Reverse File/Stream Reader

A file/stream reader that is designed to iterate over a file or a stream line by line in reverse order in a way that does not read all of the lines into memory at one time. This supports .NET Standard, the full framework as well as the Windows Universal Platform (UWP) apps.

Although this is geared towards files it will read any Stream line by line in reverse order.

.Net Framework Support

• .NET 6.0
• .NET 5.0
• .NET Standard 2.1
• .NET Standard 2.0
• .NET Framework 4.7.2
• .NET Framework 4.7.1
• .NET Framework 4.7
• .NET Framework 4.6.2
• .NET Framework 4.6.1
• .NET Framework 4.6
• .NET Framework 4.5.2
• .NET Framework 4.5.1
• .NET Framework 4.5
• .NET Framework 4

Read a file with CrLf line endings

C#

var sb = new StringBuilder();

using (var reader = new Argus.IO.ReverseFileReader(@"C:\Temp\log-file.txt"))
{
    while (!reader.StartOfFile)
    {
        sb.AppendLine(reader.ReadLine());
    }
}

Read a file with Lf line endings (Cr also supported with a change to the LineEnding property)

C#

var sb = new StringBuilder();

using (var reader = new Argus.IO.ReverseFileReader(@"C:\Temp\log-file.txt"))
{
    reader.LineEnding = Argus.IO.LineEnding.Lf;

    while (!reader.StartOfFile)
    {
        sb.AppendLine(reader.ReadLine());
    }
}

Windows Universal Platform Example (UWP)

The FileStream is available in UWP but for all intensive purposes is unusable due to the apps being sandboxed. If you instantiate the ReverseFileReader with a file path it will likely end with a permissions exception. Instead, you'll get a StorageFile and then pass its Stream off to the ReverseFileReader which will work.

One thing to note that could cause a related issue is that since Windows 10 build 14393 the TextBox and RichEditBox use only a carriage return (when those save they only save with a carriage return unless you intervene). Pay attention to that if it becomes an issue. In the near future I will provide an auto-detect method that will attempt to infer what line endings the file is using.

C#

/// <summary>
/// Opens a document and puts the contents into a TextBox named TextBoxMain.
/// </summary>
/// <returns></returns>
private async Task OpenDocument()
{
    var picker = new Windows.Storage.Pickers.FileOpenPicker
    {
        SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.DocumentsLibrary
    };

    picker.FileTypeFilter.Add(".txt");
    var OpenStorageFile = await picker.PickSingleFileAsync();

    if (OpenStorageFile != null)
    {
        var sb = new StringBuilder();

        using (var stream = await OpenStorageFile.OpenStreamForReadAsync())
        {
            using (var reader = new Argus.IO.ReverseFileReader(stream))
            {
                reader.LineEnding = Argus.IO.LineEnding.CrLf;

                while (!reader.StartOfFile)
                {
                    sb.AppendLine(reader.ReadLine());
                }

            }
        }

        TextBoxMain.Text = sb.ToString();
        return;
    }
}

Tail

I have provided a console program that is a lightweight tail utility. It reads lines of the end of the provided file. If more lines are requested than the file has the entire file is returned.

Syntax: tail <filename>
        tail <filename> <number of lines to fetch>