Creating an IConfigurationBuilder method extension to load env files

I recently needed some boilerplate code to load env files into IConfigurationBuilder. is notorious for having amazingly fragmented APIs, so I needed 3 static classes, simply for one method that loads the env file configuration and loads it to the configuration.

The extension has the same license as dotnet due to me lifting the code from dotnet itself.

// Licensed under the Apache License, Version 2.0.
// See for license information.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using DotNetEnv;
using Microsoft.Extensions.Configuration;

namespace DroHub.ProgramExtensions {
    public static class EnvFileExtension {
        public static IConfigurationBuilder LoadFromEnvFile(this IConfigurationBuilder builder,
            string path, string prefix, LoadOptions options) {
            return builder.Add(new EnvFileConfigurationSource(path, prefix, options));

    public class EnvFileConfigurationSource : IConfigurationSource {
        private readonly LoadOptions _load_options;
        private readonly string _path;
        private readonly string _prefix;

        public EnvFileConfigurationSource(string path, string prefix, LoadOptions load_options) {
            _load_options = load_options;
            _path = path;
            _prefix = prefix;

        public IConfigurationProvider Build(IConfigurationBuilder builder)
            return new EnvFileConfigurationProvider(_path, _prefix, _load_options);
    public class EnvFileConfigurationProvider : ConfigurationProvider{
        private readonly LoadOptions _options;
        private readonly string _path;
        private readonly string _prefix;

        public EnvFileConfigurationProvider(string path, string prefix, LoadOptions options) {
            _options = options;
            _path = path;
            _prefix = prefix;

        public override void Load() {
            var env = Env.Load(_path, _options).ToDictionary();

            var data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
            var filtered_variables = env
                .Where(entry => ((string)entry.Key).StartsWith(_prefix, StringComparison.OrdinalIgnoreCase));

            foreach (var envVariable in filtered_variables)
                var key = ((string)envVariable.Key)[_prefix.Length..];
                data[key] = (string)envVariable.Value;

            Data = data;

        private static IEnumerable<DictionaryEntry> NormalizeKey(KeyValuePair<string, string> e) {
            var (key, value) = e;
            yield return new DictionaryEntry() {
                Key = NormalizeKey(key),
                Value = value

        private static string NormalizeKey(string key)
            return key.Replace("__", ConfigurationPath.KeyDelimiter);

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