no streams for native C++ method inside mixed DLL called from C# GUI

[OneDanShow]

Hello everyone,

I'm working on a school project and am having the hardest time with it. Here's what I have:

First, C++ Class Library project in VS.NET 2003 with unmanaged C++ code that contains the functionality and managed C++ wrappers. This is all compiled to a mixed DLL.

Second, I have a C# Windows App project in VS.NET 2003 that has the above DLL as a reference and creates instances of the managed C++ wrapper classes to access the native C++ functionality.

This is all fine, and the C# and native C++ talk, except when a native C++ methods contains any streams (i.e. ofstream, stringstream, even cout!). It returns a System.NullPointerExeception. What is the deal?

Here is the method in native C++ that I'm trying to call (just tying to get it to print out a dummy XML file right now):


bool tigerHawk::generateVisualizationXML() {
bool result = false;
// open the XML file for output

ofstream outputfile ("visualizationData.xml");
if(outputfile.is_open()) {
string url = "http://www.onedanshow.com";
string link = "http://www.creativedaniel.com";
// write the <xml> header and <data> nodes
outputfile << "<xml blah blah>" << endl << "<data>" << endl;

int size = 5; //snapShotList.size();
int links = 2;
// for each webSnapShot object
for(int i = 0; i < size; i++) {
outputfile << "\t<page>" << endl;
outputfile << "\t\t<url>" << url << "</url>" << endl;
outputfile << "\t\t<url>" << snapShotList[i]->getURL(); << "</url>" << endl;

//stringstream os(snapShotList[i]->getLinks()); //a standard stringstream which parses 'line
//string link; //a temporary string
//while (os >> link)
for(int i = 0; i < links; i++) {
outputfile << "\t\t<link>" << link << "</link>" << endl;
}

outputfile << "\t</page>" << endl;
}
// close the </data> node and file writer
outputfile << "</data>";
outputfile.close();
result = true;

return result;
}



The code in the managed C++ to call the file:


bool TigerHawkWrap::generateVisualizationXML() {
try {
return (m_pTigerHawk->generateVisualizationXML());
}
catch(NullReferenceException* e) {
return false;
}
}


Then I simply all the method like so in C# (with the DLL reference, using the namespace, etc.)


tigerHawk.generateVisualizationXML()


Thanks for any help! This thing is due soon.

[Alex F]

Remove catch(NullReferenceException* e) block and run the program under debugger. Debugger will break on the line which causes this exception, this will give you more information. I think this is managed exception and problem is not in unmanaged code.

[Alex F]

Generally, it is not a good idea to catch NullReferenceException. You should catch exceptions which can normally happen like I/O exceptions, format exceptions reading user input etc. NullReferenceException is programmer's bug, and it is better to get exception message during program development rather than hide it with exception handler.

[NoHero]

Btw why not using System::Xml namespace classes?