Re: P/Invoke "why" question - .NET (Core and Framework) Discussion Boards

Gordon Kushner9-Aug-10 8:07

9-Aug-10 8:07

Here is one for you Win32 experts. When I as a Framework developer need to use a Win API function, I can declare the function with parameters I need rather than with strict type checking.

For example, check out the underlined code in SendMessage()

<br />
[DllImport("user32.dll", CharSet = CharSet.Auto)]<br />
public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);<br />

<br />
[DllImport("user32.dll", CharSet = CharSet.Auto)]<br />
public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, StringBuilder lParam);<br />

StringBuilder? Hello? What perplexes me is that the API is expecting a pointer but we pass in a .NET StringBuilder.

Please explain to a semi-newbie what dark magic allows this to happen.

Thank you.

Gordon

Re: P/Invoke "why" question

Luc Pattyn9-Aug-10 8:22

Luc Pattyn

9-Aug-10 8:22

That one is pretty simple, at least on the surface: all the compiler does is check consistency between your calling code and the prototype(s) you provide (both in C# or VB.NET), as if the native method were a managed one (but with the implementation lacking).

The compiler never looks at the DLL file at all, it does not even have to be present while building. At run-time what gets passed is not type-checked at all, so whether it is an array pointer, a StringBuilder reference, or an IntPtr, as long as it points to the actual data, it could work.

FYI: there may be some data conversion going on behind the scene, e.g. when the data passed is a string and the native method uses ANSI (8-bit!), the "Marshaling" will convert to/from Unicode (16-bit) for you automaticaally (using a temporary buffer, so also faking the pointer value!).

Smile | :)

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modified on Monday, August 9, 2010 3:05 PM

Re: P/Invoke "why" question

Gordon Kushner9-Aug-10 8:54

Gordon Kushner

9-Aug-10 8:54

Thank you, Luc!

Re: P/Invoke "why" question

Luc Pattyn9-Aug-10 9:06

Luc Pattyn

9-Aug-10 9:06

you're welcome.

Smile | :)

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Re: P/Invoke "why" question

DaveyM6910-Aug-10 22:15

DaveyM69

10-Aug-10 22:15

Luc's given you a brilliant answer!

In my experience, anything that takes the same memory footprint can be used. This makes int/uint (4 bytes), or even long/Size (8 bytes) interchangable.

In the case of pointers, any reference type (or value type if used with ref [or out depending on the situation]) will work as it's the pointer to the object that's passed not the value pointed to.

Whether the value stored at the address can be understood on the native side or managed side once returned is another matter of course!

Dave

If this helped, please vote & accept answer!

Binging is like googling, it just feels dirtier.
Please take your VB.NET out of our nice case sensitive forum.(Pete O'Hanlon)

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Re: P/Invoke "why" question

Luc Pattyn11-Aug-10 1:44

Luc Pattyn

11-Aug-10 1:44

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Re: P/Invoke "why" question

Gordon Kushner11-Aug-10 3:02

Gordon Kushner

11-Aug-10 3:02

Thank you, Dave.

Re: P/Invoke "why" question

Daniel Grunwald11-Aug-10 1:55

Daniel Grunwald

11-Aug-10 1:55

The .NET P/Invoke marshaller has special knowledge about some types.
For example when passing a String, it passes the pointer to the string data, not the pointer to the managed string object.
When passing a StringBuilder, the .NET marshaller passes a pointer to the StringBuilder's internal buffer.

Another useful idiom is the SafeHandle: you can create your own SafeHandle-derived class and directly use that in P/Invoke signatures. The .NET marshaller will pass the underlying handle (IntPtr) instead. This is extremely useful because it avoids a number of problems: for example the finalizer could run when there are no active references to your object, but the underlying handle is still being used in a P/Invoke call. Using a SafeHandle directly in the P/Invoke signature makes the GC aware that the handle is used during the lifetime of the P/Invoke call, and it'll avoid finalizing the object until the call is complete.

Re: P/Invoke "why" question

Gordon Kushner11-Aug-10 3:02

Gordon Kushner

11-Aug-10 3:02

When passing a StringBuilder, the .NET marshaller passes a pointer to the StringBuilder's internal buffer.

This is the voodoo that had me baffled the most. To the .NET developer, the StringBuilder is a sophisticated managed class, but somehow the API is able to treat is like a char* (I think).

Thank you!

Customizable menu

Member 32222648-Aug-10 22:14

Member 3222264

8-Aug-10 22:14

Hi,
I am in need of customizable menu,where user can change the parent and child menu at any time in a UI.Also he can grant or revoke permission for a menus or sub-menu based on user group.

Need some url ,,,,,

Thanks
Guhananth.S

Re: Customizable menu

Luc Pattyn9-Aug-10 1:31

Luc Pattyn

9-Aug-10 1:31

You should start here[^]. In the top 10 is a CodeProject article, I suggest you read it. And some of the others as well.

Smile | :)

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Re: Customizable menu

JF201511-Aug-10 18:59

JF2015

11-Aug-10 18:59

Hi,

we always use the Developer Express components (not free of charge). These provide all the functionality we ever needed and also have very good examples for each control:

http://www.devexpress.com/Products/NET/Controls/WinForms/[^]

Read/Write image metadata in VB2008 [modified]

Archdeacon6-Aug-10 23:11

Archdeacon

6-Aug-10 23:11

I need to read the Comments, Keywords (Description) in the metadata of a JPG file and then be able to write a modified comment. This Confused | :confused:

is proving tricky and help would be appreciated. This metadata is that which appears when the image is right-clicked and these tags can be changed there.
What I am after is to be able to edit EXIF in JPG images, but I cant find any information in VB.

modified on Monday, August 9, 2010 8:28 AM

asp.net updatepanel with dropdownlists-can not select a different item

Martin_6-Aug-10 8:09

Martin_

6-Aug-10 8:09

I have an updatepanel with 3 dropdownlist and the selected item from the 1st one binds the 2nd, the selected item from the 2nd binds the 3rd and so on. For example you select a state and then the 2nd ddl has the list of cities in that state.
Well when a state is selected, the 2nd shows all the cities but the problem is that I can not select another city. The first on will always be selected and cb2_SelectedIndexChanged doesnt fire.
Thanks,
Martin

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional" >
<ContentTemplate>
<asp:DropDownList ID="cb1" runat="server" AutoPostBack="true" />
<asp:DropDownList ID="cb2" runat="server" AutoPostBack="true" />
<asp:DropDownList ID="cb3" runat="server" AutoPostBack="true" />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="cb1" EventName="SelectedIndexChanged" />
</Triggers>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="cb2" EventName="SelectedIndexChanged" />
</Triggers>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="cb3" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>

Re: asp.net updatepanel with dropdownlists-can not select a different item

Not Active6-Aug-10 8:48

Not Active

6-Aug-10 8:48

1) This should have been posted to the ASP.NET forum
2) Format posted code using pre tags, look at forum guidelines
3) Use http://www.asp.net/ajax/ajaxcontroltoolkit/samples/CascadingDropDown/CascadingDropDown.aspx[^]

I know the language. I've read a book. - _Madmatt

Re: asp.net updatepanel with dropdownlists-can not select a different item

T M Gray9-Aug-10 6:02

T M Gray

9-Aug-10 6:02

Don't use multiple Triggers tags. If that isn't the source of your problem it is as least making it tougher to read your code. And if these dropdowns are the only things in the UpdatePanel you should consider just settings ChildrenAsTriggers and not even bother with the individual ASyncPostBackTrigger elements.

Message Removed

5-Aug-10 23:51

sr159

5-Aug-10 23:51

Message Removed

Re: i created mysqldump application using C#. it is saving to some path which i given. but how to make that application to save dump automatically at every 3 or 4 hours once daily [modified]

Pete O'Hanlon6-Aug-10 0:26

Pete O'Hanlon

6-Aug-10 0:26

<quote>*Contents of removed message*
i created mysqldump application using C#. it is saving to some path which i given. but how to make that application to save automatically like every 3 or 4 hours daily. i written code like below
public void Dump()
{
try
{
DateTime Time = DateTime.Now;
int year = Time.Year;
int month = Time.Month;
int day = Time.Day;
int hour = Time.Hour;
int minute = Time.Minute;
int second = Time.Second;
int millisecond = Time.Millisecond;

string path;
path = "E:\\BackupMysql1\\" + year + "-" + month + "-" + day + "-" + hour + "-" + minute + "-" + second + "-" + millisecond + "_temple.sql";
StreamWriter file = new StreamWriter(path);
ProcessStartInfo psi = new ProcessStartInfo();
psi.FileName = @"C:\Program Files\MySQL\MySQL Server 5.0\bin\mysqldump";
psi.RedirectStandardInput = false;
psi.RedirectStandardOutput = true;
psi.Arguments = string.Format(@"-u{0} -p{1} -h{2} {3}", "xyz", "abcd", "localhost", "database");
psi.UseShellExecute = false;
Process process = Process.Start(psi);
string output;
output = process.StandardOutput.ReadToEnd();
file.WriteLine(output);
process.WaitForExit();
file.Close();
process.Close();
}
catch (IOException ex)
{
MessageBox.Show("Error , unable to backup!");
}
}
anybody knows please reply me.
*End*
The easy way to do this is to use a scheduled job in the windows scheduler that is set to run at a set interval. Don't muck about attempting to write code to do this when the OS provides a perfectly serviceable method.

"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.

My blog | My articles | MoXAML PowerToys | Onyx

modified on Friday, August 13, 2010 12:24 PM

Expan/Collapse Table

All Time Programming5-Aug-10 23:12

All Time Programming

5-Aug-10 23:12

Hello,

I have a special requirement where I need to show grouped data in expandable form. For eg:
Order Date Total Amt
+ 1 05/06/2010 100.00
Item Qty Rate Amt
A 5 10 50.00
B 2 25 50.00

This format we normally see in Access when the data is grouped or a primary key of table A is the foreign key of table B. If I clikc "+" sign next to order #, I get to see the item details belonging to that order. If the order is already expanded "-" sign lies and if their are no item details pertaining to any order their should be no sign.

I know I can do this format using Tree, but that looks too off profesional.

Can you suggest me any idea to implement this using Vb.NET.

Thanks,

Thanks & Regards,

Re: Expan/Collapse Table

Prosanta Kundu online5-Aug-10 23:24

Prosanta Kundu online

5-Aug-10 23:24

See this [http://mosesofegypt.net/post/Building-a-grouping-Grid-with-GridView-and-ASPNET-AJAX-toolkit-CollapsiblePanel.aspx]

Re: Expan/Collapse Table

All Time Programming6-Aug-10 0:02

All Time Programming

6-Aug-10 0:02

Thanks.

But I need it for VB.NET application and not a web application. It doesn't seem that I can use this component in Vb.NET desktop application. Any idea or suggestions are welcome.

Thanks & Regards,

Re: Expan/Collapse Table

Not Active6-Aug-10 8:55

Not Active

6-Aug-10 8:55

Perhaps you should have specified the technology was Windows not Web.

There are plenty of examples here. Like this one, Collapsible Panels in VB.NET[^]

All Time Programming wrote:
I need it for VB.NET

Not every example will be in VB.NET and you should learn to read and understand other languages and how to translate it to your needs.

I know the language. I've read a book. - _Madmatt

Re: Expan/Collapse Table

All Time Programming6-Aug-10 19:47

All Time Programming

6-Aug-10 19:47

Thanks alot, Mark. Thumbs Up | :thumbsup:

Your links answered my query much better. Thanks once again.

Yes Mark, I can read and translate from any to any among C#, VB, Java. But needed some guideline for start up also.

Thanks & Regards,

Compare column names in Excel in each sheet by SSIS

Member 32222645-Aug-10 3:35

Member 3222264

5-Aug-10 3:35

Hi,
Is it possible to compare two excel sheets ie column names in each sheet by using SSIS package?

Need some urls or some examples

Notify user when there is no matching in column names and exlude that file and pick the next excel file placed in server.mappath.

Thanks
Guhananth.S

Re: Compare column names in Excel in each sheet by SSIS

Not Active5-Aug-10 6:27

Not Active

5-Aug-10 6:27

Stop posting and read the guidelines

I know the language. I've read a book. - _Madmatt

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.