open a text file that in use by another process

i have this code but it wont open the log file of a program thats running and using that log file how can i watch that log file and store it in a string when it changes?

Code:

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int count;
 String^ sFileName;       
 String^ sLine;
               
        private: System::Void b1_Click(System::Object^  sender, System::EventArgs^  e) {
                        count = 0;
                                ofd1->ShowDialog();
                                sFileName = ofd1->FileName;
                                filenamelbl->Text=sFileName;        // set the path to the file we are going to open
                                StreamReader^ din = File::OpenText(sFileName);

                                while ((sLine = din->ReadLine()) != nullptr) {
        count++;
                sLine = ("line {0}: {1}", count, sLine );
                logtb->Text =logtb->Text  + sLine + "\r\n";
    }

                        }
                       


        };
}

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