the faster way to filter all emails end with a special caractere in a big text file using C#

Question

i want a very faster way to filter all emails that end with a special caracter in a big text file (more than 200MB) using c#

var lines = File.ReadAllLines(file path);

foreach(var line in lines)
{
  if (line.EndsWith(myWord))
  {
    outputEmails.Text += line + Environment.NewLine;
  }
}

this code is very slow to achieve my goal.

Answer 1

Use File.ReadLines:

foreach (var line in File.ReadLines("file path"))
{
    if (line.EndsWith(myWord))
    {
        outputEmails.Text += line + Environment.NewLine;
    }
}

Internally it uses StreamReader so you don't load the entire file into memory before reading it.

---

Even better performance may be achieved with memory mapped files. This saves extra memory copy operations when reading file contents.

string line;
var stringBuilder = new StringBuilder();
using (var memoryMappedFile = System.IO.MemoryMappedFiles.MemoryMappedFile.CreateFromFile(@"D:\Temp\test.txt"))
using (var viewStream = memoryMappedFile.CreateViewStream())
using (var streamReader = new StreamReader(viewStream))
{
    while ((line = streamReader.ReadLine()) != null)
        if (line.EndsWith(myWord))
            stringBuilder.AppendLine(line);
}
outputEmails.Text = stringBuilder.ToString();

And yes, as the other answers say, using StringBuilder instead to raw string concatenation will also make code faster, especially when many lines match filter condition.