Question

I have a 1 GB text file which I need to read line by line. What is the best and fastest way to do this?

private void ReadTxtFile()
{            
    string filePath = string.Empty;
    filePath = openFileDialog1.FileName;
    if (string.IsNullOrEmpty(filePath))
    {
        using (StreamReader sr = new StreamReader(filePath))
        {
            String line;
            while ((line = sr.ReadLine()) != null)
            {
                FormatData(line);                        
            }
        }
    }
}

In FormatData() I check the starting word of line which must be matched with a word and based on that increment an integer variable.

void FormatData(string line)
{
    if (line.StartWith(word))
    {
        globalIntVariable++;
    }
}

Answer 1

If you are using .NET 4.0, try MemoryMappedFile which is a designed class for this scenario.

You can use StreamReader.ReadLine otherwise.