How to read specific amount of data from a large file

One option is to use a StreamReader to read the lines, but check the Position on the underlying stream:

List<string> lines = new List<string>();
using (var reader = File.OpenText("file.txt"))
{
    string line;
    while (reader.BaseStream.Position < DataLimit &&
           (line = reader.ReadLine()) != null)
    {
        lines.Add(line);
    }
}

There are two problems here:

• The StreamReader is likely to buffer data, so the Stream will actually be further on than the data you've read. You'll need to add some extra buffer to your limit in order to cope with this, and even then it's still not going to be very precise.
• Calling Position on each line is likely to slow things down significantly.

Another alternative is to copy as much data as you definitely want to consume into a MemoryStream, then keep reading (and converting to text) until you find the next line break, then appending that final partial-line data to the MemoryStream and finally creating a StreamReader around the MemoryStream, but again that's quite fiddly.

Yet another alternative would be to create some kind of "length-limiting" stream wrapper where you'd set the limit large enough to definitely include the last line - again, this would overread somewhat, potentially.