Initializing Lookup

Question

How do i declare a new lookup class for a property in the object initializer routine in c#?

E.g.

new Component() { ID = 1, Name = \"MOBO\", Category          


        

Answer 1

Lookups work with the same concept as Dictionaries, the difference is that Dictionaries map a key to a single value, whereas a Lookup map a key to many values.

This also means that:

ILookup

could be seen as:

IDictionary>

You basically would want to use ILookup when you want to map many objects/values to a same key. You can build an ILookup from any list of objects, where you want to group these objects by some property. See:

public class Product
{
    public string Name { get; set; }
    public string Category { get; set; }
    public decimal Price { get; set; }
}

var products = new List();
products.Add(new Product { Name = "TV", Price = 400, Category = "Electronics" });
products.Add(new Product { Name = "Computer", Price = 900, Category = "Electronics" });
products.Add(new Product { Name = "Keyboard", Price = 50, Category = "Electronics" });
products.Add(new Product { Name = "Orange", Price = 2, Category = "Fruits" });
products.Add(new Product { Name = "Grape", Price = 3, Category = "Fruits" });

// group by category
ILookup lookup = products.ToLookup(prod => prod.Category);

foreach (var item in lookup)
{
    // this first loop would run two times
    // because there are two categories: Electronics and Fruits
    string category = item.Key;
    decimal totalPriceForCategory = item.Sum(i => i.Price);

    foreach (var product in item)
    {
        // for the electronics, this would loop three times
        // for the fruits, this would loop two times
        string name = product.Name;
        decimal price = product.Price;
    }
}

You could also get all the products for a category like this:

IEnumerable eletronics = lookup["Electronics"];
IEnumerable fruits = lookup["Fruits"];