How can I create a new instance of ImmutableDictionary?

Question

I would like to write something like this:

var d = new ImmutableDictionary { { \"a\", 1 }, { \"b\", 2 } };

(using

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 { { "a", 1 }, { "b", 2 } }; you'll get

IL_0000: newobj instance void class [mscorlib]System.Collections.Generic.Dictionary`2::.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::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::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 { { "a", 1 }, { "b", 2 } }.ToImmutableDictionary();

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