Object initializer simply calls Add
for each item.
var list = new List<string>{ "One", "Two", "Three" };
As you can see, in this case parameterless constructor is called:
L_0000: nop
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor()
L_0006: stloc.1
L_0007: ldloc.1
L_0008: ldstr "One"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_0012: nop
L_0013: ldloc.1
L_0014: ldstr "Two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_001e: nop
L_001f: ldloc.1
L_0020: ldstr "Three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_002a: nop
L_002b: ldloc.1
So, you should set capacity manually:
var list = new List<string>(5){ "One", "Two", "Three" };
Compiles into:
L_0000: nop
L_0001: ldc.i4.5
L_0002: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor(int32)
// rest is same
So, algorithm is pretty obvious - it calls constructor which you specified (parameterless by default) and then calls Add
for each item.
NOTE: I know, that default capacity is 4 for List<T>
and I verified what happens if we pass more than 4 items in initializer (e.g. maybe compiler determines which constructor to call depending on items count) but result is same - parameterless constructor is called by default.
I think purpose of collection initializers is creating small collections (1 - 8 items), thus there will be a little performance impact (only one resize if you will pass 8 items into initializer). Nobody expects you will use in-place initialization with 100 items. And if you are going to do that, you should use appropriate constructor of collection.
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