HtmlLogViewer

A lightweight ASP.NET Core log viewer for Serilog rolling log files.
Adds a single browser-accessible route that lists every rolling log file it can discover, lets users choose how many lines to display, and renders a self-contained HTML page with dark / light mode toggle — zero external dependencies.

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Installation

dotnet add package HtmlLogViewer

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Quick Start

1 — Register the viewer

// Program.cs
builder.Services
    .AddControllers()
    .AddLogViewer();          // reads all settings from appsettings.json "LogViewer" section

MapControllers() must also be called, as with any ASP.NET Core controller:

app.MapControllers();

2 — Add configuration to appsettings.json

{
  "LogViewer": {
    "RoutePrefix"      : "logs",
    "PageTitle"        : "My Application — Log Viewer",
    "DefaultLineCount" : 200,
    "LineCountOptions" : [ 100, 200, 300, 500 ],
    "ExtraPaths"       : [ "logs", "C:\\OtherApp\\Logs" ]
  }
}

Why the Serilog section matters
The viewer auto-discovers log file paths by walking every "path" property found anywhere inside the Serilog configuration section.
If Serilog is configured only in code (not in appsettings.json), use ExtraPaths to point the viewer at the log directory instead.

3 — Wire Serilog from configuration (recommended)

// Program.cs
builder.Host.UseSerilog((ctx, cfg) =>
    cfg.ReadFrom.Configuration(ctx.Configuration));

This keeps the "Serilog" section as the single source of truth for both the file sink and the viewer's path auto-discovery.

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LogViewer appsettings Reference

Key	Type	Default	Description
RoutePrefix	string	"logs"	URL path for the viewer — https://host/{RoutePrefix}
PageTitle	string	"Log Viewer"	Browser-tab title and page <h1>
DefaultLineCount	int	100	Number of lines shown on first load
LineCountOptions	int[]	[100, 200, 300]	Choices in the Lines dropdown; "All" is always appended automatically
ExtraPaths	string[]	[]	Extra file or directory paths scanned for *.log / *.txt files. Supports %ENV% variables. Useful when Serilog is configured in code rather than appsettings.json

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Programmatic Override

Settings from appsettings.json can be overridden in code via the optional configure delegate. The delegate runs after the config section is bound, so only the properties you touch are overridden:

builder.Services
    .AddControllers()
    .AddLogViewer(options =>
    {
        options.PageTitle        = "My Application — Logs";
        options.DefaultLineCount = 200;
        options.LineCountOptions = [100, 200, 300, 500];
        options.ExtraPaths       = [@"C:\OtherService\Logs"];
    });

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Log File Discovery

The viewer collects candidate log paths from three sources on every request, then merges and de-duplicates them:

Priority	Source	How to configure
1	Serilog auto-discovery	Any "path" key anywhere in the "Serilog" appsettings section
2	ExtraPaths	LogViewer.ExtraPaths in appsettings.json or code
3	Runtime directories	Entered by the user in the Log Directories text box in the UI

Each resolved path may be a file path (its parent directory is used) or a directory path (scanned directly). All *.log and *.txt files are collected, de-duplicated, and sorted newest-first.

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Log Level Colours

Log lines are coloured by severity based on the token inside [...]:

Token	Meaning	Colour (dark mode)
[INF]	Information	Blue
[DBG]	Debug	Green
[WRN]	Warning	Yellow
[ERR]	Error	Red-orange
[FTL]	Fatal	Bold red

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Dark / Light Mode

The viewer ships with a dark/light mode toggle button in the header.
The chosen theme is persisted in localStorage and restored automatically on the next page load — no server round-trip needed.

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License

© Copyright 2025 ABB Ltd. All rights reserved.