using System;
using System.Configuration;

namespace Successful.Misc
{
    public static class AppSettingsHelper
    {
        private static bool ThrowException { get; set; }
        private static object DefaultValue { get; set; }
        public static TResult Get<TResult>(string key, Func<TResult> defaultValue = null, bool throwException = false)
        {
            ThrowException = throwException;
            DefaultValue = defaultValue;

            var result = default(TResult);
            var valueOfKey = ConfigurationManager.AppSettings[key];

            if (string.IsNullOrWhiteSpace(valueOfKey))
            {
                if (ThrowException && DefaultValue == null)
                {
                    throw new ArgumentNullException(valueOfKey,
                        $"Failed to get a value for the key {key} in the App.config file.");
                }

                result = GetDefaultValue(defaultValue);
            }
            else
            {
                result = ConvertToResult<TResult>(key, valueOfKey);
            }

            return result;
        }

        private static TResult ConvertToResult<TResult>(string key, string valueOfKey)
        {
            TResult result;

            try
            {
                result = (TResult)Convert.ChangeType(valueOfKey, typeof(TResult));
            }
            catch (Exception ex)
            {
                if (ThrowException && DefaultValue == null)
                {
                    throw new Exception($"Failed to convert {valueOfKey} from type {valueOfKey.GetType()} to the type {typeof (TResult)}", ex);
                }

                result = GetDefaultValue((Func<TResult>)Convert.ChangeType(DefaultValue, typeof(Func<TResult>)));
            }

            return result;
        }

        private static TResult GetDefaultValue<TResult>(Func<TResult> defaultValue)
        {
            var result = default(TResult);

            if (defaultValue != null)
            {
                try
                {
                    result = defaultValue.Invoke();
                }
                catch (Exception ex)
                {
                    if (ThrowException)
                    {
                        throw new Exception($"Failed to to invoke delegate {defaultValue} of type {defaultValue.GetType()}", ex);
                    }
                }
            }

            return result;
        }
    }
}