[Pnet-developers] [bugs #11998] Problems with casting arrays, isinst, and the as operator

[Marcus Urban]

URL:
  <http://savannah.gnu.org/bugs/?func=detailitem&item_id=11998>

                 Summary: Problems with casting arrays, isinst, and the as
operator
                 Project: DotGNU Portable.NET
            Submitted by: mathpup
            Submitted on: Wed 02/16/2005 at 07:23
                Category: None
                Severity: 5 - Average
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open

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Details:

There is something wrong with the with the C# "as" and "is" operators, which
seem to be compiled into CIL "isinst" instructions.

The following sequence prints "False" on Pnet but true on MS:

        Array x = new string[1];
        object[] y = x as object[];
        Console.WriteLine(y == null );

I was trying to debug this myself, but I do not think that I have enough
knowledge of the runtime engine internals. Also, I do not quite understand
the correspondence between CVM instructions and similarly-named CIL
instructions. I looked around to cvm_ptr.c and found this:

VMCASE(COP_ISINST):
{
        /* Determine if the object on the stack top is an
           instance of a particular class */
        classInfo = CVM_ARG_PTR(ILClass *);
        if(stacktop[-1].ptrValue != 0 &&
           !ILClassInheritsFrom(GetObjectClass(stacktop[-1].ptrValue), 
classInfo))
        {
                stacktop[-1].ptrValue = 0;
        }
        MODIFY_PC_AND_STACK(CVM_LEN_PTR, 0);
}

However, according to the CIL spec, ISINST is supposed to attempt casting
from source-type to result-type, and replace the top element of the stack
with the result (if casting is successful) or null (if casting fails). I do
not quite understand what it going on here because it seems as though the
COP_ISINST case should look more like COP_CASTCLASS case.

I'm very confused about all this, but I think that someone more familiar with
the runtime could fix it easily.






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File Attachments:


-------------------------------------------------------
Date: Wed 02/16/2005 at 07:23  Name: array-cast.cs  Size: 165B   By: mathpup
Test case
<http://savannah.gnu.org/bugs/download.php?item_id=11998&item_file_id=2202>

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  <http://savannah.gnu.org/bugs/?func=detailitem&item_id=11998>

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