File.ReadAllText with an Offset in .NET

What do you do when you need to read text from a file in .NET, but you want to start from an offset? This is a slightly niche scenario, but it does happen. Below is a solution to that problem.

To summarize the implementation, you open a file stream, seek to your offset, and then read in the bytes from there. While loading the result I read small chunks into a buffer, decoded them, and then added the decoded string to a string buffer for storage. Please note that if you are using a multibyte encoding then this helper will only work if you use the correct offset.

Helper Code

public class FileHelper

{

    private const int BufferSize = 1024;

    public static string ReadAllTextFromOffset(

        string path, 

        Encoding encoding, 

        int offset, 

        out int totalLength)

    {

        using (var fs = new FileStream(path, FileMode.Open))

        {

            totalLength = offset;

            if (offset > 0)

            {

                fs.Seek(offset, SeekOrigin.Begin);

            }

            var sb = new StringBuilder();

            var buffer = new byte[BufferSize];

            int readCount;

            do

            {

                readCount = fs.Read(buffer, 0, buffer.Length);

                totalLength += readCount;

                var subString = encoding.GetString(buffer, 0, readCount);

                sb.Append(subString);

            }

            while (readCount == buffer.Length);

            return sb.ToString();

        }

    }

}

Test Code

public class FileHelperTests

{

    private const string Path = "C:/Code/";

    [Fact]

    public void ReadAllTextWithOffset()

    {

        const string path = Path + "ReadAllTextWithOffset.txt";

        var encd = Encoding.Default;

        try

        {

            File.WriteAllText(path, "Hello World Goodnight Moon", encd);

            int al;

            var a = FileHelper.ReadAllTextFromOffset(path, encd, 0, out al);

            int bl;

            var b = FileHelper.ReadAllTextFromOffset(path, encd, 2, out bl);

            Assert.True(a.EndsWith(b));

            Assert.Equal(a.Length - 2, b.Length);

            int cl;

            var c = FileHelper.ReadAllTextFromOffset(path, encd, 12, out cl);

            Assert.True(a.EndsWith(c));

            Assert.Equal(a.Length - 12, c.Length);

        }

        finally

        {

            File.Delete(path);

        }

    }

    [Fact]

    public void ReadAllTextWithOffsetTooFar()

    {

        const string path = Path + "ReadAllTextWithOffsetTooFar.txt";

        var encd = Encoding.Default;

        try

        {

            File.WriteAllText(path, "Hello World Goodnight Moon", encd);

            int l;

            var s = FileHelper.ReadAllTextFromOffset(path, encd, 128, out l);

            Assert.Equal(string.Empty, s);

            Assert.Equal(128, l);

        }

        finally

        {

            File.Delete(path);

        }

    }

    [Fact]

    public void ReadAllTextWithOffsetWithUpdates()

    {

        const string path = Path + "ReadAllTextWithOffsetWithUpdates.txt";

        var encd = Encoding.Default;

        try

        {

            var a = "Hello World" + Environment.NewLine;

            File.AppendAllText(path, a, encd);

            int al;

            var at = FileHelper.ReadAllTextFromOffset(path, encd, 0, out al);

            Assert.Equal(a, at);

            var b = "Goodnight Moon" + Environment.NewLine;

            File.AppendAllText(path, b, encd);

            int bl;

            var bt = FileHelper.ReadAllTextFromOffset(path, encd, al, out bl);

            Assert.Equal(b, bt);

        }

        finally

        {

            File.Delete(path);

        }

    }

}

Enjoy,
Tom