newb generic ?: How to cast reference to type parameter

[sklett]

I'm revisiting an old series of overloaded methods I have and I would like
to convert them to a single generic method. Here is the method in it's
current, overloaded implementation:
<code>
public StringCustomFie ldRef GetStringField( string fieldID, CustomRecord
record)
{
if(record.custo mFieldList == null)
{
return null;
}

foreach(CustomF ieldRef fieldRef in record.customFi eldList)
{
if(fieldRef is StringCustomFie ldRef &&
((StringCustomF ieldRef)fieldRe f).internalId == fieldID)
{
return fieldRef as StringCustomFie ldRef;
}
}

return null;
}
</code>

I then tried to change it like so:
<code>
public TFieldType GetStringField< TFieldType>(str ing fieldID, CustomRecord
record)
{
if (record.customF ieldList == null)
{
return default(TFieldT ype);
}

foreach (CustomFieldRef fieldRef in record.customFi eldList)
{
if (fieldRef is TFieldType && ((TFieldType)fi eldRef).interna lId ==
fieldID)
{
return (TFieldType)fie ldRef;
}
}

return default(TFieldT ype);
}
</code>

The above code throws to compiler errors that have me stumped:
"Cannot convert type 'PMDOutAddin.co m.netsuite.webs ervices.CustomF ieldRef'
to 'TFieldType'"

It could be that what I'm trying to do isn't possible, like I said, I'm very
new to generics. I've seen generic classes before that have something like
" : where ClassName" after them, it looks like it's a base class of
interface requirement. Maybe something like this on a method would be
needed?

Any suggestions welcome.
Thanks for reading,
Steve

[Jon Skeet [C# MVP]]

On Jun 4, 3:41 pm, "sklett" <[e-mail] e:

<snip>

It could be that what I'm trying to do isn't possible, like I said, I'm very
new to generics. I've seen generic classes before that have something like
" : where ClassName" after them, it looks like it's a base class of
interface requirement. Maybe something like this on a method would be
needed?

You'll need some kind of constraint so that the internalId member is
available. I'm not immediately sure why it's complaining in quite the
way it is though - could you come up with a short but complete program
that demonstrates the problem, and we can take it from there?

See http://pobox.com/~skeet/csharp/complete.html for what I mean by
that.

Jon

[Christof Nordiek]

Hi Steve,

My Question would be, how the types CustomFieldRef and StringCustomFie ldRef
(and other types, for wich there is an overload) relate to one another.
Does StringCustomFie ldRef derive from CustomFieldRef (and do the other types
also)?
If so, a first step could be to add a type constraint.

public TFieldType GetStringField< TFieldType>(str ing fieldID, CustomRecord
record) where TFieldType : CustomFieldRef
{
...
}

This ensures, that TFieldType is a class deriving from CustomFieldRef and
the cast is possible.

Then still remains the problem of internalId. This property (or field?)
seems be defined in the deriving classes. If you can change the definition
of that classes, you should try to move it into the common base class.
Atleast there should be an abstract or virtual declaration of it, and maybe
overrides in the derived classes.
If that's not possible, you would be bound to use reflection, but that vere
likely will be worse, than having seperate declarations for each type.

HTH
Christof

[sklett]

Hi Christof,

The constraint is what I was looking for, thanks. I'm not able to change
the implementation of CustomFieldRef, but I can suggest the modification to
the developer(s) of the Web Service that it is generated from.

You are correct, StringCustomFie ldRef inherits from the abstract base
CustomFieldRef

Thank you for your suggestion,
Steve
"Christof Nordiek" <[e-mail] ote in message
news:%2******** ********@TK2MSF TNGP05.phx.gbl. ..

Hi Steve,

My Question would be, how the types CustomFieldRef and
StringCustomFie ldRef (and other types, for wich there is an overload)
relate to one another.
Does StringCustomFie ldRef derive from CustomFieldRef (and do the other
types also)?
If so, a first step could be to add a type constraint.

public TFieldType GetStringField< TFieldType>(str ing fieldID, CustomRecord
record) where TFieldType : CustomFieldRef
{
...
}

This ensures, that TFieldType is a class deriving from CustomFieldRef and
the cast is possible.

Then still remains the problem of internalId. This property (or field?)
seems be defined in the deriving classes. If you can change the definition
of that classes, you should try to move it into the common base class.
Atleast there should be an abstract or virtual declaration of it, and
maybe overrides in the derived classes.
If that's not possible, you would be bound to use reflection, but that
vere likely will be worse, than having seperate declarations for each
type.

HTH
Christof

[sklett]

Thank you for the reply Jon,

where TType : Base/Interface is what I was looking for. Learn something
every day :)

Take care,
Steve
"Jon Skeet [C# MVP]" <sk***@pobox.co mwrote in message
news:11******** *************@g 4g2000hsf.googl egroups.com...

On Jun 4, 3:41 pm, "sklett" <[e-mail] e:

<snip>

>It could be that what I'm trying to do isn't possible, like I said, I'm
very
new to generics. I've seen generic classes before that have something
like
" : where ClassName" after them, it looks like it's a base class of
interface requirement. Maybe something like this on a method would be
needed?

You'll need some kind of constraint so that the internalId member is
available. I'm not immediately sure why it's complaining in quite the
way it is though - could you come up with a short but complete program
that demonstrates the problem, and we can take it from there?

See http://pobox.com/~skeet/csharp/complete.html for what I mean by
that.

Jon