Help: The as operator must be used with a reference type ??

[GiJeet]

Problem using AS operator to cast Tag property

Hello, I’m using the Tag property of a menu item to hold an enum value
of a PictureBoxSizeM ode.

Eg: this.menuImageS tretch.Tag =
System.Windows. Forms.PictureBo xSizeMode.Stret chImage;

now in the ProcessImageCli ck method I check to make sure the Tag
propety is not null and cast from type Object to PictureBoxSizeM ode
enum. The code below works as long as the value in Tag is
PictureBoxSizeM ode enum value but when not...error.

private void ProcessImageCli ck(ToolStripIte mClickedEventAr gs e)
{
ToolStripItem item = e.ClickedItem;
if (item.Tag != null)
{
pbxPhoto.SizeMo de = (System.Windows .Forms.PictureB oxSizeMode)
item.Tag;
}
}

However, since the Tag property is of type object it can hold ANY
value so I want to prevent a runtime error and us the AS operator to
cast the tag property value to type
System.Windows. Forms.PictureBo xSizeMode but it throws an error: Error
1 The as operator must be used with a reference type
('System.Window s.Forms.Picture BoxSizeMode' is a value type)

so if I use: item.Tag as System.Windows. Forms.PictureBo xSizeMode //
throws error above

How to cast beforehand to prevent runtime error?

Thx
G

[Hans Kesting]

It happens that GiJeet formulated :

Problem using AS operator to cast Tag property

Hello, I’m using the Tag property of a menu item to hold an enum value
of a PictureBoxSizeM ode.

Eg: this.menuImageS tretch.Tag =
System.Windows. Forms.PictureBo xSizeMode.Stret chImage;

now in the ProcessImageCli ck method I check to make sure the Tag
propety is not null and cast from type Object to PictureBoxSizeM ode
enum. The code below works as long as the value in Tag is
PictureBoxSizeM ode enum value but when not...error.

private void ProcessImageCli ck(ToolStripIte mClickedEventAr gs e)
{
ToolStripItem item = e.ClickedItem;
if (item.Tag != null)
{
pbxPhoto.SizeMo de = (System.Windows .Forms.PictureB oxSizeMode)
item.Tag;
}
}

However, since the Tag property is of type object it can hold ANY
value so I want to prevent a runtime error and us the AS operator to
cast the tag property value to type
System.Windows. Forms.PictureBo xSizeMode but it throws an error: Error
1 The as operator must be used with a reference type
('System.Window s.Forms.Picture BoxSizeMode' is a value type)

so if I use: item.Tag as System.Windows. Forms.PictureBo xSizeMode //
throws error above

How to cast beforehand to prevent runtime error?

Thx
G

You could test with the "is" operator whether that Tag really holds a
PictureBoxSizeM ode.

An other option could be (C#2 and on)

pbxPhoto.SizeMo de = item.Tag as PictureBoxSizeM ode? ??
PictureBoxSizeM ode.Normal;

That is:
* use "as" to cast to a Nullable<Pictur eBoxSizeMode>,
* then use the "??" operator to convert a possible null value to a
default value (I have chosen "Normal")

Hans Kesting

[GiJeet]

>On Nov 13, 8:53*am, Hans Kesting <news.han...@sp amgourmet.comwr ote:

That is:
* use "as" to cast to a Nullable<Pictur eBoxSizeMode>,
* then use the "??" operator to convert a possible null value to a *
default value (I have chosen "Normal")

Hans, thanks for responding. I'm a little confused. I see 3 question
marks...please explain.
Thanks again!
G

[Marc Gravell]

I'm a little confused. I see 3 question marks...please explain.

Ooh, that is evil...

PictureBoxSizeM ode? means Nullable<Pictur eBoxSizeMode- i.e. it can be
either a PictureBoxSizeM ode or null (but nothing else).

?? is the null-coalescing operator; the first non-null value is used as
the result.

To be honest, I probably would *not* recommend the use of the suggested
syntax:

as {type}? ?? {default};

While terse (and quite clever), IMO this adds confusion. Which is never
a good idea. I would simply use the longer:

PictureBoxSizeM ode result;
if(val != null && val is PictureBoxSizeM ode) {
result = (PictureBoxSize Mode) val;
} else {
result = PictureBoxSizeM ode.Normal; // etc
}

YMMV

Marc

[Ben Voigt [C++ MVP]]

Marc Gravell wrote:

>I'm a little confused. I see 3 question marks...please explain.

Ooh, that is evil...

PictureBoxSizeM ode? means Nullable<Pictur eBoxSizeMode- i.e. it can
be either a PictureBoxSizeM ode or null (but nothing else).

?? is the null-coalescing operator; the first non-null value is used
as the result.

To be honest, I probably would *not* recommend the use of the
suggested syntax:

as {type}? ?? {default};

This should be clearer:

(... as Nullable<{type} >).GetValueOrDe fault(...);

>
While terse (and quite clever), IMO this adds confusion. Which is
never a good idea. I would simply use the longer:

PictureBoxSizeM ode result;
if(val != null && val is PictureBoxSizeM ode) {
result = (PictureBoxSize Mode) val;
} else {
result = PictureBoxSizeM ode.Normal; // etc
}

YMMV

Marc

[GiJeet]

>On Nov 13, 9:44*am, Marc Gravell <marc.grav...@g mail.comwrote:

Ooh, that is evil...

sorry...want's trying to be "evil", just confused.
thanks for all your help.

[Jeff Johnson]

"GiJeet" <gi****@yahoo.c omwrote in message
news:88******** *************** ***********@o4g 2000pra.googleg roups.com...

>On Nov 13, 9:44 am, Marc Gravell <marc.grav...@g mail.comwrote:

>Ooh, that is evil...

sorry...want's trying to be "evil", just confused.
thanks for all your help.

No, he meant the ? ?? syntax was evil.