How can I create a new instance of ImmutableDictionary?

Question

I would like to write something like this:

var d = new ImmutableDictionary<string, int> { { "a", 1 }, { "b", 2 } };

(using ImmutableDictionary from System.Collections.Immutable). It seems like a straightforward usage as I am declaring all the values upfront -- no mutation there. But this gives me error:

The type 'System.Collections.Immutable.ImmutableDictionary<TKey,TValue>' has no constructors defined

How I am supposed to create a new immutable dictionary with static content?

Answer 1

You can't create immutable collection with a collection initializer because the compiler translates them into a sequence of calls to the Add method. For example if you look at the IL code for var d = new Dictionary<string, int> { { "a", 1 }, { "b", 2 } }; you'll get

IL_0000: newobj instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::.ctor()
IL_0005: dup
IL_0006: ldstr "a"
IL_000b: ldc.i4.1
IL_000c: callvirt instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1)
IL_0011: dup
IL_0012: ldstr "b"
IL_0017: ldc.i4.2
IL_0018: callvirt instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1)

Obviously this violates the concept of immutable collections.

Both your own answer and Jon Skeet's are ways to deal with this.

// lukasLansky's solution
var d = new Dictionary<string, int> { { "a", 1 }, { "b", 2 } }.ToImmutableDictionary();

// Jon Skeet's solution
var builder = ImmutableDictionary.CreateBuilder<string, int>();
builder.Add("a", 1);
builder.Add("b", 2);   
var result = builder.ToImmutable();