I have a many objects of the same type (the base type is DisplayTool) in a list. The type is not determined at compile time but could be any type that has been added via a plugin system.
Let's assume I have a property called IsActive I currently need to go through all properties in the list an set the value. Additionally a single value of one object can be set.
Currently I need to do this for every property but I don't know them in advance so I was thinking of some WrapperObject that bundles the attached objects in a list and builds the properties dynamically. The ExpandoObject would be useful in this case but it doesn't bundle the properties into one.
The WinForms Property Inspector is a good example for the behavior I want. If the properties of a property of a all attached objects are the same, the value is shown, if not all values are the same it is not shown.
I have created my own MockObject derived from ExpandoObject that collects the properties and displays them. I only wanted to ask if there is already an implementation from the framework that does the same?
This question already has answers here:
Why does WPF support binding to properties of an object, but not fields?
(2 answers)
Closed 5 years ago.
I was googling for this point where why the local or global variables are not allowed as sources for WPF data binding; only the wrapping property can be bound to.
So the question is where properties are the wrapper over the variables then why these properties are allowed and the variables are not allowed.
I need to know under the hood system.
The mechanisms used for binding (PropertyDescriptor, DependencyProperties etc.) only use properties, that's the reason you can't bind to fields. Properties can provide validation, change notification and more, which is another reason to prefer properties. Fields provide none of these.
Also, using public fields is usually bad practice, so why should they bother implementing binding to fields? It would only promote those bad practices.
Well, it's not completely true. You can use a field for a binding, but first you have to declare it like a resource and after access it via resource key in binding declaration.
When you're going to bind to code binding to a property is done for be able to iject also OnPorpertyChanged notification too.
It is said that the designers of WPF have made it economical or higher performance. Can someone please explain with an example of what happens under the hood that makes the WPF property system more economical?
You are probably referring to the fact that Dependency Properties are "cheaper" than normal CLR properties.
In a few words:
Dependency properties are implemented using sparse data structures that only allocate memory for the property value if it is set on an object. In contrast, a standard CLR property value is stored as a field inside every object of the class in which the property is defined, even if all of these objects have the property set to its default value.
So for example, if we have 100 objects with 100 CLR properties of type int each, then we are using 10000 ints worth of memory even if all of those have the same default value (0).
If the property were a dependency property instead, we would not be using any additional memory at all: WPF does not need to remember the value of any property since it knows that you didn't change it from the default.
Of course the above is rather a simplistic explanation and does not cover all the advantages of dependency properties over CLR properties, but it should explain the "DPs have higher performance" statement adequately.
Most of the 'properties' of a WPF control are in fact not present on the Control itself. Instead of adding dozens of (mostly unused) properties to the (base-)classes, they elected to add a "property bag" instead, a Dictionary holding only the properties that are actually set.
As a bonus it allows for ambient and injected properties.
The WPF dependency property system stores actual property values in optimized data structures behind the scenes.
This has several advantages over storing property values as fields:
The dependency property system by NOT storing default values for properties for each object instance it can save allot of memory (basically if a property has a default value for a target object, space is not allocated for the value. This is opposed to the having properties with backing fields where the values are always stored, and the memory is always reserved for the object).
The dependency property system can have optimized event mechanism that avoids storing handler references on a per object basis (like using backing field based events), this means more space savings can be made.
There is of course a small overhead for such a system. Property access is not as lightweight as using a normal property, but the WPF team has decided the small overhead more than makes it up due to lower memory usage.
In addition to the other answers:
Dependency properties in WPF support property value inheritance. With normal CLR properties it is much harder to push values down to any "child" objects without modifying the child object. This can obviously be done with attached methods and a static mapping, but would probably not be a very generic solution. While there is some overhead with inherited properties, they are fairly efficient at passing down values.
It is said here:
http://msdn.microsoft.com/en-us/library/ms753358.aspx
Sometimes, the typical technique of backing your property with a private field is adequate.
However, you should implement your property as a dependency property whenever you want your property to support one or more of the following WPF capabilities ...
If I look at all the capabilities they are very obvious needs so I can't really see when I should NOT create dependency property.
So if I systematically create a dependency property instead of just creating a private field what would be the drawbacks?
there are in fact some drawbacks:
Creating a Dependency property is more verbose than a private field backed property.
When you use a Dependency property, your data is stored inside a Dictionary. It has some performance implications: The access lookup is done at runtime, the conversion too, and boxing/unboxing will occu because every dependency property values are stored as objects. Furthermore, all these capabilities come for a price (performance price) which you will pay with each access (read or write). Event will be raised (like PropertyChanged) validation will be performed according to your DP configuration...
Your class must inherit from DependencyObject
This is a price I am willing to pay when a need these capabilities. But they aren't needed in a lot of cases=> Use a DP when appropriate, a classic private field elsewhere.
There are case when you will regret that a property is not bindable: When it happens, replace the field with a DP implementation under the hood.
It depends on what you need.
For example if you need binding you MUST create a DependencyProperty.
If you have to work with data that doesn't "communicate" with the graphical interface using a DependencyProperty is worthless and less performant because you don't need binding, animation, etc.
The purpose of dependency properties is to provide a way to compute the value of a property based on the value of other inputs. These other inputs might include system properties such as themes and user preference, just-in-time property determination mechanisms such as data binding and animations/storyboards, multiple-use templates such as resources and styles, or values known through parent-child relationships with other elements in the element tree. In addition, a dependency property can be implemented to provide self-contained validation, default values, callbacks that monitor changes to other properties, and a system that can coerce property values based on potentially runtime information. Derived classes can also change some specific characteristics of an existing property by overriding dependency property metadata, rather than overriding the actual implementation of existing properties or creating new properties.
DP should be used only when you are making / modifying new WPF control. Overhead is too much to be used as property on VM (and makes VM dependant on WPF). For normal usage you should implement INotifyPropertyChanged
Quick question: When do you decide to use properties (in C#) and when do you decide to use methods?
We are busy having this debate and have found some areas where it is debatable whether we should use a property or a method. One example is this:
public void SetLabel(string text)
{
Label.Text = text;
}
In the example, Label is a control on a ASPX page. Is there a principle that can govern the decision (in this case) whether to make this a method or a property.
I'll accept the answer that is most general and comprehensive, but that also touches on the example that I have given.
From the Choosing Between Properties and Methods section of Design Guidelines for Developing Class Libraries:
In general, methods represent actions and properties represent data. Properties are meant to be used like fields, meaning that properties should not be computationally complex or produce side effects. When it does not violate the following guidelines, consider using a property, rather than a method, because less experienced developers find properties easier to use.
Yes, if all you're doing is getting and setting, use a property.
If you're doing something complex that may affect several data members, a method is more appropriate. Or if your getter takes parameters or your setter takes more than a value parameter.
In the middle is a grey area where the line can be a little blurred. There is no hard and fast rule and different people will sometimes disagree whether something should be a property or a method. The important thing is just to be (relatively) consistent with how you do it (or how your team does it).
They are largely interchangeable but a property signals to the user that the implementation is relatively "simple". Oh and the syntax is a little cleaner.
Generally speaking, my philosophy is that if you start writing a method name that begins with get or set and takes zero or one parameter (respectively) then it's a prime candidate for a property.
Searching through MSDN, I found a reference on Properties vs Methods that provides some great guidelines for creating methods:
The operation is a conversion, such as Object.ToString.
The operation is expensive enough that you want to communicate to the
user that they should consider caching
the result.
Obtaining a property value using the get accessor would have an observable
side effect.
Calling the member twice in succession produces different results.
The order of execution is important. Note that a type's properties should
be able to be set and retrieved in any
order.
The member is static but returns a value that can be changed.
The member returns an array. Properties that return arrays can be
very misleading. Usually it is
necessary to return a copy of the
internal array so that the user cannot
change internal state. This, coupled
with the fact that a user can easily
assume it is an indexed property,
leads to inefficient code.
If you're setting an actual property of your object then you use a property.
If you're performing a task / functionality then you use a method.
In your example, it is a definite property being set.
If however, your functionality was to AppendToLabel then you would use a method.
Properties are a way to inject or retrieve data from an object. They create an abstraction over variables or data within a class. They are analogous to getters and setters in Java.
Methods encapsulate an operation.
In general I use properties to expose single bits of data, or small calculations on a class, like sales tax. Which is derived from the number of items and their cost in a shopping cart.
I use methods when I create an operation, like retrieving data from the database. Any operation that has moving parts, is a candidate for a method.
In your code example I would wrap it in a property if I need to access it outside it's containing class:
public Label Title
{
get{ return titleLabel;}
set{ titleLabel = value;}
}
Setting the text:
Title.Text = "Properties vs Methods";
If I was only setting the Text property of the Label this is how I would do it:
public string Title
{
get{ return titleLabel.Text;}
set{ titleLabel.Text = value;}
}
Setting the text:
Title = "Properties vs Methods";
Symantically properties are attributes of your objects.
Methods are behaviors of your object.
Label is an attribute and it makes more sense to make it a property.
In terms of Object Oriented Programming you should have a clear understanding of what is part of behavior and what is merely an attribute.
Car { Color, Model, Brand }
A car has Color, Model and Brand attributes therefore it does not make sense to have a method SetColor or SetModel because symantically we do not ask Car to set its own color.
So if you map the property/method case to the real life object or look at it from symantic view point, your confusion will really go away.
You need only look at the very name... "Property". What does it mean? The dictionary defines it in many ways, but in this case "an essential or distinctive attribute or quality of a thing" fits best.
Think about the purpose of the action. Are you, in fact, altering or retrieving "an essential or distinctive attribute"? In your example, you are using a function to set a property of a textbox. That seems kind of silly, does it not?
Properties really are functions. They all compile down to getXXX() and setXXX(). It just hides them in syntactic sugar, but it's sugar that provides a semantic meaning to the process.
Think about properties like attributes. A car has many attributes. Color, MPG, Model, etc.. Not all properties are setable, some are calculatable.
Meanwhile, a Method is an action. GetColor should be a property. GetFile() should be a function. Another rule of thumb is, if it doesn't change the state of the object, then it should be a function. For example, CalculatePiToNthDigit(n) should be a function, because it's not actually changing the state of the Math object it's attached to.
This is maybe rambling a bit, but it really boils down to deciding what your objects are, and what they represent. If you can't figure out if it should be a property or function, maybe it doesn't matter which.
Properties should only be simple set and get one liners. Anything more and it should really be moved to a method. Complex code should always be in methods.
I only use properties for variable access, i.e. getting and setting individual variables, or getting and setting data in controls. As soon as any kind of data manipulation is needed/performed, I use methods.
As a matter of design Properties represent Data or Attributes of class object, While methods are actions or behaviors of class object.
In .Net, world there are other implications of using Properties:
Properties are used in Databinding, while get_ / set_ methods are not.
XML serialization user properties as natural mechanism of serilization.
Properties are accessed by PropertyGrid control and intern ICustomTypeDescriptor, which can be used effectively if you are writing a custom library.
Properties are controlled by Attributes, one can use it wisely to design Aspect Oriented softwares.
Misconceptions (IMHO) about Properties' usage:
Used to expose small calculations: ControlDesigner.SelectionRules's get block runs into 72 lines!!
Used to expose internal Data structures: Even if a property does not map to an internal data member, one can use it as property, if its an attribute of your class. Viceversa, even if its an attribute of your class properties are not advisable, to return array like data members (instead methods are used to return deep copy of members.)
In the example here it could have been written, with more business meaning as:
public String Title
{
set { Label.Text = text; }
}
Also big plus for Properties is that value of property can be seen in Visual Studio during debugging.
I prefer to use properties for add/set methods with 1 parameter. If parameters are more, use methods.
Properties are really nice because they are accessible in the visual designer of visual studio, provided they have access.
They use be used were you are merely setting and getting and perhaps some validation that does not access a significant amount of code. Be careful because creating complex objects during validation is not simple.
Anything else methods are the preferred way.
It's not just about semantics. Using properties inappropriate start having weirdness occur in the visual studio visual designer.
For instance I was getting a configuration value within a property of a class. The configuration class actually opens a file and runs an sql query to get the value of that configuration. This caused problems in my application where the configuration file would get opened and locked by visual studio itself rather than my application because was not only reading but writing the configuration value (via the setter method). To fix this I just had to change it to a method.
Here is a good set of guidelines for when to use properties vs methods from Bill Wagner
Use a Property when all these are true:
The getters should be simple and thus unlikely to throw exceptions. Note that this implies no network (or database) access. Either might fail, and therefore would throw an exception.
They should not have dependencies on each other. Note that this would include setting one property and having it affect another. (For example, setting the FirstName property would affect a read-only FullName property that composed the first name + last name properties implies such a dependency )
They should be settable in any order
The getter does not have an observable side effect Note this guideline doesn't preclude some forms of lazy evaluation in a property.
The method must always return immediately. (Note that this precludes a property that makes a database access call, web service call, or other similar operation).
Use a method if the member returns an array.
Repeated calls to the getter (without intervening code) should return the same value.
Repeated calls to the setter (with the same value) should yield no difference from a single call.
The get should not return a reference to internal data structures (See item 23). A method could return a deep copy, and could avoid this issue.
*Taken from my answer to a duplicate question.
This is simple.
1: use property when you want your data should be validated before storing in field. So in this way property provides encapsulation for your fields. Because if you leave your fields public end user may assign any value which may or may not be valid as per your business requirement like age should be greater than 18. So before value is store corresponding field we need to check its validity. In this way properties represent data.
2: Use method when you want perform some action like you are supplying some data as parameter and your method is doing some processing on the basis of supplied values and returning processed value as output. Or you want to change value of some field by this calculation. "In this way method represents action".
I come from java an i used get.. set.. method for a while.
When i write code, i don't ask to my self: "accessing this data is simple or require a heavy process?" because things can change (today retrive this property is simple, tomonrow can require some or heavy process).
Today i have a method SetAge(int age) tomonrow i will have also method SetAge(date birthdate) that calculate the age using the birthdate.
I was very disappointed that the compiler transform property in get and set but don't consider my Get... and Set.. methods as the same.