Enum question

[Kimmo Laine]

Hi,

stupid question: Why can´t i do this:

public enum eMy: byte {
Item1 = 1,
Item2 = 127,
Item3 = 128,
Item4 = 255,
}

void MyFunc() {
eMy b = eMy.Item3;
Foo( b );
}

void Foo( object o ) {
int i = ( int )o;
}

I get the "Specified cast is not valid."-exception. I must do double cast:

int i = ( int )( eMy )o;

And another thing: if i look the variable b in Debugger - it shows that the
value is "-128" not 128? Its a little confusing - when i code something like
this...

static void Foo( object o ) {
int i = (int)(eMy)o;

eMy d = eMy.Item3;
if( i == ( int )d ) {
// we go here
}
}

....and debug it: it clearly states that "c" is "-128" and "i" is "128" - i
== d -> 128 == -128!


Kimmo Laine

 

[Jon Skeet [C# MVP]]

stupid question: Why can´t i do this:

public enum eMy: byte {
Item1 = 1,
Item2 = 127,
Item3 = 128,
Item4 = 255,
}

void MyFunc() {
eMy b = eMy.Item3;
Foo( b );
}

void Foo( object o ) {
int i = ( int )o;
}

I get the "Specified cast is not valid."-exception. I must do double cast:

int i = ( int )( eMy )o;

Yup - because when you box a value, its exact type is boxed with it. In
this case, the boxed type is eMy. When you unbox, you have to unbox to
the same type, and *then* you can do conversions.

And another thing: if i look the variable b in Debugger - it shows that the
value is "-128" not 128? Its a little confusing - when i code something like
this...

static void Foo( object o ) {
int i = (int)(eMy)o;

eMy d = eMy.Item3;
if( i == ( int )d ) {
// we go here
}
}

...and debug it: it clearly states that "c" is "-128" and "i" is "128" - i
== d -> 128 == -128!

b should certainly not be shown as -128 - that's not a valid value for
a byte in the first place. Not sure what you mean about "c" in your
description though...

 

[Kimmo Laine]

My bad! "c" = "d"!

- Kimmo

stupid question: Why can´t i do this:

public enum eMy: byte {
Item1 = 1,
Item2 = 127,
Item3 = 128,
Item4 = 255,
}

void MyFunc() {
eMy b = eMy.Item3;
Foo( b );
}

void Foo( object o ) {
int i = ( int )o;
}

I get the "Specified cast is not valid."-exception. I must do double cast:

int i = ( int )( eMy )o;

Yup - because when you box a value, its exact type is boxed with it. In
this case, the boxed type is eMy. When you unbox, you have to unbox to
the same type, and *then* you can do conversions.

And another thing: if i look the variable b in Debugger - it shows that
the
value is "-128" not 128? Its a little confusing - when i code something
like
this...

static void Foo( object o ) {
int i = (int)(eMy)o;

eMy d = eMy.Item3;
if( i == ( int )d ) {
// we go here
}
}

...and debug it: it clearly states that "c" is "-128" and "i" is "128" - i
== d -> 128 == -128!

b should certainly not be shown as -128 - that's not a valid value for
a byte in the first place. Not sure what you mean about "c" in your
description though...

 

[Nick Malik]

that may just be something the debugger is doing, and may not be reflective
of the actual logic.

--- Nick

stupid question: Why can´t i do this:

public enum eMy: byte {
Item1 = 1,
Item2 = 127,
Item3 = 128,
Item4 = 255,
}

void MyFunc() {
eMy b = eMy.Item3;
Foo( b );
}

void Foo( object o ) {
int i = ( int )o;
}

I get the "Specified cast is not valid."-exception. I must do double cast:

int i = ( int )( eMy )o;

Yup - because when you box a value, its exact type is boxed with it. In
this case, the boxed type is eMy. When you unbox, you have to unbox to
the same type, and *then* you can do conversions.

And another thing: if i look the variable b in Debugger - it shows that the
value is "-128" not 128? Its a little confusing - when i code something like
this...

static void Foo( object o ) {
int i = (int)(eMy)o;

eMy d = eMy.Item3;
if( i == ( int )d ) {
// we go here
}
}

...and debug it: it clearly states that "c" is "-128" and "i" is "128" - i
== d -> 128 == -128!

b should certainly not be shown as -128 - that's not a valid value for
a byte in the first place. Not sure what you mean about "c" in your
description though...