C#

while(!allDone)
    lock the streamreader (or some other object)
    read one line from the streamreader, and set allDone true if no more data available
    unlock again
    process the line read, if any
}

StreamReader sr;
bool allDone;

void handleFileWithManyThreads(string filename, bool waitTillDone) {
    allDone=false;
    sr=File.Open(filename);
    List<Thread> threads=new List<Thread>();
    // launch N threads
    for(int i=0; i<Environment.NumberOfProcessors; i++) {
        Thread thread=new Thread();
        thread.Start(runner);
        threads.Add(thread);
    }
    if (waitTillDone) {
        // now wait for all these threads to finish
        foreach(Thread thread in threads) thread.Join();
        sr.Close();
    }
}

void runner() {
    while(!allDone) {
        string line=null;
        lock(sr) {
            line=sr.ReadLine();
            if (line==null) allDone=true;
        }
        if (line!=null) {
            // process line here
        }
    }
}

ElementHost host = new ElementHost();
host.Dock = DockStyle.Fill;
host.Child = myWpfControl;