How to make inline array initialization work like

问题:

Why is it possible to initialize a Dictionary<T1,T2> like this:

var dict = new Dictionary<string,int>() { 
    { "key1", 1 },
    { "key2", 2 }
};

...but not to initialize, say, an array of KeyValuePair<T1,T2> objects in exactly the same way:

var kvps = new KeyValuePair<string,int>[] {
    { "key1", 1 },
    { "key2", 2 }
};
// compiler error: "Array initializers can only be used in a variable 
// or field initializer.  Try using a new expression instead."

I realize that I could make the second example work by just writing new KeyValuePair<string,int>() { "key1", 1 }, etc for each item. But I'm wondering if it's possible to use the same type of concise syntax that is possible in the first example.

If it is not possible, then what makes the Dictionary type so special?

回答1:

The collection initializer syntax is translated into calls to Add with the appropriate number of parameters:

var dict = new Dictionary<string,int>();
dict.Add("key1", 1);
dict.Add("key2", 2);

This special initializer syntax will also work on other classes that have an Add method and implements IEnumerable. Let's create a completely crazy class just to prove that there's nothing special about Dictionary and that this syntax can work for any suitable class:

// Don't do this in production code!
class CrazyAdd : IEnumerable
{
    public void Add(int x, int y, int z)
    {
        Console.WriteLine(x + y + z); // Well it *does* add...
    }

    public IEnumerator GetEnumerator() { throw new NotImplementedException(); }
}

Now you can write this:

var crazyAdd = new CrazyAdd
{
    {1, 2, 3},
    {4, 5, 6}
};

Outputs:

6
15

See it working online: ideone

As for the other types you asked about:

• It doesn't work on an array because it has no Add method.
• List<T> has an Add method but it has only one parameter.

回答2:

It does work with the Dictionary, because it has an overload for Add that takes two arguments. Arrays dont even have an Add method, let alone one with two arguments.

The Dictionary class is specially designed to work with KeyValuePair<,> internally, that is the only reason you do not need the call the constructor manually, instead the two-argument Add is called and constructs the KeyValuePair under the hood.

Every other IEnumerable<KeyValuePair<,>> does not have this special implementation and therefore has to be initialized this way:

var array = new KeyValuePair<int, int>[] {
    new KeyValuePair<int, int>(1, 2),
    new KeyValuePair<int, int>(3, 4)
};

You can create the same behaviour with your own classes, like lets say you implement this:

class ThreeTupleList<T1, T2, T3> : List<Tuple<T1, T2, T3>>
{
    public void Add(T1 a, T2 b, T3 c)
    {
        this.Add(new Tuple<T1, T2, T3>(a, b, c));
    }

    // You can even implement a two-argument Add and mix initializers
    public void Add(T1 a, T2 b)
    {
        this.Add(new Tuple<T1, T2, T3>(a, b, default(T3)));
    }
}

you can initialize it like this, and even mix three-, two- and one-argument initializers:

var mylist = new ThreeTupleList<int, string, double>()
{
    { 1, "foo", 2.3 },
    { 4, "bar", 5.6 },
    { 7, "no double here" },
    null
};

回答3:

Your problem stems from the fact that it is an array, not a collection.

var kvps = new KeyValuePair<string,int>[] {
    { "key1", 1 },
    { "key2", 2 }
};

should really be:

var kvps = new KeyValuePair<string, int>[] {
    new KeyValuePair<string, int>("key1", 1),
    new KeyValuePair<string, int>("key2", 2)
};

The giveaway is the brackets. [] is an array. {} is a collection.

回答4:

Thanks to multiple answerers for pointing out that the Add method is the magical thing secret sauce that makes the initialization syntax work. So I could achieve my goal by inheriting the class in question (KeyValuePair):

public class InitializableKVPs<T1,T2> : IEnumerable<KeyValuePair<T1,T2>>
{
    public void Add(T1 key, T2 value) 
    {
        throw new NotImplementedException();
    }

    public IEnumerator<KeyValuePair<string,string>>  GetEnumerator()
    {
        throw new NotImplementedException();
    }

    IEnumerator  IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

This now is accepted by the compiler:

var kvps = new InitializableKVPs<string,int> {
    { "key1", 1 },
    { "key2", 2 }
};

Edit: Philip Daubmeier's answer has an actual, concise implementation of this.

回答5:

Override IDictionary for the C# 6.0 syntax

In case someone comes here, as I did, looking to save some keystrokes for the new C# 6.0 dictionary initializer syntax, it can be done, but requires deriving from IDictionary instead. You only need to implement the this[] set method to get this to work, which leaves a ton of non-implemented methods.

The class implementation looks like this:

// Seriously!  Don't do this in production code! Ever!!!
public class CrazyAdd2 : IDictionary<string, int>
{
    public int this[string key]
    {
        get { throw new NotImplementedException(); }
        set {Console.WriteLine($"([{key}]={value})"); }
    }

#region NotImplemented
// lots of empty methods go here
#endregion
}

then to use it:

var crazyAdd2 = new CrazyAdd2
{
    ["one"] = 1,
    ["two"] = 2,
};

and the output:

([one]=1)
([two]=2)

And here's a fiddle demonstrating the whole thing:

https://dotnetfiddle.net/Lovy7m

回答6:

You might not see it but it is the same syntax. The only difference is that an array takes elements as an input where a dictionary takes a 2 dimensional array.

int[] a = new int[]{5,6};
int[,] a = new int[]{{5,6},{3,6}};
Dictionary<int,int> a = new Dictionary<int,int>{{5,6},{3,6}};