Structures in C#

Hello Everyone,

I have made a DLL in VC++, in which one function requires a struct variable to be passed, and i want to use that DLL in my c# application. When i try to use that DLL in c# application, it is giving error as Bad Pointer...
Does anybody have solution to this....


Thanks in Advance,
Poornima .S

either use C++/CLI to expose the C++ dll to C#, or p/invoke it and create structures in C# that are laid out the same as the structures in C++ and use that to pass data into the p/invoked function.

Quote:

---

Originally Posted by MadHatter

either use C++/CLI to expose the C++ dll to C#, or p/invoke it and create structures in C# that are laid out the same as the structures in C++ and use that to pass data into the p/invoked function.

---

What do you mean by "use C++/CLI to expose the C++ dll to C#". Is not it the same thing the poster tried.

no, you can create a .net assembly using C++ which is different than a native C++ dll which the OP has.

Quote:

---

Originally Posted by MadHatter

no, you can create a .net assembly using C++ which is different than a native C++ dll which the OP has.

---

Now I get your point. You wanted to say Managed C++.

http://www.google.com/search?q=C%2B%2B%2FCLI

paste your code here did you used the extern when prototyping the function in c#?

If you want C# to talk to a native C++ dll then try using the interop assistant from Microsoft.

Darwen.

Hello Everyone thanks for your help..

This what my code looks like

//Structure declared in the VC++ DLL
struct EmpDetails
{
unsigned char * bNAME;
unsigned char * bAGE;
unsigned char * bDOJ;
unsigned char * bDESIGNATATION;
unsigned char * bGENDER;
unsigned char * bSALARY;

};

//A Function in the same DLL that takes the above declared structure as input
int __stdcall FnWriteEmpDetails(const EmpDetails &stEmpDtls, unsigned char * bReturnVal);

I have done a Wrapper class for that, the function declaration is as below.

[DllImport("TEST_DLL.dll")]
//int __stdcall FnWriteEmpDetails(const EmpDetails &stEmpDtls, unsigned char * bReturnVal);
public static extern int FnWriteEmpDetails(EmpDetails stEmpDtls, byte[] bReturnVal);

And my structure definition in Wrapper class in c# is :

public struct EmpDetails
{
public byte [] bNAME;
public byte [] bAGE;
public byte [] bDOJ;
public byte [] bDESIGNATATION;
public byte [] bGENDER;
public byte [] bSALARY;

};

When i try to use this function in C#, it is not taking the structure variable, my test app is giving the following error

An unhandled exception of type 'System.NullReferenceException' occurred in TEST.exe

Additional information: Object reference not set to an instance of an object.

Is there any solution for this?

Quote:

---

Originally Posted by nabeelisnabeel

Now I get your point. You wanted to say Managed C++.

---

Technically speeking "Managed C++" is the language that was used with .NET 1.X and is no longer used.

The current language (.NET 2.0 thru 4.0) is C++/CLI.

The two are radically different. Originally Microsoft did not follow the "Extensibility guidelines" for C++ and created a very strange beast. C++/CLI on the other hand properly (well almost) follows the guidelines.

C++/CLI is integration of native and managed programming.

Your marshalling is incorrect.

I'm presuming that your structure is made up of null terminated C++ strings ? In which case your structure should look like :

Code:

---

[StructLayout(LayoutKind.Sequential)]
struct EmpDetails
{
    [MarshalAs(UnmanagedType.LPStr)]
    string bNAME;

    [MarshalAs(UnmanagedType.LPStr)]
    string bAGE;

    [MarshalAs(UnmanagedType.LPStr)]
    string bDOJ;

    [MarshalAs(UnmanagedType.LPStr)]
    string bDESIGNATATION;

    [MarshalAs(UnmanagedType.LPStr)]
    string bGENDER;

    [MarshalAs(UnmanagedType.LPStr)]
    string bSALARY;
};

---

The reason why you were getting a null-reference exeption in your case was you probably didn't assign anything to the byte [] arrays, so they were all null when passed into the method hence 'null exception'.

I could write a book on interop so I'm not going into details about this solution. Just remember you have to set each of the fields in the structure to a string before you pass it into your function.

Your function signature is also incorrect : but without knowing what 'bReturnVal' is (whether it is a single character to be output, or whether you have to pass an already allocated block of unsigned char memory to be filled by the function) I can't give you an interop signature for this.

i.e. in C++ whether you do

Code:

---

EmpDetails details;
char returnVal = 0;
FnWriteEmpDetails(stEmpDtls, &returnVal);

---

or

Code:

---

EmpDetails details;
std::vector<char> returnVal;
returnVal.resize(10); // or some size
FnWriteEmpDetails(stEmpDtls, &returnVal.front());

---

If it's a single character it would be :

Code:

---

[DllImport("TEST_DLL.dll")]
public static extern int FnWriteEmpDetails(ref EmpDetails stEmpDtls, [Out] out char bReturnVal);

---

If it's a character array it would be :

Code:

---

[DllImport("TEST_DLL.dll")]
public static extern int FnWriteEmpDetails(ref EmpDetails stEmpDtls,
    [In][MarshalAs(UnmanagedType.LPArray)] byte [] bReturnVal);

// call like this :
EmpDetails stEmpDtls = new EmpDetails();
stEmpDtls.bNAME = "Name";
// fill in the other fields

byte [] buffer = new byte[10]; // or whatever size
FnWriteEmpDetails(ref EmpDetails, buffer);

---

Notice the 'ref' ? This is needed because the parameter stEmpDtls is being passed by reference.

As I said before for most cases you can either use the microsoft interop assistant of go to www.pinvoke.net for information.

Darwen.

Hello Darwen,

My Structure in C++ looks as below

struct EmpDetails
{
unsigned char * bNAME;
unsigned char * bAGE;
unsigned char * bDOJ;
unsigned char * bDESIGNATATION;
unsigned char * bGENDER;
unsigned char * bSALARY;

};

So in C# wrapper class, for unsigned char array, which type i should take for marshalling?

I tried taking different types and but it didn't work.
bReturnVal is an output parameter...there is no problem with that

I gave you the EmpDetails struct in C# in my last post......

Darwen.

Hello Darwen,

See if i want to give bDOJ as 22/01/2009, then in my dll function i want the day as 22, but if i pass as string in c# it won't take it as 22 instead it will take that as 2 so is there any other way to pass a byte value using marshalling.

Thanks & Regards,
Poornima.

Sorry, I don't understand your last post.

If you pass bDOJB as "22/01/2009" then it's a string right ?

Or is there some kind of encoding which is used in C++ to turns the date 22/01/2009 into an unsigned char buffer ?

Can you show the C++ code for doing this ?

Darwen.

Hello Darwen,

I will explain in detail..

I have created a DLL Which has the following structure

struct EmpDetails
{
unsigned char * bNAME;
unsigned char * bAGE;
unsigned char * bDOJ;
unsigned char * bDESIGNATATION;
unsigned char * bGENDER;
unsigned char * bSALARY;

};

and a Function to which the above said structure is passed as input the function signature is as follows:

int __stdcall FnWriteEmpDetails(const EmpDetails &stEmpDtls, unsigned char * bReturnVal);

Here, the bReturnval is an output parameter there is no problem with this..

So now i want to use the DLL in C# application.
I have done a Wrapper Class in C# for the DLL.

The structure in c# looks like below:

public struct EmpDetails
{
[MarshalAs(UnmanagedType.LPStr)]
public string bNAME;

[MarshalAs(UnmanagedType.LPStr)]
public string bAGE;

[MarshalAs(UnmanagedType.LPStr)]
public string bDOJ;

[MarshalAs(UnmanagedType.LPStr)]
public string bDESIGNATATION;

[MarshalAs(UnmanagedType.LPStr)]
public string bGENDER;

[MarshalAs(UnmanagedType.LPStr)]
public string bSALARY;

};

And the function signature in C# is

[DllImport("TEST_DLL.dll")]
//int __stdcall FnWriteEmpDetails( const EmpDetails &stEmpDetails, unsigned char * bReturnVal);
public static extern int FnWriteEmpDetails(ref EmpDetails stEmpDetails, byte[] bReturnVal);


The DLL function internally just takes the input provided and send it to some device, here the data needs to be sent in bytes,

iCount = strlen((char*)stEmpDetails.bDOJ);
for(i, j=0; j< (iCount+1); j++,i++)
bData[i] = stEmpDetails.bDOJ[j];

so if i pass the bDOJ as string "22/1/2009" incorrect data will be taken. I want to the day as "22" in one byte.

I hope i have explained the problem properly...

Thanks & Regards,
Poornima .S

Please use code tags in future.

Try this for the struct :

Code:

---

using System.Runtime.InteropServices;

public struct EmpDetails
{
    [MarshalAs(UnmanagedType.LPStr)]
    public string bNAME;

    [MarshalAs(UnmanagedType.LPStr)]
    public string bAGE;

    public IntPtr bDOJ;

    [MarshalAs(UnmanagedType.LPStr)]
    public string bDESIGNATATION;

    [MarshalAs(UnmanagedType.LPStr)]
    public string bGENDER;

    [MarshalAs(UnmanagedType.LPStr)]
    public string bSALARY;
};

// code to translate byte [] to bDObj

byte [] byteArray = new byte[4];
GCHandle handle = GCHandle.Alloc(byteArray, GCHandleType.Pinned);

try
{
    EmpDetails empDetails = new EmpDetails();
    empDetails.bDOJ = handle.AddrOfPinnedObject();
    // make function call
}
finally
{
    handle.free();
}

---

Usually I'd have bDOJ as a byte [] array in the struct, but since you have to pass it by reference I don't believe this will work. P/Invoke will try to marshal the byte [] array from native to managed code on the way out of the function which it can't do because it doesn't know how big it is. So P/Invoke will most probably throw an exception at this point.

Darwen.