IO.cs

// Variant 1

var lines = File.ReadAllLines("in.txt");
var output = new StringBuilder();

foreach (var line in lines)
{
	if (ShouldUseThisLine(line))
	{
		output.AppendLine(line);
	}
}

File.WriteAllText("out.txt", output.ToString());

// Variant 2
string line;

using (var writer = new StreamWriter("out.txt"))
{
	using (var reader = new StreamReader("in.txt"))
	{
		while ((line = reader.ReadLine()) != null)
		{
			if (ShouldUseThisLine(line))
			{
				writer.WriteLine(line);
			}
		}
	}
}