How does dictionary initialization work in C#?

Question

var maxDictionary = new Dictionary<int, double> { { 10, 40000 } }; 

In the above code, does the compiler uses a constructor? Or does the compiler create a KeyValuePair and add to the dictionary? I'm trying to understand how the compiler interprets it.

Answer 1

Yes, compiler uses default parameterless constructor and then adds all values specified in collection initializer via Dictionary.Add method. As Jon pointed, your code is compiled into

Dictionary<int, double> maxDictionary2; Dictionary<int, double> maxDictionary;  maxDictionary2 = new Dictionary<int, double>(); maxDictionary2.Add(10, 40000.0); maxDictionary = maxDictionary2; 

Generated IL:

.maxstack 3 .locals init (      [0] class [mscorlib]Dictionary`2<int32, float64> maxDictionary,      [1] class [mscorlib]Dictionary`2<int32, float64> maxDictionary2) L_0000: nop  L_0001: newobj instance void [mscorlib]Dictionary`2<int32, float64>::.ctor() L_0006: stloc.1  L_0007: ldloc.1  L_0008: ldc.i4.s 10 L_000a: ldc.r8 40000 L_0013: callvirt instance void [mscorlib]Dictionary`2<int32, float64>::Add(!0, !1) L_0018: nop  L_0019: ldloc.1  L_001a: stloc.0  

I.e. created dictionary assigned to temporary variable maxDictionary2, filled with values, and only then reference to created and filled dictionary is copied to maxDictionary variable.

Keep in mind that you can specify any other constructor, if you don't want to use parammeterless one. E.g. you can use one which sets initial capacity:

var maxDictionary = new Dictionary<int, double>(10) { { 10, 40000 } };