I have read a bit about Design-Time Attributes for Components. There I found an attribute called CategoryAttribute. On that page it says that
The CategoryAttribute class defines the following common categories:
And then lists up a number of common categories. One of them are for example Appearance. I thought, brilliant! Then I can use [Category.Appearance] instead of [Category("Appearance")]! But apparently I couldn't? Tried to write it, but Intellisense wouldn't pick it up and it wouldn't compile. Am I missing something here? Was it maybe not this those properties were for? If not, what are they for? If they are, how do I use them?
And yes, I do have the correct using to have access to the CategoryAttribute, cause [Category("Whatever")] do work. I'm just wondering how I use those defined common categories.
As you can see on MSDN it's only a getter property, not a setter.
public static CategoryAttribute Appearance { get; }
In fact, here's what the code looks like using Reflector:
public static CategoryAttribute Appearance
{
get
{
if (appearance == null)
{
appearance = new CategoryAttribute("Appearance");
}
return appearance;
}
}
So it doesn't do a heck of a lot.
The only use I can see for it, is something like this:
foreach (CategoryAttribute attrib in prop.GetCustomAttributes(typeof(CategoryAttribute), false))
{
bool result = attrib.Equals(CategoryAttribute.Appearance);
}
Basically, when using reflection to look at the class, you can easily check which category this belongs to without having to do a String comparison. But you can't use it in the manner you're trying to unfortunately.
The static property is accessed via CategoryAttribute.Appearance. But the attribute system does not allow you to invoke code in an attribute declaration and I guess that is why it wont compile for you. You will probably have to settle for [Category("Appearance")].
Starting from C# 6 there is finally a better alternative: [Category(nameof(CategoryAttribute.Appearance))].
Related
Is it possible to add attributes at runtime or to change the value of an attribute at runtime?
This really depends on what exactly you're trying to accomplish.
The System.ComponentModel.TypeDescriptor stuff can be used to add attributes to types, properties and object instances, and it has the limitation that you have to use it to retrieve those properties as well. If you're writing the code that consumes those attributes, and you can live within those limitations, then I'd definitely suggest it.
As far as I know, the PropertyGrid control and the visual studio design surface are the only things in the BCL that consume the TypeDescriptor stuff. In fact, that's how they do about half the things they really need to do.
Attributes are static metadata. Assemblies, modules, types, members, parameters, and return values aren't first-class objects in C# (e.g., the System.Type class is merely a reflected representation of a type). You can get an instance of an attribute for a type and change the properties if they're writable but that won't affect the attribute as it is applied to the type.
You can't. One workaround might be to generate a derived class at runtime and adding the attribute, although this is probably bit of an overkill.
Well, just to be different, I found an article that references using Reflection.Emit to do so.
Here's the link: http://www.codeproject.com/KB/cs/dotnetattributes.aspx , you will also want to look into some of the comments at the bottom of the article, because possible approaches are discussed.
No, it's not.
Attributes are meta-data and stored in binary-form in the compiled assembly (that's also why you can only use simple types in them).
I don't believe so. Even if I'm wrong, the best you can hope for is adding them to an entire Type, never an instance of a Type.
If you need something to be able to added dynamically, c# attributes aren't the way. Look into storing the data in xml. I recently did a project that i started w/ attributes, but eventually moved to serialization w/ xml.
Why do you need to? Attributes give extra information for reflection, but if you externally know which properties you want you don't need them.
You could store meta data externally relatively easily in a database or resource file.
Like mentionned in a comment below by Deczaloth, I think that metadata is fixed at compile time. I achieve it by creating a dynamic object where I override GetType() or use GetCustomType() and writing my own type. Using this then you could...
I tried very hard with System.ComponentModel.TypeDescriptor without success. That does not means it can't work but I would like to see code for that.
In counter part, I wanted to change some Attribute values.
I did 2 functions which work fine for that purpose.
// ************************************************************************
public static void SetObjectPropertyDescription(this Type typeOfObject, string propertyName, string description)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(DescriptionAttribute)] as DescriptionAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("description", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, description);
}
}
}
// ************************************************************************
public static void SetPropertyAttributReadOnly(this Type typeOfObject, string propertyName, bool isReadOnly)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(ReadOnlyAttribute)] as ReadOnlyAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("isReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, isReadOnly);
}
}
}
When faced with this situation, yet another solution might be questioning you code design and search for a more object-oriented way. For me, struggling with unpleasant reflection work arounds is the last resort. And my first reaction to this situation would be re-designing the code. Think of the following code, which tries to solve the problem that you have to add an attribute to a third-party class you are using.
class Employee {} // This one is third-party.
And you have code like
var specialEmployee = new Employee();
// Here you need an employee with a special behaviour and want to add an attribute to the employee but you cannot.
The solution might be extracting a class inheriting from the Employee class and decorating it with your attribute:
[SpecialAttribute]
class SpecialEmployee : Employee
{
}
When you create an instance of this new class
var specialEmployee = new SpecialEmployee();
you can distinguish this specialEmployee object from other employee objects. If appropriate, you may want to make this SpecialEmployee a private nested class.
Is it possible to add attributes at runtime or to change the value of an attribute at runtime?
This really depends on what exactly you're trying to accomplish.
The System.ComponentModel.TypeDescriptor stuff can be used to add attributes to types, properties and object instances, and it has the limitation that you have to use it to retrieve those properties as well. If you're writing the code that consumes those attributes, and you can live within those limitations, then I'd definitely suggest it.
As far as I know, the PropertyGrid control and the visual studio design surface are the only things in the BCL that consume the TypeDescriptor stuff. In fact, that's how they do about half the things they really need to do.
Attributes are static metadata. Assemblies, modules, types, members, parameters, and return values aren't first-class objects in C# (e.g., the System.Type class is merely a reflected representation of a type). You can get an instance of an attribute for a type and change the properties if they're writable but that won't affect the attribute as it is applied to the type.
You can't. One workaround might be to generate a derived class at runtime and adding the attribute, although this is probably bit of an overkill.
Well, just to be different, I found an article that references using Reflection.Emit to do so.
Here's the link: http://www.codeproject.com/KB/cs/dotnetattributes.aspx , you will also want to look into some of the comments at the bottom of the article, because possible approaches are discussed.
No, it's not.
Attributes are meta-data and stored in binary-form in the compiled assembly (that's also why you can only use simple types in them).
I don't believe so. Even if I'm wrong, the best you can hope for is adding them to an entire Type, never an instance of a Type.
If you need something to be able to added dynamically, c# attributes aren't the way. Look into storing the data in xml. I recently did a project that i started w/ attributes, but eventually moved to serialization w/ xml.
Why do you need to? Attributes give extra information for reflection, but if you externally know which properties you want you don't need them.
You could store meta data externally relatively easily in a database or resource file.
Like mentionned in a comment below by Deczaloth, I think that metadata is fixed at compile time. I achieve it by creating a dynamic object where I override GetType() or use GetCustomType() and writing my own type. Using this then you could...
I tried very hard with System.ComponentModel.TypeDescriptor without success. That does not means it can't work but I would like to see code for that.
In counter part, I wanted to change some Attribute values.
I did 2 functions which work fine for that purpose.
// ************************************************************************
public static void SetObjectPropertyDescription(this Type typeOfObject, string propertyName, string description)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(DescriptionAttribute)] as DescriptionAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("description", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, description);
}
}
}
// ************************************************************************
public static void SetPropertyAttributReadOnly(this Type typeOfObject, string propertyName, bool isReadOnly)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(ReadOnlyAttribute)] as ReadOnlyAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("isReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, isReadOnly);
}
}
}
When faced with this situation, yet another solution might be questioning you code design and search for a more object-oriented way. For me, struggling with unpleasant reflection work arounds is the last resort. And my first reaction to this situation would be re-designing the code. Think of the following code, which tries to solve the problem that you have to add an attribute to a third-party class you are using.
class Employee {} // This one is third-party.
And you have code like
var specialEmployee = new Employee();
// Here you need an employee with a special behaviour and want to add an attribute to the employee but you cannot.
The solution might be extracting a class inheriting from the Employee class and decorating it with your attribute:
[SpecialAttribute]
class SpecialEmployee : Employee
{
}
When you create an instance of this new class
var specialEmployee = new SpecialEmployee();
you can distinguish this specialEmployee object from other employee objects. If appropriate, you may want to make this SpecialEmployee a private nested class.
Alright, so after a few hours of me playing around to no avail, I built a model:
[AttributeUsage(AttributeTargets.All)]
public class PublicAttribute : System.Attribute
{
public enum Access { Public, Private }
public PublicAttribute(string Name, Access acs)
{
}
public PublicAttribute(string Name, Access acs, Action get, Action set)
{
}
}
So that if somebody were to do something like this:
[Public("PublicProperty", PublicAttribute.Access.Public)]
private string PrivateProperty = "hello";
or
[Public("PublicProperty", PublicAttribute.Access.Public, ()=>{return PrivateProperty;}, ()=>{PrivateProperty = value})]
private string PrivateProperty = "hello";
and then if somebody was trying to access PrivateProperty, they could just go:
ContainingClass.PublicProperty = //ect
"PublicProperty". and that is because of the attribute, and it would use those get/set accessors.
What I'd like to know:
Is this even possible?
Is there something that already does this?
If its possible, (even if there is something else) How do i do this?
Basically no to all 3, as C# is a strongly typed language. Even with duck typing what you're trying to achieve doesn't fit the language.
The attributes you've written allow you to interrogate the properties that have those attributes in the class, but you still need to use Reflection to discover which properties of the attribute class are set. The syntax you want to use is checked at compile-time.
No, this is not possible using attributes. Properties are part of the class metadata emitted by the C# compiler, and the C# compiler does not consider custom attributes.
You may be able to do this by using a post-processor such as PostSharp, which can rewrite your assembly after the fact, and can be instructed to consider custom attributes. However, you still wouldn't be able to include a delegate in the attribute: the set of types that can be stored in attribute state is extremely limited.
Microsoft made the WebMethodAttribute in a way reminiscent of what you're trying to describe making it represent more permission than C# public, effectively making a method available outside the application domain to the entire Internet (a very global scope). You might read it to get real implementation insight and ideas.
But you're hitting it very simply. You'll have to program some infrastructure to make it work. It's not automatic and you don't have access to Microsoft's source code for all the details.
I have a class I am working with:
public sealed class WorkItemType
It's ToString is weak (Just shows Microsoft.TeamFoundation.WorkItemTracking.Client.WorkItemType).
Is there any way to override this to show the name of the WorkItemType?
Normally I would just aggregate the value in a new class, but I am using this for bindings in WPF (I want to have a list of WorkItemTypes in a combo box and assign the selected value to a bound WorkItemType variable.)
I think I am out of luck here, but I thought I would ask.
A fairly neat way to do it might be to add an extenesion method to the WorkItemType object. Something like this:
public static class ToStringExtension
{
public static string MyToString(this WorkItemType w)
{
return "Some Stuff"
}
}
Then you could call something like
WorkItemType w = new WorkItemType;
Debug.WriteLine(w.MyToString();)
Do you need to override ToString? If you are in control of the code where the object is displayed, you can always provide a FormatWorkItemType method, or something to that effect.
WPF provides a few different built-in ways to do this right in the UI. Two I'd recommend:
You can use ComboBox's
DisplayMemberPath to display a single
property value but still select from
the WorkItemType objects.
If you want to display a composite of
a few properties you can change the
ComboBox's ItemTemplate to make it
look pretty much however you want -
formatting text, adding borders,
colors, etc. You can even set up the
DataTemplate to automatically be
applied to any WorkItemType object
that gets bound anywhere in your UI
(same basic effect from UI
perspective as changing ToString) by
putting it into Resources and giving
it only a DataType with no x:Key.
You're out of luck :-(
You could write your own class that wraps the WorkItemType and delegate down to it (a proxy) expect for the ToString:
class MyWorkItemType
{
private WorItemType _outer;
public MyWorkItemType(WorkItemType outer)
{
_outer=outer;
}
public void DoAction()
{
_outer.DoAction();
}
// etc
public override string ToString()
{
return "my value"
}
}
I don't have any C# knowledge, but can't you wrap your extended class inside another class? Proxy all method calls to the extended class, except toString(), Also very hackish, but I thought I'ld bring it up anyway.
Doing some sorta magic with reflection is probably your only hope. I know you can instantiate private constructors with it, so maybe you can override a sealed class... Note, this should be your last resort if there is seriously no other way. Using reflection is a very hackish/improper way of doing it.
In addition to the other WPF-specific answer you could use an IValueConverter in the binding to format / display the WorkItemType however you want. This has an advantage of being reusable (if you want to display the object in some other control, for instance.)
There are many examples of using converters here. This other question should be pretty similar to the ComboBox usage mentioned here. The answers note that you can either make the converter work on the entire collection of objects, or work on one item at a time. The latter might be the more reusable approach.
Is there any way to dynamically set the property name of an anonymous type?
Normally we'd do like this:
var anon = new { name = "Kayes" };
Now I'd like to set the name (or identifier) of the property dynamically, so that this name can come from an XML file or a database.
Thanks guys for your answers. No, my need is definitely not worth all the tedious alternatives. I just wanted to make my code comply with an existing library that was developed by my team lead. But we decided to update the library to support Dictionary types so that it can easily be solved.
Pete, I'm really excited to learn about dynamic types in .NET 4.0
Thanks.
That is not possible because even though the type is anonymous, it is not a dynamic type. It is still a static type, and properties about it must be known at compile time.
You might want to check out the .NET 4.0 "dynamic" keyword for generating true dynamic classes.
Not without a massive amount of reflection, probably moreso than you're interested in leveraging for what you're doing. Perhaps you should look into instead using a Dictionary, with the key value being the property name.
As has been mentioned, you can't change the property name; how would you code against it, for example? However, if this is for data-binding, you can do some tricks to bend the display name of properties at runtime - for example ICustomTypeDescriptor/TypeDescriptionProvider (and a custom PropertyDescriptor).
Lots of work; it would need to really be worth it...
Another option is a custom attribute:
using System;
using System.ComponentModel;
using System.Windows.Forms;
class MyDisplayNameAttribute : DisplayNameAttribute {
public MyDisplayNameAttribute(string value) : base(value) {}
public override string DisplayName {
get {
return #"/// " + base.DisplayNameValue + #" \\\";
}
}
}
class Foo {
[MyDisplayName("blip")]
public string Bar { get; set; }
[STAThread]
static void Main() {
Application.EnableVisualStyles();
using (Form form = new Form {
Controls = {
new PropertyGrid {
Dock = DockStyle.Fill,
SelectedObject = new Foo { Bar = "abc"}}
}
}) {
Application.Run(form);
}
}
}
The compiler must know about the property name, otherwise it can't create the anonymous type for you.
So, no, this isn't possible, unless the actual property name is known at compile-time (through some VS-generated magic with the database or something).
Dynamic properties can be done with compiling code at runtime but this would be overkill for your needs I guess...
Nope. You want do construct a static structure from dynamic information. Won't work, think about it. Use a dictionary for your case.