Marshaling the structure array member of Structure in C#

[Vini Deep]

Heath Stewart wrote: // Define the callback.[return: MarshalAs(UnmanagedType.SysUInt)]public delegate IntPtr MyFunc(ref SecondStruct Info); This should work. Thanks Very much. But I am not fully clear. __declspec (dllexport) unsigned int CALLBACK MyFunc(SEC_STRUCT * Info); This is the function exported by the C++ Dll. How should I call this function from my C# client?

[DllImport("MyDll",CallingConvention=CallingConvention.Cdecl)]
public extern static System.UInt32  MyFunc(ref SecondStruct Info) ;
  

private void button1_Click(object sender, System.EventArgs e)
{
    FirstStruct[] first= new FirstStruct[40];
    ...
    SecondStruct info = new SecondStruct(....,first,...);
    System.UInt32 uRet= MyFunc(ref info);
    MessageBox.Show(info.Model);
    ...
}

Is this code correct? How to marshal the SecondStruct to IntPtr using Marshal.StructureToPtr and back using Marshal.PtrToStructure? Pls Help. Thanks :rose: Vini

[Heath Stewart]

What I did is an alternative to Marshal.StructureToPtr and Marshal.PtrToStructure (along with using Marshal.AllocHGlobal). Either way should work. The reason I define MyFunc as a delegate is because the native signature declared it as a CALLBACK. This indicates (though for your circumstances, I may be wrong) that you don't actually call this method, but simply pass it as a callback to some other function. If you were to declare it as a P/Invoke method, it would look like this:

[DllImport("mydll.dll")]
private static extern uint MyFunc(ref SecondStruct s);

Microsoft MVP, Visual C# My Articles

[Vini Deep]

Thanks.. Couldn't quite get it correct. The structure is passed to the function in dll as reference. If I declare the structure array variable as IntPtr, should I use pointer arithematics to read the value back from the structure array member of the struct? Tnx :) Vini

[Heath Stewart]

More than likely, this is going to be necessary. See the Marshal class documentation, though, for some methods that may ease the process a little. One more thing you should consider if you can modify the unmanaged code is a thunking layer. Basically, these come in handy when you have marshaling problems such as this. For example, you could define a method with takes all the fields as parameters for the first and second struct and then assembles them internally into the second struct, which it then calls the unmanaged function you are now. Using this approach, you'd be able to easily marshal members of both of the structs.

Microsoft MVP, Visual C# My Articles

[Vini Deep]

Sorry about this. I very much appretiate your help :rose:. Still a basic doubt: Heath Stewart wrote:

[StructLayout(LayoutKind.Sequential)]
public struct FirstStruct
{
[MarshalAs(UnmanagedType.R4)] public double XSpeed;
[MarshalAs(UnmanagedType.SysInt)] public IntPtr DRate;
[MarshalAs(UnmanagedType.SysUInt)] public IntPtr RotCtrl;
// ctor added for convenience and example.
public FirstStruct(double XSpeed, long DRate, long RotCtrl)
{
this.XSpeed = XSpeed;
this.DRate = new IntPtr(DRate);
this.RotCtrl = new IntPtr(RotCtrl);
}
}

Is there any problem if I declare the structure as [StructLayoutAttribute(LayoutKind.Sequential)] public struct FirstStruct { public System.Double XSpeed; public System.Int32 DRate; public System.UInt32 RotCtrl; } If I want to call a function from a dll written in C++ from my c# client, what should I do? Should I create a similar C# structure with managed DataTypes and use that to call the c++ dll function from my C# client, then use it to get the values returned by the Dll **OR** Should I create a similar structure in C# with Unmanaged Type data and then pass it to the function exported by the Dll and use the values returned? Is the above both declaration one and the same? I have used [MarshalAs(UnmanagedType.ByValTStr, ....)] only in places where the variable declared was char[].

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)] public string Vendor;

When I debug the program, at the function call I get an 'ExecutionEngineException'. Is it the problem which the marshaling of the structure array? Kindly help. Vini

[Heath Stewart]

First, in your declaration it is more desirable to use the intrinsic keywords instead of their actual types, like double instead of System.Double. Either way works, but the former makes your code more readable. They are the same thing, after all. The problem with declaring the last two as int and uint is that a native int and unsigned int are actual processor dependent. So, if this was running on a 64-bit processor, your struct wouldn't marshal correctly. An Int32 (int) will always be 32 bits, meaning that your field alignment will be incorrect and you'll get invalid values. That's why I declared them as IntPtr and made the parameters long (to allow for large enough numbers, although your values may never be more than 32 bits). No matter how you declare your structs, the problem is that nested structures can't be marshalled by the CLR. That's why I suggested a thunk. You could declare a native function and P/Invoke that like so:

[DllImport("whatever.dll", CharSet=CharSet.Ansi)]
private static extern IntPtr MyFuncThunk(
[MarshalAs(UnmanagedType.ByValTStr, SizeConst=20)] string Model,
[MarshalAs(UnmanagedType.SysUInt)] IntPtr SuptText,
FirstStruct CurSpd,
FirstStruct MaxRdSpd,
[MarshalAs(UnmanagedType.SysUInt)] IntPtr cNumSpd,
[MarshalAs(UnmanagedType.ByValArray, SizeConst=40)] FirstStruct[] CrwWrd,
[MarshalAs(UnmanagedType.ByValArray, SizeConst=20)] FirstStruct[] DWrd);
 
[DllImport("whatever.dll")]
private static extern IntPtr MyFunc(IntPtr Info);

You pass your data to the thunk which allocs and initializes the struct (SEC_STRUCT), then returns the address to the struct (a pointer). You can use that pointer then in the call to MyFunc, although I still wonder if you really want to P/Invoke this since the function is most likely not exported, being that it's a CALLBACK. The one way to find out is to load your DLL into depends.exe (comes with the Platform SDK tools, which is installed by default with Visual Studio) and look at the exports.

Microsoft MVP, Visual C# My Articles

[Vini Deep]

Thanks. But my function eventhough declared as a call back, is called by the UI. I already have a c++ client which uses this dll and calls the said function. I am trying to build a similar client in C#. As far as writing another method is concerned, I have got only the .dll file and the .h file with me. source code is not available. I have used the depends.exe, which shows me an entry to the above mentioned function as _MyFunc@4 in the dll. Any idea?? Vini

[Heath Stewart]

In your DllImportAttribute, set EntryPoint="_MyFunc@4". If you can't modify the source, you'll have to manually marshal the nested structs as I mentioned before by either pinning the first struct or using the appropriate Marshal methods.

Microsoft MVP, Visual C# My Articles

[Vini Deep]

Thank You very much... :rose: Finally I think I am getting everything right. I used the method of pinning the first structure , which u mentioned in one of your previous post. Tnx a lot. cheers :) Vini

[poda]

Hai Vini Deep, My need is same as your problem. Please reply if you have solved it. Thanks friend.