C# - custom explicit conversion with checked/unchecked operator

Question

I'm learning to write custom type conversions in C# and I have a question I can't manage to resolve with Google / MSDN / earlier posted SO items.

Normally, a C# program that narrows a numeric type does that via unchecked explicit conversion, e.g.:

int i = 256;
byte b = (byte)i; // b == 0

however, the following will give an overflow exception:

byte b = checked((byte)i);

My question is as follows: is the behavior of the checked / unchecked keyword implementable in an custom type conversion, e.g.:

class Foo {
  public static explicit operator int(Foo bar) {
    if (checked)
      throw someEception
    else
      return some Foo to int conversion
  }
}

Of course, the code above is not the answer, but does anyone know if something like this is possible?

Answer 1

checked is a compile time thing. That is, its sole effect will be on the code block directly surrounded by a checked statement, not the methods called in that block. Consequently, there wouldn't be a checked and unchecked context at run time that you may want to adjust the behavior of a function according to it.

Answer 2

Section 14.5.12 of the C# language spec explicitly enumerates all the operators on which the checked and unchecked keywords can have an effect.

User defined operators are not part of that list, so no, you cannot write a user-defined conversion operator that takes checked/unchecked into account.

Answer 3

checked causes a "check for overflow" version of the appropriate CIL instruction to be generated by the compiler.

Here's a dump from IL Disassembler:

//000012:       Int32 i = 42;
  IL_0001:  ldc.i4.s   42
  IL_0003:  stloc.0
//000013:       Byte b1 = (Byte) i;
  IL_0004:  ldloc.0
  IL_0005:  conv.u1
  IL_0006:  stloc.1
//000014:       Byte b2 = checked ((Byte) i);
  IL_0007:  ldloc.0
  IL_0008:  conv.ovf.u1
  IL_0009:  stloc.2