Can this be done via .net generics? How?

[Andy Bell]

Can this be done via .net generics? How? The % signs below are just to show
how I want to do it, I realise they're not valid syntax.

public abstract class BaseSelectionRe quirement {
...
protected Type mControlType;
protected string mFieldName;
protected Type mFieldType;
protected UserControl mControl;
...

public %mFieldType% Value {
get {
return ( ( %mControlType% ) mControl ).%mFieldName%;
}
set {
( ( %mControlType% ) mControl ).%mFieldName% = value;
}
}
}

So, say my descended class was:

public class HatSelectionReq uirement : BaseSelectionRe quirement {
public HatSelectionReq uirement( ) {
mControlType = HatUserControl;
mFieldType = typeof( Int32 );
mFieldName = "Size";
}
...
}

Then I could get/set the int HatUserControl. Size via
HatSelectionReq uirement.Value

[Frans Bouma [C# MVP]]

Andy Bell wrote:

Can this be done via .net generics? How? The % signs below are just
to show how I want to do it, I realise they're not valid syntax.

public abstract class BaseSelectionRe quirement {
...
protected Type mControlType;
protected string mFieldName;
protected Type mFieldType;
protected UserControl mControl;
...

public %mFieldType% Value {
get {
return ( ( %mControlType% ) mControl ).%mFieldName%;
}
set {
( ( %mControlType% ) mControl ).%mFieldName% = value;
}
}
}

So, say my descended class was:

public class HatSelectionReq uirement : BaseSelectionRe quirement {
public HatSelectionReq uirement( ) {
mControlType = HatUserControl;
mFieldType = typeof( Int32 );
mFieldName = "Size";
}
...
}

Then I could get/set the int HatUserControl. Size via
HatSelectionReq uirement.Value

THat's not the area where generics should be used. Generics should be
used to generalize a piece of algoritmic code by making the containing
class generic. THings like the fieldname aren't producable in generics,
generics is solely about types.

First of all, I'd like to recommend to you to NOT use protected fields
for members. Use private fields and protected properties. This allows
you to obtain the encapsulation that's often needed.

There's another problem with the design: there's a hard reference to
the control the object works on. This is odd to me, wouldn't it be
better to have a class which can work with any object of a given
control type?

I was half-way rewriting your abstract-nonabstract class tandem with a
property descriptor approach when it became really awkward, because:
where is the control instance passed in? If you want to read a value
from a live control, you have to pass it in. In your code that's not
done anywhere so I don't know how you want to do that.

The context of your code is also not very clear, so a good anwser is
abit hard to give.

FB
--
------------------------------------------------------------------------
Lead developer of LLBLGen Pro, the productive O/R mapper for .NET
LLBLGen Pro website: http://www.llblgen.com
My .NET blog: http://weblogs.asp.net/fbouma
Microsoft MVP (C#)
------------------------------------------------------------------------

[Linda Liu [MSFT]]

Hi Andy,

I suggest you define the BaseSelectionRe quirement class as a generic class.
As for the implementation of the Value property, we don't know either the
field name or its type until at run time. So we need to use reflection to
get/set the field's value at run time.

The following is a sample.

public class BaseClass<mCont rolType,mFieldT ype>
{
protected string mFieldName;
protected UserControl mControl;

public mFieldType Value
{
get
{
PropertyDescrip tor pd =
TypeDescriptor. GetProperties(m Control)[mFieldName];
if (pd != null)
{
return (mFieldType)pd. GetValue(mContr ol);
}
else
{
return default(mFieldT ype);
}

}
set
{
PropertyDescrip tor pd =
TypeDescriptor. GetProperties(m Control)[mFieldName];
if (pd != null)
{
pd.SetValue(mCo ntrol, value);
}
}
}
}
public class DerivedClass:Ba seClass<UserCon trol,Int32>
{
private UserControl uc = new UserControl();
public DerivedClass()
{
this.mFieldName = "Height";
this.mControl = uc;
}

}

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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[Andy Bell]

Linda Liu [MSFT] wrote:

Hi Andy,

I suggest you define the BaseSelectionRe quirement class as a generic
class. As for the implementation of the Value property, we don't know
either the field name or its type until at run time. So we need to
use reflection to get/set the field's value at run time.

That is exactly what I needed. It was the PropertyDescrip tor that was
eluding me. Thank you!

[Ben Voigt [C++ MVP]]

"Andy Bell" <an************ @newsgroup.nosp amwrote in message
news:OE******** ******@TK2MSFTN GP03.phx.gbl...

Linda Liu [MSFT] wrote:

>Hi Andy,

I suggest you define the BaseSelectionRe quirement class as a generic
class. As for the implementation of the Value property, we don't know
either the field name or its type until at run time. So we need to
use reflection to get/set the field's value at run time.

That is exactly what I needed. It was the PropertyDescrip tor that was
eluding me. Thank you!

You should get a big performance improvement by defining "open" delegate
types:

mFieldType PropertyGetter< mControlType>(m ControlType that);
void PropertySetter< mControlType>(m ControlType that, mFieldType value);

and instantiating them using Delegate.Create Delegate the results of
PropertyInfo.Ge tGetMethod and PropertyInfo.Ge tSetMethod, respectively, the
first time they are needed. Reflection is slow, if you can manage to avoid
using Invoke then you'll only pay the reflection cost the first time, when
you set up the delegates.

Highly recommended to define methods like:

class MissingAccessor s<T{
static T PropertyNotGett able(object that) { throw new
NotSupportedExc eption(); }
static void PropertyNotSett able(object that, T value) { throw new
NotSupportedExc eption(); }
}

and use these to fill in your delegate members if reflection fails.

[Linda Liu [MSFT]]

Hi Ben,

Thank you very much for your sharing!

I'm interested in your solution, but I couldn't understand it fully.

If possible, could you please give us a complete sample code?

Thank you for your time!

Sincerely,
Linda Liu
Microsoft Online Community Support

[Ben Voigt [C++ MVP]]

"Linda Liu [MSFT]" <v-****@online.mic rosoft.comwrote in message
news:MU******** ******@TK2MSFTN GHUB02.phx.gbl. ..

Hi Ben,

Thank you very much for your sharing!

I'm interested in your solution, but I couldn't understand it fully.

If possible, could you please give us a complete sample code?

Thank you for your time!

Something like this (not compile tested, but at least 99% there):

internal class MissingAccessor s<T{
static T PropertyNotGett able(object that) { throw new
NotSupportedExc eption(); }
static void PropertyNotSett able(object that, T value) { throw new
NotSupportedExc eption(); }
}

public class ArbitraryProper tyProxy<mContro lType, mFieldType>
{
public delegate mFieldType PropertyGetter( mControlType that);
public delegate void PropertySetter( mControlType that, mFieldType
value);

public readonly PropertyGetter getFrom;
public readonly PropertySetter setOnto;

public ArbitraryProper tyProxy(Propert yInfo pi)
{
if (pi.PropertyTyp e != mFieldType) throw new ArgumentExcepti on();
try {
getFrom =
(PropertyGetter )Delegate.Creat eDelegate(typeo f(PropertyGette r),
pi.GetGetMethod ());
}
catch {
getFrom = MissingAccessor s<mFieldType>.P ropertyNotGetta ble;
}
try {
setOnto =
(PropertySetter )Delegate.Creat eDelegate(typeo f(PropertySette r),
pi.GetSetMethod ());
}
catch {
getFrom = MissingAccessor s<mFieldType>.P ropertyNotSetta ble;
}
}
}

public class PropertyLateBin der<mControlTyp e, mFieldType>
{
private readonly static Dictionary<stri ng,
ArbitraryProper tyProxy<mContro lType, mFieldType>cach edProps = new
Dictionary<stri ng, ArbitraryProper tyProxy<mContro lType, mFieldType>>();

public static ArbitraryProper tyProxy<mContro lType, mFieldType>
BindProperty(st ring propName)
{
ArbitraryProper tyProxy<mContro lType, mFieldTypeproxy ;
if (!cachedProps.T ryGetValue(prop Name, out proxy)) {
try {
PropertyInfo pi =
typeof(mControl Type).GetProper ty(propName);
if (pi != null)
proxy = new ArbitraryProper tyProxy<mContro lType,
mFieldType>(pi) ;
}
catch {}
cachedProps[propName] = proxy;
}
return proxy;
}

public static mFieldType Get(mControlTyp e instance, string propName) {
ArbitraryProper tyProxy<mContro lType, mFieldTypeproxy =
BindProperty(pr opName);
if (proxy == null) throw new ArgumentExcepti on("propName") ;
return proxy.getFrom(i nstance);
}

public static void Set(mControlTyp e instance, string propName,
mFieldType newValue) {
ArbitraryProper tyProxy<mContro lType, mFieldTypeproxy =
BindProperty(pr opName);
if (proxy == null) throw new ArgumentExcepti on("propName") ;
proxy.setOnto(i nstance, value);
}
}

void ChangeColor(For m f, Color c, string whichColor) {
foreach (Control c in f.Controls)
PropertyLateBin der<Control, Color>.Set(c, whichColor, c);
}

ChangeColor(thi s, Color.Black, "BackColor" );
ChangeColor(thi s, Color.Red, "ForeColor" );

Imagine that whichColor could be a dropdown list or something like that, so
it's not feasible to do early binding.

[Linda Liu [MSFT]]

Hello Ben,

Thank you very much for your great code!

It will definitly improve the performance. And I think it benefit all of us.

Thank you again for your support of our MSDN managed Newsgroup!

Sincerely,
Linda Liu
Microsoft Online Community Support