C# PInvoke for 2 ** pointers

Question

I am working with a 3. party SDK, which is made up from .dll, .lib and .h files. I am using the .dll's to PInvoke against. And the .h files to see the function names and parameters. (So I am not using the .lib files).

The SDK is rather complex, so making the PInvoke wrappers have proven to be a challenge. All the functions/structs/enums is defined in the .h files.

My question is how to implement a pinvoke for a function with 2 **.

I expect it is my C# function definition that is wrong. When I call the function it simplely crashes, no exception throw or anything. The program just stops.

Function: GetInformatiuon(...)

//C Function: GetInformatiuon(...)
ERROR GetInformatiuon(
    Component comp, 
    struct Information** Info);

//C# Function: GetInformatiuon(...)
[DllImport("externalSDK.dll", EntryPoint = "GetInformatiuon", CallingConvention = CallingConvention.Cdecl)]
    public static extern ERROR GetInformatiuon(Component comp, ref Information Info);
);

Enum: ERROR

//C Enum: ERROR 
typedef enum ERROR_E {
    OK = 0, //Everything is ok
    E_ARG = 1, //Error in the Arguments 
    E_DATA = 2 //Data error
    //And more...
 } ERROR;

 //C# Enum: ERROR
 public enum ERROR
 {
    OK = 0, //Everything is ok
    E_ARG = 1, //Error in the Arguments 
    E_DATA = 2 //Data error
    //And more...
 }

Struct: Component

//C struct: Component
typedef struct Component_S
{
    void* ObjPointer;    
    unsigned long number; 
} Component;

//C# class: Component
[StructLayout(LayoutKind.Sequential)]
public class Component
{
    public IntPtr ObjPointer;
    public uint number; //uint because usigned long C is 4 bytes (32 bits) and C# ulong is 8 bytes (64 bits), where C# uint is 4 bytes(32 bits)
}

Struct: Information

 //C struct: Information
 typedef struct Information_S {
   char* language;           
   unsigned long sampleFrequency;  
   unsigned long frameShift;       
 }Information;

//C# struct: Information
[StructLayout(LayoutKind.Sequential)]
public struct Information
{
    public string language;   
    public uint sampleFrequency;  //uint because usigned long C is 4 bytes (32 bits) and C# ulong is 8 bytes (64 bits), where C# uint is 4 bytes(32 bits)
    public uint frameShift;       //uint because usigned long C is 4 bytes (32 bits) and C# ulong is 8 bytes (64 bits), where C# uint is 4 bytes(32 bits)
}

You'll have to declare it as out IntPtr. Then use Marshal.PtrToStructure() to convert it. Then fret a great deal on how you are going to get the memory that was allocated for the structure released again. It is a lousy signature, change the C code if you can. — Hans Passant
– Hans Passant, Commented Nov 21, 2013 at 16:39
As Hans said above, struct Information** in the signature means that you're pointing to the location in memory of another pointer and your C# extern function call doesn't get to deal with an actual Information struct unless you do some Marshalling — nvuono
– nvuono, Commented Nov 21, 2013 at 16:42
Sadly I cannot change the C signature. But I will try the "out IntPtr" solution Hans mentioned — Mr. JWolf
– Mr. JWolf, Commented Nov 21, 2013 at 16:50

Mr. JWolf · Accepted Answer · 2013-11-21 17:01:12Z

1

I found the solution! (aka Hans Passant comment)

I changed the function to:

Function: GetInformatiuon(...)

//C Function: GetInformatiuon(...)
ERROR GetInformatiuon(
    Component comp, 
    struct Information** Info);

//C# Function: GetInformatiuon(...)
[DllImport("externalSDK.dll", EntryPoint = "GetInformatiuon", CallingConvention = CallingConvention.Cdecl)]
    public static extern ERROR GetInformatiuon(Component comp, out IntPtr InfoPtr);
);

Calling the function

IntPtr infoPtr;
lhErr = SDK.GetInformatiuon(component, out infoPtr);
Information info = (Information)Marshal.PtrToStructure(infoPtr, typeof(Information)) ;

answered Nov 21, 2013 at 17:01

Mr. JWolf

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