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Pointers in C#

Posted on 2004-08-01
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Last Modified: 2010-05-18
Hi there - I am accessing a 3rd party database product via its api (written in C++).  One of the functions returns a pointer to a string value. Is there an easy way to use pointers in C#.

I just need the string value. I've never really messed around with pointers before.

The api declaration is as follows

[DllImport("myThirdPartyDll.dll")]  private  static extern  char * TM1ValStringGet  ( int hUser,   int vString) ;

thanks in advance

Graham
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Question by:GillyTheTaffy
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11 Comments
 

Expert Comment

by:monotonic
ID: 11691721
In c# the best way to use pointers is by using the "ref" keyword in front of the variable, also I think you can use the ampersand im not sure about this though.
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LVL 37

Expert Comment

by:gregoryyoung
ID: 11698378
strings are immutable objects ... you can either use an IntPtr and then marshal yourself or you can pass a stringbuilder (I know that sounds odd:) )

http://www.dotnet247.com/247reference/msgs/49/246277.aspx has an example but I will put it here for PAQ.

// DLL import in my "UnsafeNativeMethods" class.
[DllImport("user32.dll", EntryPoint = "SendMessage", CharSet =
CharSet.Auto)]
public static extern IntPtr SendMessage(IntPtr hWnd, int MSG, int wParam,
System.Text.StringBuilder lParam);

....

// Here we get the text from a text box.
int length = (int)UnsafeNativeMethods.SendMessage(this.Handle,
UnsafeNativeMethods.WM_GETTEXTLENGTH, IntPtr.Zero, IntPtr.Zero);

if (length > 0)
{
StringBuilder sb = new StringBuilder(length + 1);
UnsafeNativeMethods.SendMessage(this.Handle,
UnsafeNativeMethods.WM_GETTEXT, sb.Capacity, sb);
buffer = sb.ToString();
}

if your input string will not be modified by the dll use
string as the parameter type. For output strings, use the
System.Text.StringBuilder class. Either way, the parameter should be
passed by value, leave out the "ref".

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpcondefaultmarshalingforstrings.asp is worth taking a look at.
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Author Comment

by:GillyTheTaffy
ID: 11699634
ok - I'm a bit confused with a couple of things here

Is the second parameter to the SendMessage function the pointer returned from my api call - eg presuming all is in the same class

// Database provided api that returns a pointer in its documentation
[DllImport("myThirdPartyDll.dll")]  private  static extern  char * TM1ValStringGet  ( int hUser,   int vString) ;

// Send message to populate string builder
DllImport("user32.dll", EntryPoint = "SendMessage", CharSet =
CharSet.Auto)]
public static extern IntPtr SendMessage(IntPtr hWnd, int MSG, int wParam,
System.Text.StringBuilder lParam);

// Presume that hUser and hString are correct parameters for the api to return a valid pointer
unsafe{
  char* myPointer=TM1ValStringGet(hUser,hString);
 // Documentation provides us with the max length of any strings returned
 // by the database 256
  System.Text.StringBuilder sb=new System.Text.Stringbuilder(256);
  SendMessage(This.Handle,myPointer,sb.Capacity,sb);
  string myVal=sb.ToString();
}

What is the first handle being passed to the SendMessage api (this.handle) - as I am not using a form to display the data (just compiling an xml document)  therefore there is no handle property in This.

Otherwise am I on the right track ?

TIA

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LVL 37

Expert Comment

by:gregoryyoung
ID: 11699735
it was just an example ... but yes you would be looking to use a stringbuilder ... it is explained in fairly good detail in the link above.
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Author Comment

by:GillyTheTaffy
ID: 11699972
thanks for your help. I have found an alternative for what I need

if I change the return type to a Stringbuilder then yes it works - however some of the string values have odd characters like { _ , - etc so it falls over when I try and populate the string builder. So - I did some more reading and all I needed to do was make the return type IntPtr and then use the Marshal.Class To convert it to ANSI text

see below

[DllImport("myThirdPartyDll.dll")]  private  static extern  IntPtr TM1ValStringGet  ( int hUser,   int vString) ;

IntPtr myHandle = TM1ValStringGet(hUser,hValue);

String myVal = Marshall.PtrToStringAnsi(myHandle);


thanks again

Graham
0
 
LVL 37

Expert Comment

by:gregoryyoung
ID: 11699979
"you can either use an IntPtr and then marshal yourself or you can pass a stringbuilder"
0
 
LVL 1

Author Comment

by:GillyTheTaffy
ID: 11700024
ah! Now you first post makes sense! lol

cheers
0
 
LVL 1

Author Comment

by:GillyTheTaffy
ID: 11724409
Hello again Gregory

I've got a similar issue when some the unamanaged api's are expecting char * as input parameters

eg

[DllImport("myThirdPartyDll.dll")]  private  static extern  int TM1SystemServerHandle( int hUser,   char * vString) ;

I thought I would just reverse what I did when retrieving pointer values using the Marshall class. I had been previously passing a string but have started getting spurious results like I was before you fixed my previous issue

eg

change the api call to


[DllImport("myThirdPartyDll.dll")]  private  static extern  int TM1SystemServerHandle( int hUser,   intPnt  vString) ;

than call using
string s="myString"
int iRet=IntPnt x=Marshall.PtrToStringAnsi(s)

this produces the same results as if I was still passing in the string direct. I have alos tried the other Marshall.PtrTo...... functions with no joy and also tried passing in a StringBuilder object

the equiv C++ code reads as follows

//Header file
typedef TM1_INDEX    * TM1V;           // value handle
TM1IMPORT TM1V TM1API TM1SystemServerHandle( TM1U hUser, CHAR * szName );

LPSTR sMyString="myString";

// assume huser is a valid handle
TM1SystemServerHandle( hUser, sMyString );

any help would be greatly appreciated

graham
0
 
LVL 1

Author Comment

by:GillyTheTaffy
ID: 11724469
think I've got it but won't know for sure till tomorrow when I can test it

change the api call to....

[DllImport("myThirdPartyDll.dll")]  private static extern  int TM1SystemServerHandle  (int hUser ,[MarshalAs(UnmanagedType.LPStr)] StringBuilder ipntName );

thanks if you even read this

Graham
0
 
LVL 37

Accepted Solution

by:
gregoryyoung earned 50 total points
ID: 11728940
yes that should work.
0
 
LVL 1

Author Comment

by:GillyTheTaffy
ID: 11731027
doh - didn't work.
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