Calling a Windows API Function

[Stewart Berman]

I am trying to develop a wrapper class for the Windows API functions in Visual Studio 2008:
GetOpenFileName
GetSaveFileName

I put together a starter class:

using System;
using System.Collecti ons.Generic;
using System.Linq;
using System.Text;
using System.Runtime. InteropServices ;

[System.Runtime. InteropServices .StructLayout(L ayoutKind.Seque ntial, CharSet=CharSet .Auto)]
public class OpenFileName
{
int lstructSize;
int hwndOwner;
int hInstance;
string lpstrFilter = null;
string lpstrCustomFilt er = null;
int lMaxCustomFilte r;
int lFilterIndex;
string lpstrFile = null;
int lMaxFile = 0;
string lpstrFiteTitle = null;
int lMaxFileTitle = 0;
string lpstrInitialDir = null;
string lpstrTitle = null;
int lFlags;
ushort nFileOffset;
ushort nFileExtension;
string lpstrDefExt = null;
int lCustData;
int lpfHook;
int lpTemplateName;
}

[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetSaveFileName ([In, Out] OpenFileName ofn);

namespace myNameSpace
{
class GetFileNames
{
}
}

The compilier complains:

Error 1 Expected class, delegate, enum, interface, or struct
E:\myNameSpace\ myNameSpace\Get FileNames.cs 33 15 myNameSpace
Error 2 Expected class, delegate, enum, interface, or struct
E:\myNameSpace\ myNameSpace\Get FileNames.cs 33 46 myNameSpace
Error 3 Expected class, delegate, enum, interface, or struct
E:\myNameSpace\ myNameSpace\Get FileNames.cs 35 15 myNameSpace
Error 4 Expected class, delegate, enum, interface, or struct
E:\myNameSpace\ myNameSpace\Get FileNames.cs 35 46 myNameSpace
Error 5 The modifier 'extern' is not valid for this item
E:\myNameSpace\ myNameSpace\Get FileNames.cs 32 70 myNameSpace
Error 6 The modifier 'extern' is not valid for this item
E:\myNameSpace\ myNameSpace\Get FileNames.cs 34 70 myNameSpace

Errors 1 and 3 refer to the bool.qualifier
Errors 2 and 4 refer to the OpenFileName qualifier
Errors 5 and 6 refer to the extern

The DLLImport statements were originally retrieved from pinvoke.net using the PInvoke.net add-in.
They were modified to use the fully qualified System.Runtime. InteropServices .DllImport reference.

What am I doing wrong?

[Rudy Velthuis]

Re: Calling a Windows API Function

Stewart Berman wrote:

I put together a starter class:
>
using System;
using System.Collecti ons.Generic;
using System.Linq;
using System.Text;
using System.Runtime. InteropServices ;
>
[System.Runtime. InteropServices .StructLayout(L ayoutKind.Seque ntial,
CharSet=CharSet .Auto)] public class OpenFileName
{
int lstructSize;
int hwndOwner;
int hInstance;
string lpstrFilter = null;
string lpstrCustomFilt er = null;
int lMaxCustomFilte r;
int lFilterIndex;
string lpstrFile = null;
int lMaxFile = 0;
string lpstrFiteTitle = null;
int lMaxFileTitle = 0;
string lpstrInitialDir = null;
string lpstrTitle = null;
int lFlags;
ushort nFileOffset;
ushort nFileExtension;
string lpstrDefExt = null;
int lCustData;
int lpfHook;
int lpTemplateName;
}
>
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetSaveFileName ([In, Out] OpenFileName ofn);
>
What am I doing wrong?

Your static extern etc. functions should be declared as static members
of a class (even if, in reality, they are extern). And I would really
make OpenFileName a struct instead, passing it by reference.

--
Rudy Velthuis http://rvelthuis.de

"There are only two tragedies in life: one is not getting what
one wants, and the other is getting it."
-- Oscar Wilde (1854-1900)

[Rudy Velthuis]

Re: Calling a Windows API Function

Stewart Berman wrote:

I am trying to develop a wrapper class for the Windows API functions
in Visual Studio 2008:

FWIW:

http://pinvoke.net/default.aspx/comdlg32/GetOpenFileName.html

http://pinvoke.net/default.aspx/comdlg32/GetSaveFileName.html

--
Rudy Velthuis http://rvelthuis.de

"Each problem that I solved became a rule which served afterwards
to solve other problems."
-- Rene Descartes (1596-1650), "Discours de la Methode"

[Mark Rae [MVP]]

Re: Calling a Windows API Function

"Stewart Berman" <saberman@nospa m.nospamwrote in message
news:mc0fh45hch 7oa8c15mhp3pinn bqmrbvdui@4ax.c om...

>I am trying to develop a wrapper class for the Windows API functions in
>Visual Studio 2008:
GetOpenFileName
GetSaveFileName

Have you seen this:?

http://www.experts-exchange.com/Programming/Languages/C_Sharp/Q_21017042.html

--
Mark Rae
ASP.NET MVP

http://www.markrae.net

[Stewart Berman]

Re: Calling a Windows API Function

The definition you pointed to is:
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);

Which is what I had as I got it from pinvoke.net:
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);

Am I missing something?

"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:

>Stewart Berman wrote:
>

>I am trying to develop a wrapper class for the Windows API functions
>in Visual Studio 2008:

>
>FWIW:
>
>http://pinvoke.net/default.aspx/comd...nFileName.html
>http://pinvoke.net/default.aspx/comd...eFileName.html

[Stewart Berman]

Re: Calling a Windows API Function

>Your static extern etc. functions should be declared as static members

>of a class (even if, in reality, they are extern).

I am new to C# and do not understand your suggestions. Can you please provide a link to code?

And I would really
>make OpenFileName a struct instead, passing it by reference.

My limited understanding is that the only difference between a class and a structure is that a class
is allocated in the heap and passed by reference while a structure is allocated on the stack and
passed by either value or reference. I have always thought that the stack should be limited to
simple arguments passed by value and pointers to complex structures. The OpenFileName structure
seems a bit much to put on the stack. However, I am new to C# and maybe a bit confused.

Is that another difference between a class and a stack?

"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:

>Stewart Berman wrote:
>

>I put together a starter class:
>>
>using System;
>using System.Collecti ons.Generic;
>using System.Linq;
>using System.Text;
>using System.Runtime. InteropServices ;
>>
>[System.Runtime. InteropServices .StructLayout(L ayoutKind.Seque ntial,
>CharSet=CharSe t.Auto)] public class OpenFileName
>{
> int lstructSize;
> int hwndOwner;
> int hInstance;
> string lpstrFilter = null;
> string lpstrCustomFilt er = null;
> int lMaxCustomFilte r;
> int lFilterIndex;
> string lpstrFile = null;
> int lMaxFile = 0;
> string lpstrFiteTitle = null;
> int lMaxFileTitle = 0;
> string lpstrInitialDir = null;
> string lpstrTitle = null;
> int lFlags;
> ushort nFileOffset;
> ushort nFileExtension;
> string lpstrDefExt = null;
> int lCustData;
> int lpfHook;
> int lpTemplateName;
>}
>>
>[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
>static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
>[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
>static extern bool GetSaveFileName ([In, Out] OpenFileName ofn);
>>
>What am I doing wrong?

>
>Your static extern etc. functions should be declared as static members
>of a class (even if, in reality, they are extern). And I would really
>make OpenFileName a struct instead, passing it by reference.

[Stewart Berman]

Re: Calling a Windows API Function

No. I don't take free trials unless I actually believe the site will be worth paying for.

On Mon, 10 Nov 2008 01:48:45 -0000, you wrote:

"Mark Rae [MVP]" <mark@markNOSPA Mrae.netwrote:

>"Stewart Berman" <saberman@nospa m.nospamwrote in message
>news:mc0fh45hc h7oa8c15mhp3pin nbqmrbvdui@4ax. com...
>

>>I am trying to develop a wrapper class for the Windows API functions in
>>Visual Studio 2008:
>GetOpenFileNam e
>GetSaveFileNam e

>
>Have you seen this:?
>http://www.experts-exchange.com/Prog..._21017042.html

[Rudy Velthuis]

Re: Calling a Windows API Function

Stewart Berman wrote:

The definition you pointed to is:
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
>
Which is what I had as I got it from pinvoke.net:
[DllImport("comd lg32.dll", SetLastError=tr ue, CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
>
Am I missing something?

That in the example (OK, it is VB, but I assume you can read that) it
is part of a class.


--
Rudy Velthuis http://rvelthuis.de

"I've always wanted to be somebody, but I should have been
more specific." -- George Carlin

[Rudy Velthuis]

Re: Calling a Windows API Function

Stewart Berman wrote:

Your static extern etc. functions should be declared as static
members of a class (even if, in reality, they are extern).

>
I am new to C# and do not understand your suggestions. Can you
please provide a link to code?
>

And I would really
make OpenFileName a struct instead, passing it by reference.

>
My limited understanding is that the only difference between a class
and a structure is that a class is allocated in the heap and passed
by reference while a structure is allocated on the stack and passed
by either value or reference. I have always thought that the stack
should be limited to simple arguments passed by value and pointers to
complex structures. The OpenFileName structure seems a bit much to
put on the stack. However, I am new to C# and maybe a bit confused.
>
Is that another difference between a class and a stack?
>
"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:
>

Stewart Berman wrote:

I put together a starter class:
>
using System;
using System.Collecti ons.Generic;
using System.Linq;
using System.Text;
using System.Runtime. InteropServices ;
>
[System.Runtime. InteropServices .StructLayout(L ayoutKind.Seque ntial,
CharSet=CharSet .Auto)] public class OpenFileName
{

Make all members public:

public int lstructSize;
public int hwndOwner;
public int hInstance;
public string lpstrFilter = null;
public string lpstrCustomFilt er = null;
public int lMaxCustomFilte r;
public int lFilterIndex;
public string lpstrFile = null;

etc...

Like I said, they should be static functions of a class:

public class APILib
{
[DllImport("comd lg32.dll", SetLastError=tr ue,
CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
[DllImport("comd lg32.dll", SetLastError=tr ue,
CharSet = CharSet.Auto)]
static extern bool GetSaveFileName ([In, Out]
OpenFileName ofn);
}

Now you can use them like below:

OpenFileName ofn = new OpenFileName();
ofn.structSize = Marshal.SizeOf( ofn);
ofn.filter = filter;
ofn.file = new string(new Char[256] {});
ofn.maxFile = ofn.file.Length ;

// etc...

if (APILib.GetOpen FileName(ofn))
{
// get data from ofn
}


--
Rudy Velthuis http://rvelthuis.de

"Maybe this world is another planet's Hell."
-- Aldous Huxley (1894-1963)

[Family Tree Mike]

Re: Calling a Windows API Function

I agree with you, in particular about that site.

"Stewart Berman" <saberman@nospa m.nospamwrote in message
news:upafh4lrlv ouip4s5076u1e92 0s3s5jc07@4ax.c om...

No. I don't take free trials unless I actually believe the site will be
worth paying for.
>

[Stewart Berman]

Re: Calling a Windows API Function

"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:

>Stewart Berman wrote:
>

Your static extern etc. functions should be declared as static
members of a class (even if, in reality, they are extern).

>>
>I am new to C# and do not understand your suggestions. Can you
>please provide a link to code?
>>

And I would really
make OpenFileName a struct instead, passing it by reference.

>>
>My limited understanding is that the only difference between a class
>and a structure is that a class is allocated in the heap and passed
>by reference while a structure is allocated on the stack and passed
>by either value or reference. I have always thought that the stack
>should be limited to simple arguments passed by value and pointers to
>complex structures. The OpenFileName structure seems a bit much to
>put on the stack. However, I am new to C# and maybe a bit confused.
>>
>Is that another difference between a class and a stack?
>>
>"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:
>>

Stewart Berman wrote:
>
>I put together a starter class:
>>
>using System;
>using System.Collecti ons.Generic;
>using System.Linq;
>using System.Text;
>using System.Runtime. InteropServices ;
>>
>[System.Runtime. InteropServices .StructLayout(L ayoutKind.Seque ntial,
>CharSet=CharSe t.Auto)] public class OpenFileName
>{

>
>Make all members public:
>

> public int lstructSize;
> public int hwndOwner;
> public int hInstance;
> public string lpstrFilter = null;
> public string lpstrCustomFilt er = null;
> public int lMaxCustomFilte r;
> public int lFilterIndex;
> public string lpstrFile = null;

>
>etc...
>
>Like I said, they should be static functions of a class:
>
public class APILib
{
[DllImport("comd lg32.dll", SetLastError=tr ue,
CharSet = CharSet.Auto)]
static extern bool GetOpenFileName ([In, Out] OpenFileName ofn);
[DllImport("comd lg32.dll", SetLastError=tr ue,
CharSet = CharSet.Auto)]
static extern bool GetSaveFileName ([In, Out]
OpenFileName ofn);
}
>
>Now you can use them like below:
>
OpenFileName ofn = new OpenFileName();
ofn.structSize = Marshal.SizeOf( ofn);
ofn.filter = filter;
ofn.file = new string(new Char[256] {});
ofn.maxFile = ofn.file.Length ;
>
// etc...
>
if (APILib.GetOpen FileName(ofn))
{
// get data from ofn
}
>

I assume the need to imbed the imports in a class is due to the dot net framework. I don't
understand the need to make the members of the OpenFileName class public. They will only be
referred to with the code that instantiated the class. Why do they need to be public?

[Peter Duniho]

Re: Calling a Windows API Function

On Sun, 09 Nov 2008 20:05:14 -0800, Stewart Berman
<saberman@nospa m.nospamwrote:

[...]
I assume the need to imbed the imports in a class is due to the dot net
framework. I don't
understand the need to make the members of the OpenFileName class
public. They will only be
referred to with the code that instantiated the class. Why do they need
to be public?

All due respect, your messages in this thread suggest that you may not
really understand C# well enough to effectively use a more advanced
feature like p/invoke. Rather than run the risk of engaging in a
variation of "cargo cult programming"
(http://en.wikipedia.org/wiki/Cargo_cult_programming) in which you engage
in canned rituals to achieve certain goals without really understanding
what's behind those rituals, you should probably spend more time learning
the basics before moving on to the more complicated things.

Practically all of these questions are simply basic rules that are part of
the C# language, fundamental to all class design. They aren't even all
that unique to C#, as they are shared in some form by other
object-oriented languages like C++ and Java as well.

I'd like to suggest that you really should spend a fair amount more time
learning OOP concepts in general, and the C# language specifically, before
you attempt to successfully use p/invoke or other advanced C#/.NET
features.

Pete

[Rudy Velthuis]

Re: Calling a Windows API Function

Stewart Berman wrote:

I assume the need to imbed the imports in a class is due to the dot
net framework.

It is actually due to the C# language. Not all languages on .NET
require this.

I don't understand the need to make the members of
the OpenFileName class public. They will only be referred to with
the code that instantiated the class. Why do they need to be public?

Because otherwise they'll be private (which is the default
accessibility for class members), and so no code in any other class
would be able to access them.
--
Rudy Velthuis http://rvelthuis.de

"Wagner's music is better than it sounds."
-- Mark Twain (1835-1910)

[\Ji Zhou [MSFT]\]

RE: Calling a Windows API Function

Hello Stewart Berman,

Thanks for using Microsoft Newsgroup Support Service, my name is Ji Zhou
[MSFT] and I will be working on this issue with you.

As you already found the imported function's declaration should be embedded
in a class. Otherwise we will receive the error you mentioned in your first
post. The reason is, in C# language, there is not a concept of global
function or global variable while the C++ does have. If we want to simulate
the global function using C#, we need to create a host class and put the
function declaration in that class as public static type. Then, we can
access it via NameSpace.Class .Function everywhere else in the project.

As to why the fields of the class OpenFileName should be declared as
public, the public fields can be accessed from the OpenFileName instance
directly. If we use the private field, after we create an instance of
OpenFileName, we cannot initialize the private fields value. The P/Invoke
may fails for the incorrect parameter.

One more thing to state is that, what is the business reason for calling
the native API GetOpenFileName and GetSaveFileName from C#. Based on my
experience, since the .NET Framework provide this function in the assembly
System.Windows. Forms.dll, why not directly use the SaveFileDialog and
OpenFileDialog in .NET?

Codes look like,

private void button1_Click(o bject sender, System.EventArg s e)
{
Stream myStream = null;
OpenFileDialog openFileDialog1 = new OpenFileDialog( );

openFileDialog1 .InitialDirecto ry = "c:\\" ;
openFileDialog1 .Filter = "txt files (*.txt)|*.txt|A ll files (*.*)|*.*" ;
openFileDialog1 .FilterIndex = 2 ;
openFileDialog1 .RestoreDirecto ry = true ;

if(openFileDial og1.ShowDialog( ) == DialogResult.OK )
{
try
{
if ((myStream = openFileDialog1 .OpenFile()) != null)
{
using (myStream)
{
// Insert code to read the stream here.
}
}
}
catch (Exception ex)
{
MessageBox.Show ("Error: Could not read file from disk. Original
error: " + ex.Message);
}
}
}

We firstly create an instance of OpenFileDialog and set its properties like
the InitialDirector y, Filter, Filter Index and so on. After that, we can
call its.ShowDialog( ) to pop up the open file dialog. You can get more
information in the following two MSDN articles.

http://msdn.microsoft.com/en-us/libr...openfiledialog.
aspx
http://msdn.microsoft.com/en-us/libr...savefiledialog.
aspx

If you have any future questions or concerns, please feel free to let me
know and I will try my best to follow up. Have a nice day, Stewart!


Best regards,
Ji Zhou (v-jzho@online.mic rosoft.com, remove 'online.')
Microsoft Online Community Support

Delighting our customers is our #1 priority. We welcome your comments and
suggestions about how we can improve the support we provide to you. Please
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[Stewart Berman]

Re: Calling a Windows API Function

"Rudy Velthuis" <newsgroups@rve lthuis.dewrote:

>Stewart Berman wrote:
>

>I assume the need to imbed the imports in a class is due to the dot
>net framework.

>
>It is actually due to the C# language. Not all languages on .NET
>require this.
>

>I don't understand the need to make the members of
>the OpenFileName class public. They will only be referred to with
>the code that instantiated the class. Why do they need to be public?

>
>Because otherwise they'll be private (which is the default
>accessibilit y for class members), and so no code in any other class
>would be able to access them.

I realized that about three seconds after I hit the send button on the message.