How can I create a new instance of ImmutableDictionary?

So far I like this most:

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

Either create a "normal" dictionary first and call ToImmutableDictionary (as per your own answer), or use ImmutableDictionary<,>.Builder:

var builder = ImmutableDictionary.CreateBuilder<string, int>();
builder.Add("a", 1);
builder.Add("b", 2);
var result = builder.ToImmutable();

It's a shame that the builder doesn't have a public constructor as far as I can tell, as it prevents you from using the collection initializer syntax, unless I've missed something... the fact that the Add method returns void means you can't even chain calls to it, making it more annoying - as far as I can see, you basically can't use a builder to create an immutable dictionary in a single expression, which is very frustrating :(

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();