Difference between call instance vs newobj instance in IL

Question

I'm delving into C# in Depth, and playing with nullable value types. Just for experimental purposes I wrote a piece of code:

    private static void HowNullableWorks()
    {
        int test = 3;
        int? implicitConversion = test;
        Nullable test2 = new Nullable(3);

        MethodThatTakesNullableInt(null);
        MethodThatTakesNullableInt(39);
    }

And I was supprised to see that implicitConversion / test2 variables are initialized with:

call       instance void valuetype [mscorlib]System.Nullable`1::.ctor(!0)

instruction, whereas when MethodThatTakesNullableInt is called I can see:

IL_0017:  initobj    valuetype [mscorlib]System.Nullable`1

and

IL_0026:  newobj     instance void valuetype [mscorlib]System.Nullable`1::.ctor(!0)

which I understand. I thought that I'll see newobj instruction for implicitConversion / test2 as well.

This is full IL code:

.method private hidebysig static void  HowNullableWorks() cil managed
{
  // Code size       50 (0x32)
  .maxstack  2
  .locals init ([0] int32 test,
           [1] valuetype [mscorlib]System.Nullable`1 implicitConversion,
           [2] valuetype [mscorlib]System.Nullable`1 test2,
           [3] valuetype [mscorlib]System.Nullable`1 CS$0$0000)
  IL_0000:  nop
  IL_0001:  ldc.i4.3
  IL_0002:  stloc.0
  IL_0003:  ldloca.s   implicitConversion
  IL_0005:  ldloc.0
  IL_0006:  call       instance void valuetype [mscorlib]System.Nullable`1::.ctor(!0)
  IL_000b:  nop
  IL_000c:  ldloca.s   test2
  IL_000e:  ldc.i4.3
  IL_000f:  call       instance void valuetype [mscorlib]System.Nullable`1::.ctor(!0)
  IL_0014:  nop
  IL_0015:  ldloca.s   CS$0$0000
  IL_0017:  initobj    valuetype [mscorlib]System.Nullable`1
  IL_001d:  ldloc.3
  IL_001e:  call       void csharp.in.depth._2nd.Program::MethodThatTakesNullableInt(valuetype [mscorlib]System.Nullable`1)
  IL_0023:  nop
  IL_0024:  ldc.i4.s   39
  IL_0026:  newobj     instance void valuetype [mscorlib]System.Nullable`1::.ctor(!0)
  IL_002b:  call       void csharp.in.depth._2nd.Program::MethodThatTakesNullableInt(valuetype [mscorlib]System.Nullable`1)
  IL_0030:  nop
  IL_0031:  ret
} // end of method Program::HowNullableWorks

Difference between call instance vs newobj instance in IL

1 Answers