Re: More marshaling woes :-( - C# Discussion Boards

16-Aug-09 8:48

This is the C code from MSDN[^]:

typedef struct { 
    DWORD dwDeltaTime; 
    DWORD dwStreamID; 
    DWORD dwEvent; 
    DWORD dwParms[]; // this is the problem!
} MIDIEVENT;

The problem is with the array as the dwParms field.

If I just want one value, no problem - I just declare the struct like this:

[StructLayout(LayoutKind.Sequential)]
public struct MIDIEVENT
{
    public int dwDeltaTime;
    private int dwStreamID;
    public uint dwEvent;
    public uint dwParms;
}

... and it works fine, but when I need an array of uints I can't make it work. If I change the last C# field to

[MarshalAs(UnmanagedType.ByValArray)]
public uint[] dwParms;

then it works fine but only gets the first element. If I specify a SizeConst then I can get all the values fine, but I have no way of knowing how many elements there will be at compile time.

Help!

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)
Why are you using VB6? Do you hate yourself? (Christian Graus)

Luc Pattyn16-Aug-09 8:58

16-Aug-09 8:58

Hi Dave,

sorry for the delay. Laugh | :laugh:

I haven't done this with a variable length array, I have some experience with ByValArray for fixed lengths, with SizeConst.

What you always can do is forget about the struct, just allocate an actual array (either int or uint, whatever suits best) with the right element count to contain the three individuals and the embedded array.
Then use GCHandle to pin it and get its IntPtr, and pass that. That is the way that always works, and never copies any data. BTW: don't forget to free the GCHandle when done.

I hope the native side will somehow figure out the length of the embedded array, and not read/write outside it.

Smile | :)

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DaveyM6916-Aug-09 9:51

16-Aug-09 9:51

Luc Pattyn wrote:
sorry for the delay

10 minutes... you are slacking Luc! Laugh | :laugh:

Cheers mate - that worked. I owe you a Beer | [beer]

or ten someday.

What are the pro's and cons of copying data to unmanaged memory vs pinning in managed memory in your opinion?

Luc Pattyn16-Aug-09 10:33

16-Aug-09 10:33

pro GCHandle:
- avoids copying, which doubles the memory load, and trashes the cache.
- pinned arrays are read/write, they pass data both ways.
- you can free the GCHandle much later (when all the native stuff is done dealing with the array, could be more than one native call there, even async stuff).
- conceptually simpler than a lot of the Marshal stuff

contra GCHandle:
- unknown cost, assumed irrelevant for large arrays (I often do megabytes of image data)
- pinned means GC can't move, however >80KB is large-object heap anyway, and that one never(hardly ever?) compacts (that is bad in general, not due to GCHandle).
- needs more code in simple cases
- coder is responsible for freeing (I have an LP_Pinner class that implements IDisposable, hence using construct). See TrayIconBuster article.

Smile | :)

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DaveyM6917-Aug-09 0:46

17-Aug-09 0:46

Just a quickie Luc, If I have a struct instance that is pinned for use in a function, and that struct has an IntPtr as a field, is there any need to pin the field, or is that taken care of automatically by pinning the struct?

Luc Pattyn17-Aug-09 1:13

17-Aug-09 1:13

I don't expect automatic recursion to occur on pinning; IMO it is your responsibility to make sure somehow that all managed objects are immobilized for as long as your native code uses pointers to them.

I actually hope pinning isn't using any reflection to figure what is inside the object.

Smile | :)

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DaveyM6917-Aug-09 1:50

17-Aug-09 1:50

Makes sense. I didn't want to unnecessarily replicate something that may have already been done by the system.

Thumbs Up | :thumbsup:

Luc Pattyn wrote:
I actually hope pinning isn't using any reflection to figure what is inside the object.

Me too, but I don't think so...

Hristo-Bojilov16-Aug-09 9:04

Hristo-Bojilov

16-Aug-09 9:04

DaveyM69 wrote:
[MarshalAs(UnmanagedType.ByValArray)]
public uint[] dwParms;

You should specify SizeConst parameter for MarshalAsAttribute.You should declare your problematic field as IntPtr,if you don't know the array size.This I wrote this too into your thread before few days.When you get pointer to your struct use Marchal.Copy to copy the data to managed array.See this sample.

Life is a stage and we are all actors!

DaveyM6916-Aug-09 10:03

16-Aug-09 10:03

The size isn't known until runtime and varies hugely. I tried several different ways using an IntPtr but the data was not correct on the receiving end (a MIDI device). In fact, with the same data in, the output was different each time so I assume the pointer address was getting stored rather than the data itself!

The MIDIHDR structure takes an IntPtr in one field to the data - this data can be one or an array of MIDIEVENTs which contains the unspecified length array.

Luc's suggestion of converting the stuct to a uint[] as a whole instead of just that one field, pinning that and sending the IntPtr of the pinned array to the field in the MIDIHDR works though Big Grin | :-D

Additional information

Pete O'Hanlon16-Aug-09 10:22

16-Aug-09 10:22

I know you know this Dave - this is purely for anybody who's reading this thread who doesn't know about the SizeParamIndex parameter. This parameter (when applied in a MarshalAs(UnmanagedType.LPArray), points to the field in the structure that has the size of the returned array.

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

Luc Pattyn16-Aug-09 10:38

16-Aug-09 10:38

I'm one of those "anybody who's reading this thread who doesn't know about the SizeParamIndex parameter".
Thanks for the info, and BTW I hope I'll never need it...

Smile | :)

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Pete O'Hanlon16-Aug-09 10:41

16-Aug-09 10:41

Luc Pattyn wrote:
Thanks for the info, and BTW I hope I'll never need it...

No probs. I'm a lot more awake tonight. Big Grin | :-D

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

DaveyM6916-Aug-09 10:52

16-Aug-09 10:52

I hadn't come accross that one Pete Thumbs Up | :thumbsup:

It doesn't help with this problem as the size (in bytes) of the uint array is held in the lower 24 bits of the uint that is the previous field. Whoever came up with that idea wants shooting!

They could have had the upper 8 bits of the other field as a separate byte parameter for the flags it relates to, and kept a field purely for the size of the array... it would have used just one extra byte of memory per struct when each struct can be thousands of bytes long already so not much difference Mad | :mad:

Pete O'Hanlon16-Aug-09 10:58

16-Aug-09 10:58

DaveyM69 wrote:
It doesn't help with this problem

I'd realised that; I just thought people might find it interesting - which shows what a bad sastard I really am.

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

Luc Pattyn16-Aug-09 11:14

16-Aug-09 11:14

Pete O'Hanlon wrote:
which shows what a bad sastard I really am.

Well Billy that's why we all like you. Together with you knowing and sharing a couple of things of course.

Big Grin | :-D

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Pete O'Hanlon16-Aug-09 11:19

16-Aug-09 11:19

Luc Pattyn wrote:
Well Billy

Great thread there; missed out a couple of favourites Cupid Stunt and Mary Hinge (you need to know about Kenny Everett to follow this character).

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

DaveyM6916-Aug-09 11:50

Selected tab that affect on listbox

16-Aug-09 11:50

All knowledge greatfully reaceived Big Grin | :-D

(They were running some of the old 'Kenny' shows on UK TV Gold a couple of months ago!)

tamir90116-Aug-09 8:15

tamir901

16-Aug-09 8:15

Hi,

I have tabcontrol that i added to it 10 tabs (named 1 to 10).
i also have listbox that contains 1 to 10 items.

I'm trying to combain between those to object,
when a some tab get focus (GotFocus event) the same name in the listbox will be selected automatic.

For exemple:
i press on the tab name 4,
the item named 4 in the kistbox will marked will blue (selected).

Someone can help me here?

tamir90116-Aug-09 9:38

tamir901

16-Aug-09 9:38

I try to do this:

void tabControl1_GotFocus(object sender, EventArgs e)
        {

            foreach (TabPage item in tabControl1.TabPages)
            {
                if (item.Focus())
                {
                    for (int i = 0; i < listBoxUsers.ListBoxControl.Items.Count; i++)
                    {
                        if (item.Text==listBoxUsers.ListBoxControl.Items[i].ToString())
                        {
                            listBoxUsers.ListBoxControl.SelectedItem = item.Text;
                        }
                    }
                }
            }
        }

But it dosen't work so good,
can i improve it?

Saksida Bojan16-Aug-09 10:57

Saksida Bojan

16-Aug-09 10:57

I woud try something this: And i am sorry, i deleted my previous answer because i didn't read correctly

void tabControl1_Selected(object sender, TabControlEventArgs e)
{
listBoxUsers.ListBoxControl.SelectedItem = tabControl1.SelectedTab.Name;     
}

Luc Pattyn16-Aug-09 11:34

16-Aug-09 11:34

Hi,

IMO you may have to put such code inside a tabPage_GotFocus() handler for each of the tab pages.
Not sure the tabControl gets a GotFocus event when switching tab pages.

Smile | :)

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