brute force attack on System.Random

I'm working on a homework. I have this code which generates random strings with System.Random class.

public string GenerateString(int length)
{
    const string valid = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
    StringBuilder res = new StringBuilder();
    Random rnd = new Random();
    while (0 < length--){
        res.Append(valid[rnd.Next(valid.Length)]);
    }
    return res.ToString();
}

What I have to do is: I need to write a brute force function which tries to find the string which is generated by GenerateString function

As far as I know, System.Random uses system time for seeding. Which is decreasing the surface of the brute force attack. But I'm not sure about the way should I follow. I think I need to do get the system time and seed my brute force function with that. But I'm not sure how to do this.