CIL unbox_any instruction - strange behavior

Question

.method public static void  Test<class T>(object A_0) cil managed
{
  // Code size       13 (0xd)
  .maxstack  1
  .locals init (!!T V_0)
  IL_0000:  ldarg.0
  IL_0001:  isinst     !!T
  IL_0006:  unbox.any  !!T
  IL_000b:  stloc.0
  IL_000c:  ret
} // end of method DemoType::Test

The equal C# code is:

public static void Test<T>(object o) where T : class
{
    T t = o as T;
}

My questions are:

1. Why unbox.any been called? if you just do

    var a = father as child 
   

   isinst intruction will call and no unbox.any, and If i'll remove the generic definition and i'll try to cast (isinst) the object to some class, no unbox.any will be called.

2. Maybe unbox.any been called because the generic definition, so in this case the unbox.any need to throw a NullReferenceException because the answer of isinst instruction return null for this casting. see unbox_any. And if you try to run this code you will see that no exception has thrown.

Update

I can understand the unbox_any becuase the object type parameter and it try to cast it to concrete type after the isinst check. Maybe the generics influence also.

My question is, why not thrown an exception in unbox.any if the the obj we try to unbox to T is null?

The documentation say: "NullReferenceException is thrown if obj is a null reference."

Answer 1

The unbox is to keep the verifier happy. The verifier is not particularly smart about knowing that a type parameter T is always going to be a reference type, and so the C# compiler emits these otherwise unnecessary unboxes.

If you do a search of the Roslyn source code for Unbox_any and IsVerifierReference you'll see that this happens in quite a few places around the code generator.

The jitter will know when generating the code whether the type parameter is a reference or not and should generate decent code regardless of the seemingly unnecessary instruction.