using System;

using System.Collections;

using System.Collections.Generic;

public class SomeEntity

{

    static int count {get;set;} = 1;

    public int id {get;}

    public string someProp {get;set;}

    public SomeEntity()

    {

        id = count;

        count++;    

    }

   public override string ToString()

   {

      return nameof(SomeEntity) + ": " + id + " - " + someProp;

   }

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var myDict = new Dictionary<string, SomeEntity>();

        // Three ways to say the same

        var x = myDict.GetOrAddNew("myKey");

        var x1 = myDict.GetOrAddNew("myKey1", null);

        var x2 = myDict.GetOrAddNew("myKey2", default);

        // Custom value

        var x3 = myDict.GetOrAddNew("myKey3", new SomeEntity{someProp = "Reee"});

        foreach(var kvp in myDict)

            Console.WriteLine(kvp.Value.ToString());

    }

}

​

    /// <summary>

    /// Extensions for dealing with <see cref="Dictionary{TKey,TValue}"/>

    /// </summary>

    public static class DictionaryExtensions

    {

        public static TValue GetOrAddNew<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, TValue defaultValue = default) 

            where TValue : new() 

            => dict.GetOrAdd(key, (values, innerKey) => EqualityComparer<TValue>.Default.Equals(default(TValue), defaultValue) ? new TValue() : defaultValue);

​

        public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, TValue defaultValue = default)

            => dict.GetOrAdd(key, (values, innerKey) => defaultValue);

​

        public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, Func<TValue> valueProvider)

            => dict.GetOrAdd(key, (values, innerKey) => valueProvider());

​

        public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, Func<TKey, TValue> valueProvider)

            => dict.GetOrAdd(key, (values, innerKey) => valueProvider(key));

​

        public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, Func<IDictionary<TKey, TValue>, TKey, TValue> valueProvider)

        {

            if (dict == null) throw new ArgumentNullException(nameof(dict));

            if (key == null) throw new ArgumentNullException(nameof(key));

            if (valueProvider == null) throw new ArgumentNullException(nameof(valueProvider));

​

            if (dict.TryGetValue(key, out var foundValue))

                return foundValue;

​

            dict[key] = valueProvider(dict, key);

            return dict[key];

        }

    }