find max value

Question

i need to find max value within a file. This means finding max value within each 1000 values in a file, i.e., if there is 5000 values within a file then there should be 5 max values. Within first 1000 values: one max value, then within next 1000 values one more max value.... and so on. 

# the size of the file will be unknown

# i had prepared a code for finding max value within first 1000 values in the file. I was thinking of iterating it, to get the desired results, but could not. The codes are as follows.

        List<Double> list = new List<Double>();

                StreamReader dat = new StreamReader(file path);

                //int count = 1;

                string dataline = "";

                for (int i = 1; i <= dat.BaseStream.Length; i+=1000)

                {

                    while ((dataline = dat.ReadLine()) != null)

                    {

                        Double sdat = Double.Parse(dataline);

                        list.Add(sdat);

                        richTextBox1.Text = list.Take(1000).Max().ToString();

                    }

                }
please help!
Thank you.