To read large file you can use streamReader.
Just do
reader.ReadToEnd() on a background thread, and display a marquee-type progress bar while processing.
or
int bufferSize = 1024;
var sb = new StringBuilder();
var buffer = new Char[bufferSize];
var length = 0L;
var totalRead = 0L;
var count = bufferSize;
using (var sr = new StreamReader(@"C:\Demofile.txt"))
{
length = sr.BaseStream.Length;
while (count > 0)
{
count = sr.Read(buffer, 0, bufferSize);
sb.Append(buffer, 0, count);
totalRead += count;
}
}