In C# itself, is there something like "attached property" used in WPF?
The short answer is no. The slightly longer answer is that this is a bit of an unfortunate story. We designed "extension properties" for C# 4 and got as far as implementing (but not testing) them when we realized, oh, wait, the thing we designed is not really compatible with WPF-style properties. Rather than redesign and reimplement the feature we ended up cutting it.
The even longer version is here:
http://blogs.msdn.com/b/ericlippert/archive/2009/10/05/why-no-extension-properties.aspx
AttachedProperties are part of the .NET Framework, not part of the C# language specification, and specifically part of the System.Activities.Presentation.Model namespace, which is WPF specific.
In WPF, an attached property allows you to do something like:
<TextBlock Grid.Row="2" Text="I know nothing about grids!" />
This would be like having a class in C# defined as:
public class TextBlock
{
public string Text { get; set; }
}
And being able to do this:
var tb = new TextBlock();
tb.Grid.Row = 2; // this line would not compile
In order to make this work, you'd need to pass a Grid object into your TextBlock class:
public class TextBlock
{
public string Text { get; set; }
public Grid Grid { get; set; }
public TextBlock(Grid grid)
{
Grid = grid;
}
}
But I don't think there's anything directly equivalent to the way attached properties work in WPF. You'd need to build it by hand.
What are you trying to accomplish?
You can use the ConditionalWeakTable<TKey, TValue> class to attach arbitrary state to an instance. You can combine it with extension methods to create a form of extension properties, but unfortunately without using the nice property syntax in C#.
I think you're thinking of getters and setters.
They are created like this:
public class Person
{
//default constructor
public Person()
{
}
private string _Name;
public string Name
{
//set the person name
set { this._Name = value; }
//get the person name
get { return this._Name; }
}
}
More on how they work here:
http://msdn.microsoft.com/en-us/library/aa287786(v=vs.71).aspx
Related
We're using durandal to convert C# models to Knockout viewmodels for rendering. I'm wondering if there's a way I can set up a C# model with properties that have a set method or something so that the bindings and dependencies are already present when I get the Knockout viewmodel.
I'd like a scenario like this to happen.
public class MyObject{
public string FirstName{get; set;}
public string LastName {get; set}
private string fullName{get; set;}
public FullName{
get{return fullName;}
set{fullName = FirstName +" "+LastName}
}
I've used the prime example Knockout uses when explaining computed observables. I understand how to achieve this in purely JS viewmodel. However, I'd like to set up my C# model similar to what I have above (I don't think this actually works) in order to get back a computed that already has it's dependencies.
I'm not entirely sure it's possible, but it sure would be nice.
Max Brodin mentions KnockoutMVC which is a huge antipattern and you lose almost all benefits with Knockout since it creates server callbacks for almost everything you do.
A better option is to use for example DuoCode or Open source alternative WootzJs
These tools will compile C# code to Javascript, I have only tested DuoCode but it was farily easy to create a Knockout binding for it. After that its easy to create ViewModels like
using Knockout;
namespace ViewModels
{
public class FooViewModel
{
private readonly Observable<string> bar;
private readonly Observable<string> computed;
public FooViewModel()
{
bar = Global.Observable("HelloWorld"); //Translates to ko.observable("HelloWorld") on client
computed = Global.Computed(() => bar.Get() + "COMPUTED");
}
public Observable<string> Bar { get { return bar; } }
public Observable<string> Computed { get { return computed; } }
}
}
I have also created bindings for ko.mapping like
Mapping.Map(new { bar = "DataFromserver" }, null, this); which translates to
ko.mapping.fromJS({ bar = "DataFromserver" }, null, this); on client
The idea is good, but it's not going to work in the real world projects. Your C# code need to be translated to javascript and it could work for simple cases like yours. But if you change your computed property to more something complicated you will have problems.
There is a knockoutmvc project that provides such functionality. They have hello world sample which looks like something you can use as a start. You just need to mark your property with attributes
public class HelloWorldModel
{
public string FirstName { get; set; }
public string LastName { get; set; }
[Computed]
[ScriptIgnore]
[JsonIgnore]
public string FullName
{
get { return FirstName + " " + LastName; }
}
}
And in razor view you should call:
#{
var ko = Html.CreateKnockoutContext();
}
#ko.Apply(Model)
You seem to want auto-translation of computed properties. That's a big ask for a small convenience. I think I could do it for trivial expressions, but how would you map .NET library methods like string.Format to javascript? You would need a client side library replicating the semantics. Big job, not worth the effort.
In C#, can you use a property without a field?
Edit for clarification:
private string _name;
public string Name
{
get { return _name; }
set { _name value; }
}
It seem's like they are always paired, is there a circumstance where we don't use the field at all?
All properties must have a field, assuming they are simple properties to store a value (*). However, the language (as of version 3.0) offers a way to declare the field implicitly. For example:
public int Value { get; set; }
That would declare a property named Value with an implicit field backing it and the getter and setter both public. You can include an accessibility keyword on either the getter or setter to restrict access to the property. For example:
public int Value { get; private set; }
In this case, only the owning type may call the setter, but any class can call the getter.
The next version of C# will have additional features for dealing with these "automatic properties", allowing you to provide a concise initialization syntax for them. For now, you have to initialize them in a constructor.
EDIT: based on your edited question, it seems worthwhile to address this specific question: "is there a circumstance where we don't use the field at all?"
The answer to that is, it's not common for no field to be involved at all. But it is possible, and it's not uncommon for a property to not use a field as storage for the property. For example, imagine a Rectangle object with an Area property:
class Rectangle
{
public double Width { get; private set; }
public double Height { get; private set; }
public double Area { get { return Width * Height; } }
}
Obviously there are fields involved (two of them), but there is not a field specifically dedicated to the Area property.
Another example would be where the property delegates. For example, in a WinForms Form subclass, it's common to expose specific control values via a property:
class MyForm : Form
{
public string EditText
{
get { return textBox1.Text; }
set { textBox1.Text = value; }
}
}
Again, the textBox1 field is being used here. But it actually represents something other than the property itself. The property is using a member of the object that field references.
I hope that clarifies the relationship between fields and properties adequately for you. Please feel free to ask for further clarifications if needed.
(*) Note that the only real rule for properties is that they have at least one of the getter or setter, and those methods can do whatever you want. I assume we are talking about simple value-based properties here.
A property is not required to have a field
public string Version
{
get
{
return "1.3.Awesome";
}
}
If you're asking what I think you are, the answer is yes, you just put get; set; inside the property declaration. C# encapsulates a variable for you.
EDIT: example
//no need for field declaration
public string Name
{
get;
set;
}
New to C#, and I understand that encapsulation is just a way of "protecting data". But I am still unclear. I thought that the point of get and set accessors were to add tests within those methods to check to see if parameters meet certain criteria, before allowing an external function to get and set anything, like this:
private string myName;
public string MyName;// this is a property, speical to c#, which sets the backing field.
private string myName = "mary";// the backing field.
public string MyName // this is a property, which sets/gets the backing field.
{
get
{
return myName;
}
set
{
if (value != "Silly Woman"){
myName = value;
}
}
}
But I've been seeing code in c# which just looks like this:
public string MyName { get; set; }
Why would you just have a get and set with nothing in there, - isn't that the same as just declaring your private backing field public? If you can just get and set it from outside, why wouldn't you just do it directly?
Indeed, creating an auto-property as follows:
public string Name { get; set; }
is identical to building a property backed by a field:
private string _name;
public string Name {
get { return _name; }
set { _name = value; }
}
The point of these properties is not to hide data. As you observed, they don't do this. Instead, these properties can do other stuff instead of just working with a field:
public string Name {
get { return _name; }
set { if (value == null) throw new Exception("GTFO!"); _name = value; }
}
Another thing is, you can make properties virtual:
public virtual string Name { get; set; }
which, if overridden, can provide different results and behaviours in a derived class.
By using public string MyName { get; set; }, you leave an ability to change its logic later without the need to recompile/change other code that uses your property.
For example, if you are making a library and v1 uses a field and v2 uses a property, applications that work with v1 will not work with v2 without recompilation (and, potentially, code changes if they are written in some .NET language that has different syntax for accessing fields).
Another important difference is in serialization scenarios -- a lot of them do not support fields. Also any interface that requires a property can not be implemented without using one, but depending on interface it may not be required to do any additional checks/logic in it.
It makes it easier to add logic later. If you have a class that has a public field that you want to change to a property, you have to recompile everything that uses your class. That's a key point that I didn't understand initially.
If you have a class:
public class MyClass
{
public string MyString;
}
You could access the value like this:
var myClass = new MyClass();
string s = myClass.MyString;
Now change that to a property:
public class MyClass
{
public string MyString { get; set; }
}
How is it accessed? The exact same way:
var myClass = new MyClass();
string s = myClass.MyString;
So no big deal, right? Well, actually....
Properties are actually compiled into getter and setter methods:
get_MyString() and set_MyString(string value)
So the two methods do produce different compiled code. Now if all your code that uses this class is in the same project, is not as big a deal, because it will all be compiled together. But if you have an API library that you've distributed, it can be a much bigger deal to update.
Because it is easier to change the Code if you want to add the checks/tests later on.
Especially if you have many inheritance and many classes in your code it is very hard to change the implementation from a public variable to a public Property.
Moreover you can add to the get and set within the property different attributes, e.g. if you are using reflection. The get and set of the property are internally different methods. If you have just a public variable /field it is not possible to added different properties to the different access ways.
Yeah, but you can easily change it to:
public string MyName { get; private set; }
Plus, properties are used in other scenarios, like DataContracts and Serialization... so, this is a nice feature... (Mostly, syntactic sugar. I think) EDIT: I take that back.. you can apply virtual to it, so it's not the same
I'm trying to do this:
public string LangofUser
{
get
{
return string.IsNullOrEmpty("how to get value?") ? "English" : "how to get value?";
}
set;
}
do I have to do this?
string _LangofUser
public string LangofUser
{
get
{
return string.IsNullOrEmpty(_LangofUser) ? "English" : _LangofUser;
}
set { _LangofUser = value};
}
This mixing of auto-implement and not-auto-implemented properties in C# is not possible. A property must be fully auto-implemented or a normal property.
Note: Even with a fully auto-implemented property there is no way to reference the backing field from C# source in a strongly typed manner. It is possible via reflection but that's depending on implementation details of the compiler.
As others have said, don't try to mix automatic and regular properties. Just write a regular property.
If you want to know what secret names we generate behind the scenes for hidden compiler magic, see
Where to learn about VS debugger 'magic names'
but do not rely on that; it can change at any time at our whim.
If you provide your own implementation of the property, it's not automatic any more. So yes, you need to do create the instance.
Check this question
What's the difference between encapsulating a private member as a property and defining a property without a private member?
If you want to keep the automatic property and still have a default value, why don't you initialize it in your constructor?
public class MyClass
{
public MyClass() { LangOfUser = "English"; }
public string LangOfUser { get; set; }
}
Since C# 6, you can also set a default value for a property as follows:
public class MyClass
{
public string LangOfUser { get; set; } = "English";
}
For C#, I hate writing out the variables and then writing out all the properties. Isn't there a way to select all variables, right click and create all the properties.
Right click on the field declaration, menu Refactor -> Encapsulate field and you go from
int n;
to
int n;
public int N
{
get { return n; }
set { n = value; }
}
Are you looking for a code refactoring tool? If so, check out ReSharper. It provides an easy to to turn simple field-backed properties into auto-properties, and vice versa.
If you simply don't want to write custom field-backed properties, you can use auto-properties, fpor example, like so:
public string MyProperty { get; set; } // generates an auto-property
which is equivalent to:
private string m_MyProperty;
public string MyProperty
{
get { return m_MyProperty; }
set { m_MyProperty = value; }
}
You can even make the accessibilty of the setter and getter difference:
public string MyProperty { get; private set; }
If you do choose to use auto-properties, be aware that you cannot access the underlying field, nor can you supply an implementation for just one portion (just the getter or just the setter). You can, however, make the property virtual.
If you're using C# 3.0 or above (VisualStudio 2008, essentially), you can use auto properties. While this isn't exactly what you're asking for, it should (hopefully) do the trick.
Rather than writing:
private string m_Name;
public string Name
{
get { return m_Name; }
set { m_Name = value; }
}
You can just write:
public string Name { get; set; }
This will give you quick, "dumb" (i.e. no retrieval or assignment logic) properties that can go on your class. If you find you need retrieval and assignment logic later, just come back and do the full property declaration syntax and you won't have to change any of the calling code.
The only real difference is that you'll have to use the property to get the value within your class, as the backing variable is generated and compile time and unavailable to your code.
FYI, simply typing "prop" (no quotes) triggers one of the snippets that comes with VS, and you just tab your way through, by far the quickest option.
Why aren't you doing:
public int SomeProperty { get; set; }
or
public int SomeOtherProperty { get; private set; }
?
from this line:
string mytest;
select the whole line "string mytest;",
then VS menu: Edit > Refactor > Encapsulate field ...
you get this:
public string Mytest { get => mytest; set => mytest = value; }
we can quickly create c# properties in visual studio using prop shortcut
and behalf of visual studio tool we can generate c# properties using a tool called c# property generator..
when class has so many properties in it , when we create a object of that class,
we have to take certain pain to assign properties so this tool will reduce your pain to certain extent this will automatically assign object with properties..
c# property assigner
You probably should be using Auto-Implemented properties in C# for most things. However, if you want 'old-style' properties with explicit backing fields you can create a Visual Studio code snippet to make them easier to write. This blog post has an example of one.