UTM Link Builder — C# source

// utm-link-builder — campaign URL builder with canonical utm_* ordering. Language: C# (.NET 7+, BCL only). Port of src/lib/utm.ts build(); lint/bulk/preset helpers live in the TypeScript/Go sources.

using System;
using System.Collections.Generic;
using System.Web;

static class UtmLinkBuilder
{
    private static readonly (string Field, string QueryKey)[] Canonical =
    {
        ("source", "utm_source"), ("medium", "utm_medium"), ("campaign", "utm_campaign"),
        ("term", "utm_term"), ("content", "utm_content"),
    };

    static string Build(string baseUrl, Dictionary<string, string> parameters)
    {
        string text = baseUrl.Contains("://") ? baseUrl : "https://" + baseUrl; // coerce scheme-less
        if (!Uri.TryCreate(text, UriKind.Absolute, out var uri) || string.IsNullOrEmpty(uri.Host))
            return null; // unparseable base

        // ParseQueryString keeps pair order; remove stale utm_* entries.
        var kept = HttpUtility.ParseQueryString(uri.Query);
        foreach (var key in kept.AllKeys)
            if (key != null && key.StartsWith("utm_", StringComparison.Ordinal))
                kept.Remove(key);

        foreach (var (field, queryKey) in Canonical)
        {
            if (!parameters.TryGetValue(field, out var value)) continue;
            if (!string.IsNullOrEmpty(value)) kept[queryKey] = value;
        }

        var builder = new UriBuilder(uri) { Query = kept.ToString() };
        return builder.Uri.ToString();
    }
}