What consumes less resources and is faster File.AppendText or File.WriteAllText storing first in StringBuilder?

Question

I have to write thousands of dynamically generated lines to a text file. I have two choices, Which consumes less resources and is faster than the other?

A. Using StringBuilder and File.WriteAllText

StringBuilder sb = new StringBuilder();

foreach(Data dataItem in Datas)
{
    sb.AppendLine(
        String.Format(
            "{0}, {1}-{2}",
            dataItem.Property1,
            dataItem.Property2,
            dataItem.Property3));
}

File.WriteAllText("C:\\example.txt", sb.ToString(), new UTF8Encoding(false)); 

B. Using File.AppendText

using(StreamWriter sw = File.AppendText("C:\\example.txt"))
{
    foreach (Data dataItem in Datas)
    {
        sw.WriteLine(
            String.Format(
                "{0}, {1}-{2}",
                dataItem.Property1,
                dataItem.Property2,
                dataItem.Property3));
    }
}

Answer 1

Your first version, which puts everything into a StringBuilder and then writes it, will consume the most memory. If the text is very large, you have the potential of running out of memory. It has the potential to be faster, but it could also be slower.

The second option will use much less memory (basically, just the StreamWriter buffer), and will perform very well. I would recommend this option. It performs well--possibly better than the first method--and doesn't have the same potential for running out of memory.

You can speed it quite a lot by increasing the size of the output buffer. Rather than

File.AppendText("filename")

Create the stream with:

const int BufferSize = 65536;  // 64 Kilobytes
StreamWriter sw = new StreamWriter("filename", true, Encoding.UTF8, BufferSize);

A buffer size of 64K gives much better performance than the default 4K buffer size. You can go larger, but I've found that larger than 64K gives minimal performance gains, and on some systems can actually decrease performance.