System.Threading.Timer: Why is it hating me?

Question

I just started messing around with C#/.NET/mono and stuff, and I'm trying to make a simple song player. For this, I am using winmm.dll (did not find an easy cross-platform solution). The problem is this: I need to update a trackbar along with the song playing. I have two functions, Player.GetLength and Player.GetCurrentPosition, which return the time in miliseconds. If I call them "normally", everything is ok. But I need to call them in a timer, like this:

new System.Threading.Timer((state) =>
{
    length = Player.GetLength();
    pos = Player.GetCurrentPosition();
    trackBar1.Value = (pos / length) * 100;
}, null, 0, 100);     

This is GetLength, and GetCurrentPosition is similar:

public static int GetLength()
{
    StringBuilder s = new StringBuilder(128);
    mciSendString("status Song length", s, s.Capacity, IntPtr.Zero);
    return int.Parse(s.ToString());
}

The problem: when one of these two functions gets called, the program just stops, without any warning or exception thrown. Note: I am using .NET

So I was wondering if you can explain to me where I got it wrong :)