I'd like to do the same in C#. Is there anyway of using properties in C# with parameters in the same way I've done with the parameter 'Key' in this VB.NET example?
Private Shared m_Dictionary As IDictionary(Of String, Object) = New Dictionary(Of String, Object)
Public Shared Property DictionaryElement(ByVal Key As String) As Object
Get
If m_Dictionary.ContainsKey(Key) Then
Return m_Dictionary(Key)
Else
Return [String].Empty
End If
End Get
Set(ByVal value As Object)
If m_Dictionary.ContainsKey(Key) Then
m_Dictionary(Key) = value
Else
m_Dictionary.Add(Key, value)
End If
End Set
End Property
Thanks
Is there anyway of using properties in C# with parameters
No. You only can provide the default property in C# with an argument, to model indexed access (as in a dictionary):
public T this[string key] {
get { return m_Dictionary[key]; }
set { m_Dictionary[key] = value; }
}
Other properties can't have arguments. Use a function instead. By the way, it's recommented to do the same in VB so other .NET languages (C# …) can use your code.
By the way, your code is unnecessarily complicated. Four things:
You don't need to escape the String identifier. Use the keyword directly.
Why not use ""?
Use TryGetValue, it's faster. You query the dictionary twice.
Your setter doesn't have to test whether the value already exists.
Public Shared Property DictionaryElement(ByVal Key As String) As Object
Get
Dim ret As String
If m_Dictionary.TryGetValue(Key, ret) Then Return ret
Return "" ' Same as String.Empty! '
End Get
Set(ByVal value As Object)
m_Dictionary(Key) = value
End Set
End Property
The "proper" way to do it in C# is to create child class specifically to access the collection. It should either hold the collection itself or have internal linkages to the parent class.
A more general-purpose, safer, and reusable solution to your problem might be implementing a generic, "parameterized" property class, like this:
// Generic, parameterized (indexed) "property" template
public class Property<T>
{
// The internal property value
private T PropVal = default(T);
// The indexed property get/set accessor
// (Property<T>[index] = newvalue; value = Property<T>[index];)
public T this[object key]
{
get { return PropVal; } // Get the value
set { PropVal = value; } // Set the value
}
}
You could then implement any number of properties within your public class so that clients could set/get the properties with an index, descriptor, security key, or whatever, like this:
public class ParameterizedProperties
{
// Parameterized properties
private Property<int> m_IntProp = new Property<int>();
private Property<string> m_StringProp = new Property<string>();
// Parameterized int property accessor for client access
// (ex: ParameterizedProperties.PublicIntProp[index])
public Property<int> PublicIntProp
{
get { return m_IntProp; }
}
// Parameterized string property accessor
// (ex: ParameterizedProperties.PublicStringProp[index])
public Property<string> PublicStringProp
{
get { return m_StringProp; }
}
}
Finally, client code would access your public class's "parameterized" properties like this:
ParameterizedProperties parmProperties = new ParameterizedProperties();
parmProperties.PublicIntProp[1] = 100;
parmProperties.PublicStringProp[1] = "whatever";
int ival = parmProperties.PublicIntProp[1];
string strVal = parmProperties.PublicStringProp[1];
Sure, this seems weird, but it definitely does the trick. Besides, from a client-code perspective, it's not weird at all -- it's simple and intuitive and acts just like real properties. It doesn't break any C# rules, nor is it incompatible with other .NET managed languages. And from the class-implementer's perspective, creating a reusable, generic, "parameterized" property template class makes component coding a relative breeze, as shown here.
NOTE: You can always override the generic property class to provide custom processing, such as indexed lookup, security-controlled property access, or whatever-the-heck you want.
Cheers!
Mark Jones
Here is a sample for you (with changes along the lines of Grauenwolf's suggestions):
using System;
using System.Collections.Generic;
public class Test
{
public FakeIndexedPropertyInCSharp DictionaryElement { get; set; }
public Test()
{
DictionaryElement = new FakeIndexedPropertyInCSharp();
}
public class FakeIndexedPropertyInCSharp
{
private Dictionary<string, object> m_Dictionary = new Dictionary<string, object>();
public object this[string index]
{
get
{
object result;
return m_Dictionary.TryGetValue(index, out result) ? result : null;
}
set
{
m_Dictionary[index] = value;
}
}
}
}
class Program
{
static void Main(string[] args)
{
Test t = new Test();
t.DictionaryElement["hello"] = "world";
Console.WriteLine(t.DictionaryElement["hello"]);
}
}
Your code sample strikes me as a very strange design and an abuse of what properties are intended for. Why not just an instance method AddOrUpdateKey:
Public Sub AddOrUpdateKey(ByVal Key As String, ByVal Value as Object)
If m_Dictionary.ContainsKey(Key) Then
m_Dictionary(Key) = Value
Else
m_Dictionary.Add(Key, Value)
End If
End Sub
Your property also returns String.Empty if the key does not exist, but claims to return an Object, nor a String.
Thanks Konrad, Alan, Grauenwolf,
In conclusion, I can't use C# properties exactly in the same way that in VB.NET... :_( Anyway, your answers has been very usefull to me, and I´ll probably take this ideas to my C# code.
In addition to the answers to the properties question, there are other good points. For example,
Use TryGetValue, it's faster. You query the dictionary twice.
Your setter doesn't have to test whether the value already exists.
Thanks Sören, too, using a method don't fits well in my initial aims, but thanks very much.
Related
In C#, is it possible to check if a Property has a Setter that references another object at runtime?
Here is some code:
private PropertyChanged _propertyChanged;
public string PropertyChangedName
{
get
{
return _propertyChanged.Name;
}
set
{
_propertyChanged.Name = value;
}
}
In the above code, the PropertyChangedName is referenced as part of the PropertyChanged object. When setting the PropertyChangedName, the _propertyChanged.Name is set.
Here is some code where the PropertyChangedName is not referenced as part of the PropertyChanged object:
private string _propertyChangedName;
public string PropertyChangedName
{
get
{
return _propertyChangedName;
}
set
{
_propertyChangedName = value;
}
}
I am not sure this is the best way, however, it might help you:
// store the current state:
var valuesbefore = new List<object>();
foreach (var r in this.GetType().GetFields(System.Reflection.BindingFlags.NonPublic))
valuesbefore.Add(r.GetValue(this));
// change the value the specified property
PropertyChangedName = PropertyChangedName + "a";
//get the new state of the class:
var valuesnow = new List<object>();
foreach (var r in this.GetType().GetFields(System.Reflection.BindingFlags.NonPublic))
valuesnow.Add(r.GetValue(this));
// check for any change
bool equal = valuesbefore.SequenceEqual(valuesnow);
If equal is false, it means that you are dealing with a case in which the setter changes the value of private field. Note that special care is required when dealing with types other than string, int, etc.
You could implement INotifyPropertyChanged and subscribe to it.
This is a really tricky question. The System.Reflection library in .NET allows you to observe the class signatures (Types, Fields, Methods, etc), invoke them, and get their values; but not see within their body.
Read through the answers in this question for some options to inspect the contents of methods. (a Property is just a method -- two if you have both get and set)
I have come across a situation where I probably needed to add properties(of a class) in a list to invoke them manually(or you can say, I need to assign there values(setter)). That is why because, I don't even know which properties is to set the values, but they are decided at runtime. So far I am trying to find out the solution here and there but still I don't get any article that even hints me a work around for this purpose.
Here's what I want to do exactly (mentioned as comments)-
public class DemoClass
{
IList<Properties> _listOfProps;
private int _iFirstProperty;
private string _iSecondProperty;
public DemoClass()
{
_listOfProps = new List<Properties>();
}
public int FirstProperty
{
get
{
return _iFirstProperty;
}
set
{
_iFirstProperty = value;
// Here I want to add this property into the list.
_listOfProps.Add(FirstProperty);
RaisePropertyChanged("FirstProperty");
}
}
public string SecondProperty
{
get
{
return _iSecondProperty;
}
set
{
_iSecondProperty = value;
RaisePropertyChanged("SecondProperty");
}
}
public void HandleChangedProperties()
{
foreach (var list in _listOfProps)
{
// Here I want to invoke the property. ie. sets the 'value' of this property.
list.Invoke(value)
}
}
}
I know, I can use Func to add in the list like- but I can't go with this.
List<Func<int>> listOfFunc = new List<Func<int>>();
listOfFunc.Add(() => { return 0; }); // Adds using lambda expression
listOfFunc.Add(temp); // Adds as a delegate invoker
private int temp()
{
return 0;
}
from MSDN
Properties can be used as if they are public data members, but they
are actually special methods called accessors.
if properties are internally methods, Why they can't be added as List of Func<>
Also, if there's no way I can do that without using Reflection (by getting PropertyInfo list), why Microsoft hasn't designed this in C#?
You can either keep a list of PropertyInfo values and later set the value of the properties using reflection, or you can keep a list of setter delegates (which effectively just forward the value to the real, hidden setter).
For example:
IList<Action<object>> listOfSetters;
listOfSetters.Add(o => this.FirstProperty = (int)o);
// and then:
listOfSetters[0](42); // FirstProperty = 42
The C# "readonly" keyword is a modifier that when a field declaration includes it, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.
Now suppose I do want this "assign value once" constraint, but I would rather allow the assignment be done outside of constructors, a lazy/late evaluation/initialization maybe.
How could I do that? and is it possible to do it in a nice way, for example, is it possible to write some attribute to describe this?
If I understand your question correctly, it sounds like you just want to set a field's value once (the first time), and not allow it to be set after that. If that is so, then all the previous posts about using Lazy (and related) may be useful. But if you don't want to use those suggestions, perhaps you can do something like this:
public class SetOnce<T>
{
private T mySetOnceField;
private bool isSet;
// used to determine if the value for
// this SetOnce object has already been set.
public bool IsSet
{
get { return isSet; }
}
// return true if this is the initial set,
// return false if this is after the initial set.
// alternatively, you could make it be a void method
// which would throw an exception upon any invocation after the first.
public bool SetValue(T value)
{
// or you can make thread-safe with a lock..
if (IsSet)
{
return false; // or throw exception.
}
else
{
mySetOnceField = value;
return isSet = true;
}
}
public T GetValue()
{
// returns default value of T if not set.
// Or, check if not IsSet, throw exception.
return mySetOnceField;
}
} // end SetOnce
public class MyClass
{
private SetOnce<int> myReadonlyField = new SetOnce<int>();
public void DoSomething(int number)
{
// say this is where u want to FIRST set ur 'field'...
// u could check if it's been set before by it's return value (or catching the exception).
if (myReadOnlyField.SetValue(number))
{
// we just now initialized it for the first time...
// u could use the value: int myNumber = myReadOnlyField.GetValue();
}
else
{
// field has already been set before...
}
} // end DoSomething
} // end MyClass
Now suppose I do want this "assign value once" constraint, but I would rather allow the assignment be done outside of constructors
Note that lazy initialization is complicated, so for all of these answers you should be careful if you have multiple threads trying to access your object.
If you want to do this inside the class
You can use the C# 4.0 built-in lazy initialization features:
http://msdn.microsoft.com/en-us/library/dd997286.aspx
http://msdn.microsoft.com/en-us/library/dd642331.aspx
http://sankarsan.wordpress.com/2009/10/04/laziness-in-c-4-0-lazyt/
Or for older versions of C#, just supply a get method, and check if you're already initialized by using a backing field:
public string SomeValue
{
get
{
// Note: Not thread safe...
if(someValue == null)
{
someValue = InitializeSomeValue(); // Todo: Implement
}
return someValue;
}
}
If you want to do this outside the class
You want Popsicle Immutability:
http://blogs.msdn.com/b/ericlippert/archive/2007/11/13/immutability-in-c-part-one-kinds-of-immutability.aspx
http://msdn.microsoft.com/en-us/library/ms750509.aspx
http://csharpindepth.com/Talks.aspx (search for "popsicle immutability" and you'll find a video)
Basically:
You make the whole class writable, but add a Freeze method.
Once this freeze method is called, if users try to call setters or mutator methods on your class, you throw a ModifyFrozenObjectException.
You probably want a way for external classes to determine if your class IsFrozen.
BTW, I made up these names just now. My selections are admittedly poor, but there is no generically followed convention for this yet.
For now I'd recommend you create an IFreezable interface, and possibly related exceptions, so you don't have to depend on the WPF implementation. Something like:
public interface IFreezable
{
void Freeze();
bool IsFrozen { get; }
}
You can use the Lazy<T> class:
private readonly Lazy<Foo> _foo = new Lazy<Foo>(GetFoo);
public Foo Foo
{
get { return _foo.Value; }
}
private static Foo GetFoo()
{
// somehow create a Foo...
}
GetFoo will only be called the first time you call the Foo property.
This is know as the "once" feature in Eiffel. It is a major oversight in C#. The new Lazy type is a poor substitute since it is not interchangeable with its non-lazy version but instead requires you to access the contained value through its Value property. Consequently, I rarely use it. Noise is one of the biggest problems with C# code. Ideally, one wants something like this...
public once Type PropertyName { get { /* generate and return value */ } }
as oppose to the current best practice...
Type _PropertyName; //where type is a class or nullable structure
public Type PropertyName
{
get
{
if (_PropertyName == null)
_PropertyName = /* generate and return value */
return _PropertyName
}
}
I have a type SearchBag that holds a bunch of strings and nullable integers to use for passing on search values. I need a way to check if the search bag contains any values.
I'm currently trying to do it like this:
public bool HasValues()
{
return GetType().GetProperties().Any(p => p.GetValue(this, null) != null);
}
But was wondering if there's a better way.
Without modifying the SearchBag type, there isn't a better way.
EDIT: You could change the type to set a boolean flag in every property setter, then check the flag instead of using Reflection.
You could use Post Sharp to intercept the request to change a property value. You could have all search classes inherit from a common class with a List<string>. Then create an aspect attribute to add a property name to that dictionary whenever the value changes. The following is just a sample, and has bugs:
[Serializable]
public class PropertyChangeAwareAttribute : OnMethodBoundaryAspect
{
public override void OnEntry(MethodExecutionEventArgs eventArgs)
{
if (eventArgs.Method.Name.StartsWith("set_"))
((SearchBagBase)eventArgs.Instance).PropertiesChanged.Add(eventArgs.Method.Name);
base.OnEntry(eventArgs);
}
}
abstract class SearchBagBase
{
public List<string> PropertiesChanged = new List<String>();
}
[PropertyChangeAware]
class RegularSearch : SearchBagBase
{
public String Key { get; set; }
}
with usage:
RegularSearch regularSearch = new RegularSearch();
regularSearch.Key = "toys";
regularSearch.PropertiesChanged.ForEach(Console.WriteLine);
I've created this "question" as a community-wiki, because there is no right or wrong answer. I only would like to know how the community feels about this specific issue.
When you have a class with instance variables, and you also created properties that are simply getters and setters for these instance variables, should you use the properties inside your own class, or should you always use the instance variable?
Having auto-properties in C# 3.0 made this an even harder decision.
Using properties:
public class MyClass
{
private string _name;
// could be an auto-property of-course
public string Name { get { return _name; } set { _name = value; } }
public void Action()
{
string localVar = Name;
// ...
Name = "someValue";
// ...
}
}
Using instance variables:
public class MyClass
{
private string _name;
public string Name { get { return _name; } set { _name = value; } }
public void Action()
{
string localVar = _name;
// ...
_name = "someValue";
// ...
}
}
(for those who hate member prefixes, I apologize)
Personally, I always use the latter (instance variables), because I feel that properties should only be used by other classes, not yourself. That's why I mostly stay away from auto-properties as well.
Of course, things change when the property setter (or getter) does a little more than just wrapping the instance variable.
Are there compelling reasons to pick one or the other?
I always use instance variables as well. The reason is because properties might be doing stuff like validating arguments (like in a setter) for not null or not empty. If you're using the variable inside your class code, there's no need to go through the extra overhead of those checks (assuming you know the variable value is valid). The properties could be doing other things as well (logging, for example), that are important for the public API, but not for internal usage, so again, it's better to avoid the overhead and just use the instance variable in my opinion.
I think it becomes more difficult to change the internal implementation if the code uses its own public interface.
Difficult to explain but consider these expressions:
mTotalPrice = mPrice * mQuantity;
mTotalPrice = Price * Quantity;
What to do in the second expression if I need to change the internals to express all prices in € instead of $ (without affecting the public interface which still uses $)?
One solution is to make the expression more complex by adding the opposite of the change in the property.
mTotalPrice = Price / Rate * Quantity
The other solution is to start to use the private field instead.
mTotalPrice = mPrice * Quantity
In the end you get a mix of private and public use. The only way to get consistent use is to always use the private field.
I don't like prefixing members either, but actually I find I can write something like this accidently and not spot it until run time. Which kinda tempts me to avoid using properties where they're not necessary... but I still do, currently!
Public String MyString
{
{ get { return this.MyString; } } //<== Stack Overflow
{ set { this.myString = value; } }
}
private String myString;
I think that there is no difference between these two approaches.
Auto-implemented properties is just a quick way to access private members which are created any way.
Example from MSDN:
class Customer
{
// Auto-Impl Properties for trivial get and set
public double TotalPurchases { get; set; }
public string Name { get; set; }
public int CustomerID { get; set; }
// Constructor
public Customer(double purchases, string name, int ID)
{
TotalPurchases = purchases;
Name = name;
CustomerID = ID;
}
// Methods
public string GetContactInfo() {return "ContactInfo";}
public string GetTransactionHistory() {return "History";}
// .. Additional methods, events, etc.
}
99% of the time I use the property rather then the instance variable. In the past, I've worked with a lot of code that used the instance variable and when there was a bug associated with that variable, I had to put a breakpoint on every line of code that referenced it.
I decided to use properties instead, either public or private, to wrap around the instance variable. Doing this means that I only have to put a breakpoint in the getter/setter of the property if I need to debug an issue with the instance variable, rather then having (potentially) a lot of breakpoints scattered all over the code.