在字典上添加“.Add”，列表为值

Question

I've been struggling to Google this question as I can't get the wording quite right (hence the title).

The gist is why do one of the below work, is there a shorthand for test3 :

  var test1 = new Dictionary<string, int>();
  test1["Derp"] = 10; // Success

  var test2 = new Dictionary<string, List<int>>();
  test2["Derp"].Add(10); // Fail

  var test3 = new Dictionary<string, List<int>>();
  test3["Derp"] = new List<int>();
  test3["Derp"].Add(10); // Success

A scenario I'm coming across often is similar to the below (this is a very basic example):

  var names = new List<string>() { "Jim", "Fred", "Fred", "Dave", "Jim", "Jim", "Jim" };

  var nameCounts = new Dictionary<string, int>();

  foreach(var name in names)
  {
    if (!nameCounts.ContainsKey(name))
      nameCounts.Add(name, 0);

    nameCounts[name]++;
  }

In other words - is there a way to skip the "ContainsKey" check, and go straight to adding to my list (and key automatically)?

Edit: to be clear, I hadn't used the below as in my real-life situation, it isn't quite as simple (unfortunately!)

var nameCounts = names.GroupBy(x => x)
                      .ToDictionary(x => x.Key, x => x.Count());

Answer 1

Perl calls this auto-vivification, and I use some extensions to Dictionary to implement various forms, you would need the one that uses a lambda to generate the initial values:

//***
// Enhanced Dictionary that auto-creates missing values with seed lambda
// ala auto-vivification in Perl
//***
public class SeedDictionary<TKey, TValue> : Dictionary<TKey, TValue> {
    Func<TValue> seedFn;
    public SeedDictionary(Func<TValue> pSeedFn) : base() {
        seedFn = pSeedFn;
    }
    public SeedDictionary(Func<TValue> pSeedFn, IDictionary<TKey, TValue> d) : base() {
        seedFn = pSeedFn;
        foreach (var kvp in d)
            Add(kvp.Key, kvp.Value);
    }

    public new TValue this[TKey key]
    {
        get
        {
            if (!TryGetValue(key, out var val))
                base[key] = (val = seedFn());
            return val;
        }
        set => base[key] = value;
    }
}

So then you could do test2 like so:

var test2 = new SeedDictionary<string, List<int>>(() => new List<int>());
test2["Derp"].Add(10); // works

For your name counts example, you could use the version that auto-creates the default value for the value type:

//***
// Enhanced Dictionary that auto-creates missing values as default
// ala auto-vivification in Perl
//***
public class AutoDictionary<TKey, TValue> : Dictionary<TKey, TValue> {
    public AutoDictionary() : base() { }
    public AutoDictionary(IDictionary<TKey, TValue> d) : base() {
        foreach (var kvp in d)
            Add(kvp.Key, kvp.Value);
    }

    public new TValue this[TKey key]
    {
        get
        {
            if (!TryGetValue(key, out var val))
                base[key] = val;
            return val;
        }
        set => base[key] = value;
    }
}

Answer 2

Another way you can do this (among many), is a little extension method (cutesy of Jon Skeet here )

public static TValue GetOrCreate<TKey, TValue>(this IDictionary<TKey, TValue> dictionary,TKey key) where TValue : new() 
{
    TValue ret;
    if (!dictionary.TryGetValue(key, out ret))
    {
        ret = new TValue();
        dictionary[key] = ret;
    }
    return ret; 
 }

Usage

strong textvar test2 = new Dictionary<string, List<int>>();
var myNewList = test2.GetOrCreate("Derp");
myNewList.Add(10); 

// or

var test2 = new Dictionary<string, List<int>>();
test2.GetOrCreate("Derp").Add(10); // winning!

---

_{Note : In all my early morning pep, i actually didn't look at this question, Eric Lippert is on the money in the comments, this can be simply done via a GroupBy and a projection to a dictionary with ToDictionary without all the extra fluff of extension methods and classes}

Cutesy of Eric Lippert

// Count occurrences of names in a list 
var nameCounts = names.GroupBy(x => x)
                      .ToDictionary(x => x.Key, x => x.Count());

---

Additional Resources

Enumerable.GroupBy Method

Groups the elements of a sequence.

Enumerable.ToDictionary Method

Creates a Dictionary<TKey,TValue> from an IEnumerable<T> .

Answer 3

I usually do something like this:

TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key)
        where TValue : new()
        => dict.TryGetValue(key, out TValue val) ? val : dict[key] = new TValue();

Edit: Another way is:

TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key)
        where TValue : new()
        => dict.ContainsKey(key) ? dict[key] : dict[key] = new TValue();

I'm not sure if this is as performant, but it works on older C# versions, where my first example doesn't.

Answer 4

Alternative with C# 7 out variable :

foreach(var name in names)
{
    nameCounts[name] = nameCounts.TryGetValue(name, out var count) ? count + 1 : 1;
}