C＃Singleton字典类

Question

I have the following class which loads a large, static XML file into an dictionary. I want it to only load it the XML once when referenced.

public class MyClass
{

    private static readonly string _xmlfile = $"{path.db}database.xml";

    public Dictionary<string, MyXML> content;

    public MyClass()
    {

        var d = XMLHelper.Deserialize<MyXMLs>(_xmlfile);

        content = d.content;

    }

}

I tried following the main function from this article https://www.codeproject.com/Articles/14026/Generic-Singleton-Pattern-using-Reflection-in-C

But couldnt quite translate is to the Dictionary

Answer 1

Use Jon Skeet's fully lazy and thread-safe singleton pattern: http://csharpindepth.com/Articles/General/Singleton.aspx

public sealed class Singleton
{
    private Singleton()
    { }

    public static Singleton Instance { get { return Nested.instance; } }

    private class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        { }

        internal static readonly Singleton instance = new Singleton();
    }
}

or his pattern with the Lazy<T> type

public sealed class Singleton
{
    private static readonly Lazy<Singleton> lazy =
        new Lazy<Singleton>(() => new Singleton());

    public static Singleton Instance { get { return lazy.Value; } }

    private Singleton()
    { }
}

Answer 2

You should use lazy initialisation. Try this:

class Program
{
    static void Main(string[] args)
    {
        MyClass myClass = new MyClass();
        MyClass.MyXML xml = myClass["someKeyValue"];
    }
}


public class MyClass
{
    private class Path
    {
        public string db;
    }
    public class MyXML { }



    private static Path path = new Path();

    private static readonly string _xmlfile = $"{path.db}database.xml";

    private Dictionary<string, MyXML> content = new Dictionary<string, MyXML>();

    public MyXML this[string key]
    {
        get
        {
            MyXML value = content[key];
            if(key == "someKeyValue" && value == null)
            {
                value = getXmlFromFile(_xmlfile);
                content[key] = value;
            }
            return value;
        }
    }

    public MyClass()
    {

    }

    private MyXML getXmlFromFile(string path)
    {
        return XMLHelper.Deserialize<MyXMLs>(path).content;
    }

}