C# Dllimport - pointer to a pointer receiving array

Question

I am trying to DLLImport the function simxGetObjects from remoteApi.dll of v-rep software. Here is the link to the function description: http://www.coppeliarobotics.com/helpFiles/en/remoteApiFunctions.htm#simxGetObjects

and here is the brief description for this function from the above link:

Description: Retrieves object handles of a given type, or of all types (i.e. all object handles)

C synopsis: simxInt simxGetObjects(simxInt clientID,simxInt objectType,simxInt* objectCount,simxInt** objectHandles,simxInt operationMode)

C parameters:
clientID: the client ID. refer to simxStart.

objectType: object type (sim_object_shape_type, sim_object_joint_type, etc., or sim_handle_all for any type of object

objectCount: pointer to a value that will receive the number of retrieved handles

objectHandles: pointer to a pointer that will receive an object handle array. The array remains valid until next remote API function is called. operationMode: a remote API function operation mode. Recommended operation mode for this function is simx_opmode_oneshot_wait

Here is the way I am importing it (simxGetObjects function):

[DllImport("remoteApi.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern int simxGetObjects(int clientID, string objectType, IntPtr objectCount, ref IntPtr objectHandles, string operationMode);

and here is how I am calling it:

int intClientID = simxStart("127.0.0.1", 19999, true, true, 5000, 5);
IntPtr intptrObjectCount = IntPtr.Zero;
IntPtr intptrObjectHandles = IntPtr.Zero;
simxGetObjects(intClientID, "sim_handle_all", intptrObjectCount, ref intptrObjectHandles, "simx_opmode_oneshot_wait");

It does not show any error, however both intptrObjectCount and intptrObjectHandles variables are zero.

I really appreciate if someone can help me on this.

The return value is a flag showing the status of the function whether it was successful or not. However, the actual outputs are objectCount and objectHandles that I need to obtain. It basically calls the software that is running in the background and retrieve the number and handle of all of the objects that are in the scene. — NESHOM
– NESHOM, Commented Jul 21, 2015 at 19:53
The return value (I mean value itself) might help to understand what you are doing wrong. — walruz
– walruz, Commented Jul 21, 2015 at 19:56
It is 0 which means it runs ok. Sorry I didn't get what you meant. — NESHOM
– NESHOM, Commented Jul 21, 2015 at 20:00

walruz · Accepted Answer · 2015-07-22 04:49:54Z

1

Try this:

[DllImport("remoteApi.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int simxGetObjects(
    int clientID, 
    int objectType, 
    out int objectCount, 
    out IntPtr objectHandles, 
    int operationMode
);

int objectCount;
IntPtr objectHandles;

int result = simxGetObjects( clientID, 
                             objectType, 
                         out objectCount, 
                         out objectHandles, 
                             operationMode );
if( result == 0 && objectHandles != IntPtr.Zero )
{
    for( int index = 0; index < objectCount; index++ )
    {
        IntPtr handle = (IntPtr)((int)objectHandles + index*4);

        // do something with handle            
    }
}

edited Jul 22, 2015 at 4:49

answered Jul 21, 2015 at 19:52

walruz

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4 Comments

NESHOM Over a year ago

Thanks for your response. The reason I am using string is that based on the description sim_handle_all is the value needs to be used to get all of the objects. So I pass sim_handle_all as string to the function. I can not find the int value for it in the documents.

walruz Over a year ago

@NESHOM: Here is C synopsys: "simxInt simxGetObjects(simxInt clientID,simxInt objectType,simxInt* objectCount,simxInt** objectHandles,simxInt operationMode)". objectType and operationMode are just enums. String shall be removed!

NESHOM Over a year ago

Thanks for this, but the issue I have now, I think, is all about objectHandles that is a pointer to a pointer that will receive an object handle array. How can I marshal this and get access to the array items?

walruz Over a year ago

@NESHOM: I updated my answer. I believe this is how you should access received array of handles.

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