Soenneker.Css.Minify

A lightweight CSS text and file minifier that removes comments and unnecessary whitespace while preserving token boundaries.

Installation

dotnet add package Soenneker.Css.Minify

Registration

using Soenneker.Css.Minify.Abstract;
using Soenneker.Css.Minify.Registrars;

services.AddCssMinifierAsSingleton();

ICssMinifier minifier = serviceProvider.GetRequiredService<ICssMinifier>();

AddCssMinifierAsScoped() is also available. Both methods register the matching IFileUtil lifetime used by file operations. The minifier itself keeps no per-call state, so singleton registration is suitable when its file utility is also singleton.

Minify CSS text

const string css = """
    /* navigation */
    .nav .item001 {
        margin: 0px  0.50rem;
        color: #001122;
        width: calc(100% - 1rem);
    }
    """;

string result = minifier.Minify(css);
// .nav .item001{margin:0 .5rem;color:#001122;width:calc(100% - 1rem)}

A ReadOnlySpan<char> overload is available when the input already resides in a span. Empty input returns an empty string.

The minifier:

• removes block comments, including /*! ... */ comments;
• removes redundant whitespace and a final semicolon before };
• preserves quoted strings and escapes;
• preserves required selector, value-list, and calc() spacing;
• normalizes numeric forms such as 00.50em to .5em and removes units from zero where supported;
• leaves digits inside identifiers, custom-property names, and hex colors unchanged.

Minify a file

await minifier.MinifyFile(
    inputPath: "wwwroot/css/site.css",
    outputPath: "wwwroot/css/site.min.css",
    cancellationToken);

The input file is read completely, minified in memory, and written to the output path through IFileUtil. Existing output is replaced according to that utility's write behavior. File errors and cancellation propagate to the caller.

Scope and validation

This is a purpose-built token minifier, not a full CSS parser. It does not report malformed CSS, resolve imports, add vendor prefixes, rewrite colors, produce source maps, or guarantee every future/proprietary CSS grammar is understood. Run representative browser or CSS parser tests before placing it in a production asset pipeline.

Minification is not sanitization. Do not treat arbitrary CSS as safe merely because it passed through this library.