

char *pTagDataBuf in c++ is not the same as string pTagDataBuf in c# for example. C# has managed strings. char* uses an unmanaged pointer. You may need to use an unsafe code block to access the function.

This error message will also show up if you are trying to load a 32bit dll into a 64bit project or vica verca.

edited Oct 21 '16 at 14:05

answered Oct 21 '16 at 13:47

Russell Trahan

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4
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They indeed are translated to strings when used with P/Invoke, the only you have to do is to set the correct marshaling. Look at this:http://stackoverflow.com/questions/30028593/const-char-in-c – Gusman Oct 21 '16 at 13:50
http://stackoverflow.com/questions/15273783/pinvoke-char-in-c-dll-handled-as-string-in-c-issue-with-null-characters Indeed, there is more than just what the OP stated. – Russell Trahan Oct 21 '16 at 13:52
Not sure what you mean, but there's no need of an unmanaged block, if the string is an input declare it as string, if it's an output as an StringBuilder with initial size, that's all. Also, that would not unbalance the stack, both are pointers so in the stack there's only 64 bit pointers, unbalanced stack happens when not all the data put on the stack is consumed. – Gusman Oct 21 '16 at 13:56
Based on the description of the unmanaged function's variable, `pTagDataBuf`, I assume that the data is stored as a char array but is not actually a string. It is a raw data buffer. He probably doesn't want a string. He probably wants to access the raw bytes. It can be achieved in multiple ways but I'm trying to give the most applicable answer, not just solve the programming problem. – Russell Trahan Oct 21 '16 at 14:04
Read the commend beside the field: (Output) The data read (ASCII representation of HEX), Its a string. And no, if it's an output as a string a pre-initialized StringBuilder is what must be used. – Gusman Oct 21 '16 at 14:07
I've never named a variable `DataBuf` and used it as a string. Semantics. The programming approach should follow what the application is. Not just whatever will compile. – Russell Trahan Oct 21 '16 at 14:18
No need for unsafe at all, and you certainly won't see this error with 32/64 bit mismatch. In short, everything in this answer is wrong. – David Heffernan Oct 21 '16 at 14:20

Importing C++ dll to C#

2 Answers2