c# - When using object initializers, why does the compiler generate an extra local variable?

Question

While answering a question on SO yesterday, I noticed that if an object is initialized using an Object Initializer, the compiler creates an extra local variable.

Consider the following C# 3.0 code, compiled in release mode in VS2008:

public class Class1
{
    public string Foo { get; set; }
}

public class Class2
{
    public string Foo { get; set; }
}

public class TestHarness
{
    static void Main(string[] args)
    {
        Class1 class1 = new Class1();
        class1.Foo = "fooBar";

        Class2 class2 =
            new Class2
            {
                Foo = "fooBar2"
            };

        Console.WriteLine(class1.Foo);
        Console.WriteLine(class2.Foo);
    }
}

Using Reflector, we can examine the code for the Main method:

.method private hidebysig static void Main(string[] args) cil managed
{
    .entrypoint
    .maxstack 2
    .locals init (
        [0] class ClassLibrary1.Class1 class1,
        [1] class ClassLibrary1.Class2 class2,
        [2] class ClassLibrary1.Class2 <>g__initLocal0)
    L_0000: newobj instance void ClassLibrary1.Class1::.ctor()
    L_0005: stloc.0 
    L_0006: ldloc.0 
    L_0007: ldstr "fooBar"
    L_000c: callvirt instance void ClassLibrary1.Class1::set_Foo(string)
    L_0011: newobj instance void ClassLibrary1.Class2::.ctor()
    L_0016: stloc.2 
    L_0017: ldloc.2 
    L_0018: ldstr "fooBar2"
    L_001d: callvirt instance void ClassLibrary1.Class2::set_Foo(string)
    L_0022: ldloc.2 
    L_0023: stloc.1 
    L_0024: ldloc.0 
    L_0025: callvirt instance string ClassLibrary1.Class1::get_Foo()
    L_002a: call void [mscorlib]System.Console::WriteLine(string)
    L_002f: ldloc.1 
    L_0030: callvirt instance string ClassLibrary1.Class2::get_Foo()
    L_0035: call void [mscorlib]System.Console::WriteLine(string)
    L_003a: ret 
}

Here, we can see that the compiler has generated two references to an instance of Class2 (class2 and <>g__initLocal0), but only one reference to an instance of Class1 (class1).

Now, I'm not very familiar with IL, but it looks like it's instantiating <>g__initLocal0, before setting class2 = <>g__initLocal0.

Why does this happen?

Does it follow then, that there is a performance overhead when using Object Initializers (even if it is very slight)?

Answer 1

Thread-safety and atomicity.

First, consider this line of code:

MyObject foo = new MyObject { Name = "foo", Value = 42 };

Anybody reading that statement might reasonably assume that the construction of the foo object will be atomic. Before the assignment the object doesn't exist at all. Once the assignment has completed the object exists and is in the expected state.

Now consider two possible ways of translating that code:

// #1
MyObject foo = new MyObject();
foo.Name = "foo";
foo.Value = 42;

// #2
MyObject temp = new MyObject();  // temp will be a compiler-generated name
temp.Name = "foo";
temp.Value = 42;
MyObject foo = temp;

In the first case the foo object is instantiated on the first line, but it won't be in the expected state until the final line has finished executing. What happens if another thread tries to access the object before the last line has executed? The object will be in a semi-initialised state.

In the second case the foo object doesn't exist until the final line when it is assigned from temp. Since reference assignment is an atomic operation this gives exactly the same semantics as the original, single-line assignment statement. ie, The foo object never exists in a semi-initialised state.