C#-ILookup 初始化

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

new Component() { ID = 1, Name = "MOBO", Category = new Lookup<int, string> } 

The category bit always get a compile error.

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I have a property called Category that is of the type Lookup<int, string> and I want to instantiate this property via

new Component() { ID = 1, Name = "MOBO", Category = new Lookup<int, string> };

But I cannot get past the compile errors.

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• I can't really follow what you are trying to do here... Perhaps add a bit more information? – Stormenet Mar 2 '10 at 22:15 

• So sad that Lookups don't have a constructor. I like them so much more than Dictionary's because the Key property is derived from the value object. Far more DRY than a dictionary where you constantly have to tell it again and again the Person.SSN is the key everytime you add an item. – AaronLS Dec 4 '12 at 20:48

• @AaronLS I guess you'll love KeyedCollection then! :) All you have to do is inherit from it, and implement the method that tells it how to retrieve the key from the value. (Its a method that takes a value as input and returns the key from it.) ...Do be warned however, it does have some pitfalls and quirks - I recommend to read up on it and do some tests before serious use. – AnorZaken Sep 19 '15 at 0:13 

• stackoverflow.com/questions/3850930/multi-value-dictionary – Slai Nov 26 '16 at 6:42

Per MSDN documentation, there is no public constructor for the Lookup class: http://msdn.microsoft.com/en-us/library/bb460184.aspx

You can create an instance of a Lookup<TKey, TElement> by calling ToLookup on an object that implements IEnumerable<T>.

You will want to do something like:

new Component { ID = 1, Name = "MOBO", Category = new[] { … }.ToLookup(…) }

Update to address comments:

I'm not sure where you are getting your category info from, so I will make something up…

new Component {
    ID = 1, 
    Name = "MOBO", 
    Category = new Dictionary<int, string> { 
        { 3, "Beverages" }
        { 5, "Produce" }
    }.ToLookup(o => o.Key, o => o.Value)
}

My guess is that your categories will come from some other source instead of instantiating a dictionary like I did here.

From MSDN:

There is no public constructor to create a new instance of a Lookup<TKey, TElement>.
Additionally, Lookup<TKey, TElement> objects are immutable, that is, you cannot add or remove elements or keys from a Lookup<TKey, TElement> object after it has been created.

Here's my attempt on this. Make sure the key is immutable (Gist).

public class MultiValueDictionary<TKey, TElement>
: Collection<TElement>, ILookup<TKey, TElement>
{
  public MultiValueDictionary(Func<TElement, TKey> keyForItem)
    : base(new Collection(keyForItem))
  {
  }

  new Collection Items => (Collection)base.Items;

  public IEnumerable<TElement> this[TKey key] => Items[key];
  public bool Contains(TKey key) => Items.Contains(key);
  IEnumerator<IGrouping<TKey, TElement>>
    IEnumerable<IGrouping<TKey, TElement>>.GetEnumerator() => Items.GetEnumerator();

  class Collection
  : KeyedCollection<TKey, Grouping>, IEnumerable<TElement>, IList<TElement>
  {
    Func<TElement, TKey> KeyForItem { get; }

    public Collection(Func<TElement, TKey> keyForItem) => KeyForItem = keyForItem;
    protected override TKey GetKeyForItem(Grouping item) => item.Key;

    public void Add(TElement item)
    {
      var key = KeyForItem(item);
      if (Dictionary != null && Dictionary.TryGetValue(key, out var collection))
        collection.Add(item);
      else
        Add(new Grouping(key) { item });
    }

    public bool Remove(TElement item)
    {
      var key = KeyForItem(item);
      if (Dictionary != null && Dictionary.TryGetValue(key, out var collection)
        && collection.Remove(item))
      {
        if (collection.Count == 0)
          Remove(key);
        return true;
      }
      return false;
    }

    IEnumerator<TElement> IEnumerable<TElement>.GetEnumerator()
    {
      foreach (var group in base.Items)
        foreach (var item in group)
          yield return item;
    }

    const string IndexError = "Indexing not supported.";
    public int IndexOf(TElement item) => throw new NotSupportedException(IndexError);
    public void Insert(int index, TElement item) => Add(item);
    public bool Contains(TElement item) => Items.Contains(item);
    public void CopyTo(TElement[] array, int arrayIndex) =>
    throw new NotSupportedException(IndexError);
    new IEnumerable<TElement> Items => this;
    public bool IsReadOnly => false;
    TElement IList<TElement>.this[int index]
    {
      get => throw new NotSupportedException(IndexError);
      set => throw new NotSupportedException(IndexError);
    }
  }

  class Grouping : Collection<TElement>, IGrouping<TKey, TElement>
  {
    public Grouping(TKey key) => Key = key;
    public TKey Key { get; }
  }
}

You can't just use ToLookup; you have to tell it how to find the keys and values:

// from ChaosPandion's code
using System.Linq; // make sure you have the using statement 

var component = new Component()  
{  
    ID = 1,  
    Name = "MOBO",  
    Category = (Lookup<int, string>)
       (new Dictionary<int, string>() { {1, "one"} })
       .ToLookup(p=>p.Key, p=>p.Value)
}  

I don't understand why you want to use a Lookup here instead of a dictionary, though.

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• 2

  Lookups can contain duplicate keys and maintain order. This is distinctly different from a dictionary. Also, the entire immutably thing aside, so while there are some equivalent map implementations .. it's not a Dictionary. – user2864740 Jun 30 '17 at 16:37 

• @Gabe he want to use 1 to Many mapping of lookup which is not possible in Dictionary – Akshay Anand Aug 27 '17 at 1:24

If you just need to return an empty ILookup for some reason you can return one from an empty dictionary. For example, to make an ILookup<string, int>, you can use this:

return new Dictionary<string, int>().ToLookup(kvp => kvp.Key, kvp => kvp.Value);

Unfortunately this is the most concise way I can see to do it without having to make a class that implements ILookup yourself.

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<string, Category>

could be seen as:

IDictionary<string, IEnumerable<Category>>

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<Product>();
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<string, Product> 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<Product> eletronics = lookup["Electronics"];
IEnumerable<Product> fruits = lookup["Fruits"];

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