Float/double precision in debug/release modes

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They can indeed be different. According to the CLR ECMA specification:

Storage locations for floating-point
numbers (statics, array elements, and
fields of classes) are of fixed size.
The supported storage sizes are
float32 and float64. Everywhere else
(on the evaluation stack, as
arguments, as return types, and as
local variables) floating-point
numbers are represented using an
internal floating-point type. In each
such instance, the nominal type of the
variable or expression is either R4 or
R8, but its value can be represented
internally with additional range
and/or precision. The size of the
internal floating-point representation
is implementation-dependent, can vary,
and shall have precision at least as
great as that of the variable or
expression being represented. An
implicit widening conversion to the
internal representation from float32
or float64 is performed when those
types are loaded from storage. The
internal representation is typically
the native size for the hardware, or
as required for efficient
implementation of an operation.

What this basically means is that the following comparison may or may not be equal:

class Foo
{
  double _v = ...;

  void Bar()
  {
    double v = _v;

    if( v == _v )
    {
      // Code may or may not execute here.
      // _v is 64-bit.
      // v could be either 64-bit (debug) or 80-bit (release) or something else (future?).
    }
  }
}

Take-home message: never check floating values for equality.

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