MultiTerm/Common/AppSettings/XmlAppSettingsProvider.cs

using Common.Logging;
using System.Xml.Serialization;

namespace Common.AppSettings;

public class XmlAppSettingsProvider : IAppSettingsProvider
{
    private readonly string settingsFilePath;
    private readonly XmlSerializer serializer;
    private List<AppSetting> settings;

    public event EventHandler<LogEntry>? LogWorthyEvent;

    public XmlAppSettingsProvider(string settingsFilePath)
    {
        // check for file path validity
        if (String.IsNullOrEmpty(settingsFilePath)) { throw new ArgumentNullException(nameof(settingsFilePath)); }

        this.settingsFilePath = settingsFilePath;
        this.settings = new List<AppSetting>();
        this.serializer = new XmlSerializer(typeof(List<AppSetting>));

        // create file if it does not exist yet
        if (File.Exists(settingsFilePath) == false)
        {
            this.Save();
        }
    }

    public void Load()
    {
        // read file by creating a stream. this will throw an exception when the file does not exist.
        using var fileStream = new FileStream(this.settingsFilePath, FileMode.Open);

        // deserialize and cast to List if not null
        var deserializedObj = this.serializer.Deserialize(fileStream);
        if(deserializedObj != null)
        {
            // create log entry if there were already some settings loaded
            if(this.settings.Count > 0) 
            {
                this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Warn, category: nameof(XmlAppSettingsProvider),
                    message: $"'{nameof(Load)}()' deserialization overwrote previously loaded settings."));
            }
            this.settings = (List<AppSetting>)deserializedObj;
        }
        else
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Warn, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(Load)}()' deserialization resulted in null object. Did not change loaded settings."));
        }
    }

    public void Save()
    {
        // if saving fails, just log the exception. user is happier if the app is useable but does not remember settings
        try
        {
            // create file and serialize data
            TextWriter writer = new StreamWriter(this.settingsFilePath);
            this.serializer.Serialize(writer, this.settings);
            writer.Close();
        }
        catch (Exception ex)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(){ LogLevel = LogLevel.Error, Category = nameof(XmlAppSettingsProvider),
                Message = $"'{nameof(Save)}()' failed to create file or serialize data.", Exception = ex });
        }
    }



    public bool WriteSetting(string key, string value)
    {
        // guard empty key or value
        if(String.IsNullOrEmpty(key)) throw new ArgumentNullException(nameof(key));
        if (String.IsNullOrEmpty(value)) throw new ArgumentNullException(nameof(value));

        // get all settings with this key
        var foundSettings = this.settings.FindAll(x => x.Key == key);

        // check if the key already exists multiple times
        if(foundSettings.Count > 1)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Error, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(WriteSetting)}()' cancelled because {foundSettings.Count} settings with the key '{key}' were found. Expected none or exactly one."));

            return false;  // cancel
        }
        // if exactly one is found => remove existing app setting
        else if(foundSettings.Count == 1)
        {
            this.settings.Remove(this.settings.Find(x => x.Key == key)!);
        }
        // none existing yet => inform via event
        else if(foundSettings.Count == 0)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Info, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(WriteSetting)}()' initialized new setting with key '{key}' and value '{value}'."));
        }

        // add new setting
        this.settings.Add(new AppSetting(key, value));
        return true;
    }

    public bool TryReadSetting(string key, out string value)
    {
        // guard empty key
        if (String.IsNullOrEmpty(key)) throw new ArgumentNullException(nameof(key));

        // get all settings with this key
        var foundSettings = this.settings.FindAll(x => x.Key == key);

        // too many values found
        if (foundSettings.Count > 1)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Error, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSetting)}()' did find {foundSettings.Count} settings with the key '{key}'. Expected only one."));

            value = string.Empty;
            return false;
        }

        // no value found
        if (foundSettings.Count == 0)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Error, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSetting)}()' did not find a setting with the key '{key}'."));

            value = string.Empty;
            return false;
        }

        // extract correct setting when found exactly one
        var setting = foundSettings[0];
        if(setting.Value != null)
        {
            value = setting.Value;
        }
        // if value is null it was probably an empty string or similar => set it to empty string
        else
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Warn, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSetting)}()' value of retrieved setting with key '{key}' was null. Returned empty string instead."));

            value = string.Empty;
        }
        return true;
    }

    public bool TryReadSettingOrAddDefault(string key, out string value, string defaultValue)
    {
        // guard empty key or defaultValue
        if (String.IsNullOrEmpty(key)) throw new ArgumentNullException(nameof(key));
        if (String.IsNullOrEmpty(defaultValue)) throw new ArgumentNullException(nameof(defaultValue));

        // get all settings with this key
        var foundSettings = this.settings.FindAll(x => x.Key == key);

        // too many values found
        if (foundSettings.Count > 1)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Error, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSettingOrAddDefault)}()' did find {foundSettings.Count} settings with the key '{key}'. Expected only one."));

            value = string.Empty;
            return false;
        }

        // no value found => initialize with the default value
        if (foundSettings.Count == 0)
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Info, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSettingOrAddDefault)}()' did not find a setting with the key '{key}'. Adding setting with value '{defaultValue}'."));

            value = defaultValue;
            return this.WriteSetting(key, defaultValue);
        }

        // extract correct setting when found exactly one
        var setting = foundSettings[0];
        if (setting.Value != null)
        {
            value = setting.Value;
        }
        // if value is null it was probably an empty string or similar => set it to empty string
        else
        {
            this.LogWorthyEvent?.Invoke(this, new LogEntry(level: LogLevel.Warn, category: nameof(XmlAppSettingsProvider),
                message: $"'{nameof(TryReadSetting)}()' value of retrieved setting with key '{key}' was null. Returned empty string instead."));

            value = string.Empty;
        }

        return true;
    }
}