Sigh, another PropertyGrid question. I thought I could get around this until I ran into a problem where I couldn't actually avoid it.
I have a boolean property that sometimes needs to be read-only and sometimes needs to be changeable depending on the object selected from a TreeView.
My question is how can I change the ReadOnlyAttribute of a property dynamically? Obviously, creating a boolean variable and then trying to set it like ReadOnlyAttribute(boolVar) doesn't work and now I'm out of ideas.
The only solution I can think of is creating separate, near-identical classes for items where this property is writable and one for read-only, but this seems a bit unelegant to me.
Help? :)
You can provide dynamic information about the properties of a class to a property grid by implementing ICustomTypeDescriptor.
The property grid will call ICustomTypeDescriptor.GetProperties() and you return a collection of objects derived from PropertyDescriptors. In your implementation you can override the PropertyDescriptor.IsReadOnly property and implement your logic.
This is quite a bit of work in the first place, but it gives you the possibility to dynamically return a property name and description (helpful for localization), dynamically mark properties as read-only, dynamiclly show and hide properties, and do a lot of other usefull things.
What I would do is create a base class with a protected version the property, then create two classes that inherit the base class that have the readonly and the non-readonly bits.
You could try something along these lines to avoid the type conversion involved with multiple classes:
class TestClass
{
private bool isMyPropertyReadOnly;
public bool IsMyPropertyReadOnly
{
get { return isMyPropertyReadOnly; }
set { isMyPropertyReadOnly = value; }
}
private int myVar;
public int MyProperty
{
get { return myVar; }
set
{
if (isMyPropertyReadOnly)
{
throw new System.Exception("The MyProperty property is read-only.");
}
else
{
myVar = value;
}
}
}
}
Related
I have a boolean variable, I want every change to its value to invoke a piece of code.
my current solution is the following:
bool _manualControl;
bool manualControl {
get {
return _manualControl;
}
set {
this._manualControl = value;
GlobalEventManager.ManualControlEvent?.Invoke(value);
}
}
this solution has two problems:
the value of "_manualControl" can be changed internally without invoking my piece of code, I want to prevent that.
I would prefer to avoid using two variables to get the desired behavior.
Is there any way to achieve what I want while avoiding these two specified issues?
You can set the property to be public and have a private backing field that can only be modified from within the class, which you have control of.
Or you could use an Aspect Oriented Programming framework like PostSharp, which would allow you to use an auto property and annotate it with the behaviour you desire. This would remove the need for you to have a backing field.
To me this sounds a bit like you want to solve an architectural problem, aka code smell. Why is it that you fear your field might be set outside your setter? Is it a particularly large class that a lot of people are chaning without really knowing what it is doing?
Why even have code in the setter? Like you could just redesign your code to have a method do what your setter code does and introduce that into your code flow / process.
And have a Unit Test validate your desired behavior.
If you want to:
ensure that the setter code always executes when a new value is assigned (inside and outside of the class)
avoid having two members in the class, that represent a single value
Then this can be approached by wrapping the value within a struct like one below:
struct Intercepted<T>
{
private readonly Action<T> _onChange;
private T _value;
public Intercepted(Action<T> onChange, T initialValue = default(T))
{
_onChange = onChange;
_value = initialValue;
}
public T Value
{
get
{
return _value;
}
set
{
_value = value;
_onChange?.Invoke(value);
}
}
}
In the class, ManualControl can now be represented with a single member, of type Intercepted<bool>:
public Intercepted<bool> ManualControl { get; } = new ManualControl(
onChange: newValue => {
GlobalEventManager.ManualControlEvent?.Invoke(newValue);
}
);
The value can be accessed like this:
// from within the class
if (ManualControl.Value) { ... }
// from outside
myObj.ManualControl.Value = true;
Now there is no way to change the value without triggering the setter code, both inside and outside the class.
Can someone tell me, what is the porpose of using a public property for a private class member but not implementing anything in the set and get part? I know that it could be just for examples and that later on you can implement something but I don't know if there is any meaning using it like that
I am going to assume you know there is a private field generated by the C# compiler as the field backing up this property. It is syntactic sugar. I am also going to assume you know this is an auto-implemented property. So what is the point of a property if it public with no logic in the get or set. In other words, any class can access it and set its value. So why choose this property over a public field?
Purpose of it is:
Today the get and set is empty but it may need code in the future. To avoid breaking a contract in the future, you use an empty get and set.
Most .NET databinding can be done against properties but not fields. So although it is empty, it still serves a far greater purpose.
The get and set can have access modifiers. For example, you can say set is private but get is public.
A field cannot be used in interfaces but properties can.
This is an "auto-implemented" property in C#. In effect, the property acts as a full property, i.e.:
This:
public int Number { get; set; }
Is equivalent to:
private int _number;
public int Number
{
get
{
return this._number;
}
set
{
this._number = value;
}
}
This prevents you from having to type out the full property declaration. This is useful when you don't have any additional logic in the getter and/or setter.
If you're creating simple data mapping and transfer classes, auto-implementing properties can make the class definition far more concise.
When you see
public int Count { get; set; }
as a member of a class, that is not nothing. You're just deferring to the default implementation of auto-properties. It's roughly,
private int _count;
public int get__Count()
{
return _count;
}
public void set__Count(int value)
{
_count = value;
}
You just have syntactic sugar in C# to use these methods roughly like a field, i.e.
instanceOfObject.Count = 42; // Not a field access, but behind the scenes a method call.
I saw some get set method to set values. Can anyone tell me the purpose of this?
public string HTTP_USER_NAME
{
get
{
return UserName;
}
set
{
UserName = value;
}
}
public string HTTP_USER_PASSWORD
{
get
{
return UserPwd;
}
set
{
UserPwd = value;
}
}
Actually why use these things. For global access, or is there some other reason for this type of thing?
They are just accessors and mutators. That's how properties are implemented in C#
In C# 3 you can use auto-implemented properties like this:
public int MyProperty { get; set; }
This code is automatically translated by the compiler to code similar to the one you posted, with this code is easier to declare properties and they are ideal if you don't want to implement custom logic inside the set or get methods, you can even use a different accessor for the set method making the property immutable
public int MyProperty { get; private set; }
In the previous sample the MyProperty will be read only outside the class where it was declared, the only way to mutate it is by exposing a method to do it or just through the constructor of the class. This is useful when you want to control and make explicit the state change of your entity
When you want to add some logic to the properties then you need to write the properties manually implementing the get and set methods just like you posted:
Example implementing custom logic
private int myProperty;
public int MyProperty
{
get
{
return this.myProperty;
}
set
{
if(this.myProperty <=5)
throw new ArgumentOutOfRangeException("bad user");
this.myProperty = value;
}
}
It seems as though you understand the functionality of getters and setters, and others answered that question. "Normal" class variables (without getters and setters) are called "fields", and "properties" (which have the getters and setters) encapsulate fields.
The purpose of properties is to control outside access to fields. If you want a variable to be read-only to outside logic, you can omit the setters, like so:
private int dataID;
public int DataID {
get { return dataID; }
}
You can also make the setter private and achieve the same read-only functionality.
If an object has a chance of being null (for whatever reason), you can guarantee an instance always exists like this:
private Object instance;
public Object Instance {
get {
if (instance == null)
instance = new Object();
return instance;
}
}
Another use for properties is defining indexers.
//in class named DataSet
private List<int> members;
public int this[int index] {
get { return members[index]; }
}
With that indexer defined, you can access an instance of DataSet like this:
int member = dataSet[3];
Check these links,.. they gives clear explanation.
http://www.dotnetperls.com/property
http://code.anjanesh.net/2008/02/property-getters-setters.html
if UserName and UserPwd are class variables, better to use like this
_userName
_userPwd
Standard way to implement properties in C#. UserName and UserPwd are private member variables (string type) of the class where these 2 methods are defined.
HTTP_USER_NAME and HTTP_USER_PASSWORD are the public properties of your class. UserName and UserPwd could be your private field. And you are allowing other people to set or get the values via these public properties. No direct accesss to private propeties. Also you can do some logic inside the get method of the property.Ex : you will have a public property called Age and in the get method of that, you may read the value of your private field called "dateOfBirth" and do some calculation ( CurrentYear-dateOfBirth) and return that as the Age.
Properties are just accessors over fields. They allow to do certain operations (if needed), and provide controlled access to fields.
If you want to know when to use Properties, and when to use Only fields, Check the link Properties vs Fields – Why Does it Matter? (Jonathan Aneja)
From Properties (C# Programming Guide)
A property is a member that provides a flexible mechanism to read, write, or compute the value of a private field. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.
In this example, the TimePeriod class stores a time period. Internally the class stores the time in seconds, but a property named Hours enables a client to specify a time in hours. The accessors for the Hours property perform the conversion between hours and seconds.
Example
class TimePeriod
{
private double seconds;
public double Hours
{
get { return seconds / 3600; }
set { seconds = value * 3600; }
}
}
class Program
{
static void Main()
{
TimePeriod t = new TimePeriod();
// Assigning the Hours property causes the 'set' accessor to be called.
t.Hours = 24;
// Evaluating the Hours property causes the 'get' accessor to be called.
System.Console.WriteLine("Time in hours: " + t.Hours);
}
}
// Output: Time in hours: 24
Properties Overview
Properties enable a class to expose a public way of getting and setting values, while hiding implementation or verification code.
A get property accessor is used to return the property value, and a set accessor is used to assign a new value. These accessors can have different access levels. For more information, see Restricting Accessor Accessibility (C# Programming Guide).
The value keyword is used to define the value being assigned by the set accessor.
Properties that do not implement a set accessor are read only.
For simple properties that require no custom accessor code, consider the option of using auto-implemented properties. For more information, see Auto-Implemented Properties (C# Programming Guide).
e.g.
[XmlAttribute("Type")]
public string DataTypeString
{
get
{
return _dataType.ToString();
}
set
{
_dataType = Type.GetType(value);
}
}
private Type _dataType;
public Type DataType
{
get { return _dataType; }
set { _dataType = value; }
}
In the above code, first property type is string, however the setter sets, the _dataType which is of System.Type
You can and often benificial to do so for get properties.
Type conversion in set properties likley be more contraversial. Properties are normally expected to be lightweight, not to throw exceptions and normally x.YYY=newValue; expected to result in x.YYY==newValue to be true - these may be hard to achieve if type conversion happens.
I.e. in your particular example allowing to set type by string as property feels like a bad idea - what is expected behavior of x.DataTypeString="42";? You may want to consider some other way to enable XML serialization of your property...
Sure you can. That's the purpose of the properties. The user of the property doesn't need to know what's behind it. He/she just sets or gets a string and that's all they need to know.
I guess its ok, but I would prefer to just do this, which less odd and less verbose:
public void SetDataType(string value) {
_dataType = Type.GetType(value);
}
public void SetDataType(Type value) {
_dataType = value;
}
public Type DataType { get; private set; }
As was already mentioned, it seems rather odd to have two getter/setter pairs for the same private variable.
It's hard to say without knowing the purpose of this class. But in general, I think I'd be confused by having two properties with the same backing field (so that changing one affects the other).
There are exceptions though. For example, in a Circle class, I would find it perfectly reasonable to have both a Radius property and a Diameter property. So again, it kind of depends on the purpose of your class.
Also, in the example you provided, it doesn't seem like the DataTypeString property provides much benefit. If you remove it, callers could achieve the same effect by simply calling
foo.DataType = Type.GetType("System.Int32");
or
string dataTypeString = foo.DataType.ToString()
.
In much of the code I have seen (on SO, thecodeproject.com and I tend to do this in my own code), I have seen public properties being created for every single private field that a class contains, even if they are the most basic type of get; set; like:
private int myInt;
public int MyInt
{
get { return myInt; }
set { myInt = value }
}
My question is: how does this differ from:
public int MyInt;
and if we should use properties instead of public fields why should we use them in this specific case? (I am not talking about more complex examples where the getters and setters actually do something special or there is only one get or set (read/write only) rather than just returning/setting a value of a private field). It does not seem to add any extra encapsulation, only give a nice icon in IntelliSense and be placed in a special section in class diagrams!
See this article http://blog.codinghorror.com/properties-vs-public-variables/
Specifically
Reflection works differently on variables vs. properties, so if you rely on reflection, it's easier to use all properties.
You can't databind against a variable.
Changing a variable to a property is a breaking change.
Three reasons:
You cannot override fields in subclasses like you can properties.
You may eventually need a more complex getter or setter, but if it's a field, changing it would break the API.
Convention. That's just the way it's done.
I'm sure there are more reasons that I'm just not thinking of.
In .Net 3.x you can use automatic properties like this:
public int Age { get; set; }
instead of the old school way with declaring your private fields yourself like this:
private int age;
public int Age
{
get { return age; }
set { age = value; }
}
This makes it as simple as creating a field, but without the breaking change issue (among other things).
When you create private field name and a simple public property Name that actually gets and sets the name field value
public string Name
{
get { return name; }
}
and you use this property everywhere outside your class and some day you decide that the Name property of this class will actually refer to the lastName field (or that you want to return a string "My name: "+name), you simply change the code inside the property:
public string Name
{
get { return lastName; //return "My name: "+name; }
}
If you were using public field name everywhere in the outside code then you would have to change name to lastName everywhere you used it.
Well it does make a difference. Public data can be changed without the object instance knowing about it. Using getters and setters the object is always aware that a change has been made.
Remember that encapsulating the data is only the first step towards a better structured design, it's not an end-goal in itself.
You have to use properties in the following cases:
When you need to serialize data in the property to some format.
When you need to override properties in derived class.
When you implement get and set methods with some logic. For example, when you implement Singleton pattern.
When you're derived from interface, where property was declared.
When you have specific issues related to Reflection.
It... depends?
I always use getters & setters, since they created this shortcut:
public int Foo { get; set; }
At compile time it is translated. Now you can't get fancy with it, but it is there, and if you need to get fancy you just spell it out later.
However public, private, protected... it's all a matter of who you want to be able to tweak the data. We use inheritance a lot and this is a very common method for us, so that only chidren can edit certain properties.
protected _foo;
public Foo
{
get { return _foo; }
} //lack of set intentional.
I can't believe that with 11 answers, nobody has said this:
Not all private fields should be exposed as public properties. You should certainly use properties for anything that needs to be non-private, but you should keep as much of your class private as possible.
There are many reasons why.
Mainly:
You can do some other functions when the variable is set
You can prevent setting and provide only get
Some 'things' only work on properties (DataBinding, for example)
You can hide the implementation of the property [perhaps it is a ViewState variable, in ASP.NET).
The point is - what if further down the line you want to make sure that every time myInt is referenced something special happens (a log file is written to, it's changed to 42 etc)? You can't do that without getters and setters. Sometimes it's wise to program for what you might need, not what you need right now.
Actually, if you're using Silverlight, you'll realise that fields cannot be set a static resources and thus you'll have to use a property (even to access a const).
I've realised that when I tried to federate the region names I use in Composite Guidance (PRISM).
However, that's just a language limitations and apart from static/const fields I alsways use properties.
The idea is you should not accidentally/unintentionally change the value of a class private field outside.
When you use get and set, that means you are changing the class private field intentionally and knowingly.
Setting a value into a private field only changes that field,but making them in property you can handle another arguments for example,you can call a method after setting a value
private string _email;
public string Email
{
get
{
return this._email;
}
set
{
this._email = value;
ReplaceList(); //**
}
}
In simpler words, answer to your question is the access modifiers i.e. public and private.
If you use:
public int myInt;
public int MyInt
{
get { return myInt; }
set { myInt = value }
}
then both MyInt property and myInt variable is available in the project to be modified.
Means, if your class suppose A is inherited by class suppose B,
then myInt and MyInt both are available for modification and no check can be applied.
Suppose you want myInt value can be set in derive class if some particular condition pass.
This can be achieved only by making field private and property to be public.
So that only property is available and conditions can be set based on that.