Is there a way to stipulate that the clients of a class should specify a value for a set of properties in a class. For example (see below code), Can i stipulate that "EmploymentType" property in Employment class should be specified at compile time? I know i can use parametrized constructor and such. I am specifically looking for outputting a custom warning or error during compile time. Is that possible?
public class Employment
{
public EmploymentType EmploymentType {get; set;}
}
public enum EmploymentType
{
FullTime = 1,
PartTime= 2
}
public class Client
{
Employment e = new Employment();
// if i build the above code, i should get a error or warning saying you should specify value for EmploymentType
}
As cmsjr stated what you need to do is this:
public class Employment
{
public Employment(EmploymentType employmentType)
{
this.EmploymentType = employmentType;
}
public EmploymentType EmploymentType { get; set; }
}
This will force callers to pass in the value at creation like this:
Employment e = new Employment(EmploymentType.FullTime);
In the situation where you need to have a default constructor (like serialization) but you still want to enforce the rule then you would need some sort of state validation. For instance anytime you attempt to perform an operation on the Employment class you can have it check for a valid state like this:
public EmploymentType? EmploymentType { get; set; } // Nullable Type
public void PerformAction()
{
if(this.Validate())
// Perform action
}
protected bool Validate()
{
if(!EmploymentType.HasValue)
throw new InvalidOperationException("EmploymentType must be set.");
}
If you're looking throw custom compiler warnings, this is not exactly possible. I asked a similar question here Custom Compiler Warnings
You could achieve what you want to do by not having a default constructor, and instead defining a constructor that takes employment type as an argument. If someone attempted to instantiate the class using a parameter-less constructor, they would get a compile error.
EDIT code sample
public Employment(EmploymentType eType)
{
this.EmploymentType = eType;
}
OO principles dictate that an object should never be in an invalid state. So this should be a constructor parameter.
There is no way to indicate that a property is required at compile time.
You mentioned in your original post that you already know about doing this with constructors, which is definitely the right way to do it. I don't believe there's going to be any way to do what you want, even with attributes (which is normally how you would manipulate compiler warnings, etc). Since the object could be created and then passed to another method to set the parameter, it's not necessarily as obvious as "client has to specify value".
You could try and create a custom FxCop or VSTS static analysis rule, but I don't think you'll be able to do this with just the C# compiler.
Interface Icontrol
' Declare an interface.
Property MustHave() As String
End Interface
Then in your user control include
Public Class mycontrol
Inherits System.Web.UI.UserControl
Implements Icontrol
It makes this property required
Related
I have an interface
public interface IIntf { string Foo { get; set; } }
and a class
class MyClass : IIntf
{
public string Foo { get; private set; }
}
. Now I get the compiler error
'MyClass' does not implement interface member 'IIntf.Foo.set'.
'MyClass.Foo.set' is not public.
So it seems that Foo.set is its own entity somehow (and Foo.get analogously). Can I do anything with them? If yes, what?
E.g. I tried implementing the missing setter property by using Foo.set - it was even suggested by Visual Studio
but I did not find any syntax that works. When following the suggestion and trying to implement the Foo.set method (without corresponding get because I have that already), I get a different type of compiler error:
But since Visual Studio shows (and, at times, suggests while typing code) Foo.set and Foo.get, I assume that something can be done with them. Any ideas what? Maybe some nice way of redirecting methods to only the getter / only the setter? Or something else?
Dealing with the issues / questions that #Kjara has raised in the comments:
Properties are implemented using get and set methods behind
the scenes (by the compiler).
You cannot easily access / pass around these
methods (without reflection).
When define properties, you must
define the get and set at the same time (you can't define the
get on one line and the set on another).
As per #FrankM and #Dirk above - the confusion here appears to stem from the fact that the IDE implies (falsely) that you can define the getter and setter separately.
Like this
public class MyClass : IIntf
{
public string Foo { get; private set; }
string IIntf.Foo { get => Foo; set => Foo = value; }
}
For the purposes of this question, a 'constant reference' is a reference to an object from which you cannot call methods that modify the object or modify it's properties.
I want something like this:
Const<User> user = provider.GetUser(); // Gets a constant reference to an "User" object
var name = user.GetName(); // Ok. Doesn't modify the object
user.SetName("New value"); // <- Error. Shouldn't be able to modify the object
Ideally, I would mark with a custom attribute (e.g. [Constant]) every method of a class that doesn't modify the instance, and only those methods can be called from the constant reference. Calls to other methods would result in an error, if possible, during compile time.
The idea is I can return a read-only reference to and be sure that it will not be modified by the client.
The technique you're referring to is called "const-correctness" which is a language feature of C++ and Swift, but not C#, unfortunately - however you're onto something by using a custom attribute because that way you can enforce it via a Roslyn extension - but that's a rabbit-hole.
Alternatively, there's a much simpler solution using interfaces: because C# (and I think the CLR too) does not support const-correctness (the closest we have is the readonly field modifier) the .NET base-class-library designers added "read-only interfaces" to common mutable types to allow a object (wheather mutable or immutable) to expose its functionality via an interface that only exposes immutable operations. Some examples include IReadOnlyList<T>, IReadOnlyCollection<T>, IReadOnlyDictionary<T> - while these are all enumerable types the technique is good for singular objects too.
This design has the advantage of working in any language that supports interfaces but not const-correctness.
For each type (class, struct, etc) in your project that needs to expose data without risk of being changed - or any immutable operations then create an immutable interface.
Modify your consuming code to use these interfaces instead of the concrete type.
Like so:
Supposing we have a mutable class User and a consuming service:
public class User
{
public String UserName { get; set; }
public Byte[] PasswordHash { get; set; }
public Byte[] PasswordSalt { get; set; }
public Boolean ValidatePassword(String inputPassword)
{
Hash[] inputHash = Crypto.GetHash( inputPassword, this.PasswordSalt );
return Crypto.CompareHashes( this.PasswordHash, inputHash );
}
public void ResetSalt()
{
this.PasswordSalt = Crypto.GetRandomBytes( 16 );
}
}
public static void DoReadOnlyStuffWithUser( User user )
{
...
}
public static void WriteStuffToUser( User user )
{
...
}
Then make an immutable interface:
public interface IReadOnlyUser
{
// Note that the interfaces' properties lack setters.
String UserName { get; }
IReadOnlyList<Byte> PasswordHash { get; }
IReadOnlyList<Byte> PasswordSalt { get; }
// ValidatePassword does not mutate state so it's exposed
Boolean ValidatePassword(String inputPassword);
// But ResetSalt is not exposed because it mutates instance state
}
Then modify your User class and consumers:
public class User : IReadOnlyUser
{
// (same as before, except need to expose IReadOnlyList<Byte> versions of array properties:
IReadOnlyList<Byte> IReadOnlyUser.PasswordHash => this.PasswordHash;
IReadOnlyList<Byte> IReadOnlyUser.PasswordSalt => this.PasswordSalt;
}
public static void DoReadOnlyStuffWithUser( IReadOnlyUser user )
{
...
}
// This method still uses `User` instead of `IReadOnlyUser` because it mutates the instance.
public static void WriteStuffToUser( User user )
{
...
}
So, these are the first two ideas I initially had, but don't quite solve the problem.
Using Dynamic Objects:
The first idea I had was creating a Dynamic Object that would intercept all member invokations and throw an error if the method being called isn't marked with a [Constant] custom attribute. This approach is problematic because a) We don't have the support of the compiler to check for errors in the code (i.e. method name typos) when dealing with dynamic objects, which might lead to a lot of runtime errors; and b) I intend to use this a lot, and searching for method names by name every time a method is called might have considerable performance impact.
Using RealProxy:
My second idea was using a RealProxy to wrap the real object and validate the methods being called, but this only works with objects that inherit from MarshalByRefObject.
Update 1
It seems like either my English is terribly awful, or people just don't give a sh... to understand what I'm asking about or simply look at the title of the post.
C#5 specification clearly states:
Because the backing field is inaccessible, it can be read and written
only through the property accessors, even within the containing type.
This means that automatically implemented read-only or write-only
properties do not make sense, and are disallowed.
public string MyProperty {get;} has no sense, yet it costs nothing for compiler to emit getter not even warring about lacking setter. Backing field will be initialized with a default value. What does it mean? It means that designers spent some effort to implement a verification, to introduce functionality that could be left out.
Let's now consider C#6:
In C#6 the initialization of auto-implemented properties was introduced.
public string FirstName { get; set; } = "Jane";
or
public string FirstName { get; } = "Jane";
In the latter case property can be set in a constructor as well:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
}
public Program()
{
ImagePath = "";
}
}
But only in constructor of the class where property was declared. Derived classes cannot set property's value.
Now ask yourself what this property means, if it was not initialized in constructor:
property string My {get;}
This is a 100% equivalent of C#5 prohibited property. It has no sense.
But such declaration being invalid in C#5 became valid in C#6. However semantics didn't change at all: this property is useless without explicit initialization.
That's why I am asking:
Why not explicitly initialized readonly auto-implemented property is valid in c# 6?
What I expect to see as an answer:
Either debunking of my initial assumptions about changes in C#6
Or the explanation of how and why compiler designers changed their mind
about what makes sense, and what does not.
I find the answer It's by design to be completely irrelevant. It is just a fact. I look for reasons. I don't believe compiler designers decide on changes in behavior of compiler with just tossing the coin.
This is an example of good answer.
Original question
In VS2015 this code is compiled without errors:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
Console.WriteLine("Hello World");
}
}
However, in VS2013 I get error:
Compilation error (line 5, col 28): 'Program.ImagePath.get' must
declare a body because it is not marked abstract or extern.
Automatically implemented properties must define both get and set
accessors.
I know about initializable auto implemented properties, and in case of VS2015 field gets default value, that is null here. But then it's interesting to know why this snippet was invalid in C# 5?
Initializable auto-implemented readonly property left without explicit initialization seems to me a bit ODD. It is likely a mistake rather than intention. I'd personally prefer compiler to require explicit initialization in this case:
public string ImagePath { get; } = default(string);
Ok, I know that such property can be also assigned in constructor:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
}
public Program()
{
ImagePath = "";
DoIt();
}
public void DoIt()
{
//ImagePath = "do it";
}
}
public class My : Program
{
public My()
{
//ImagePath = "asdasd";
}
}
But if compiler can check that local variable is not initialized, the same is possible for the property.
So why is it as it is?
The compiler is telling you that automatic properties must have both accessors defined. For example, you could fix the error with
public string ImagePath { get; private set; }
assuming that you do not intend the property to be settable outside the class.
As to why you have to declare a setter or manually implement the property -- well, what good would be a property that you can read from, but will always return the default value of its type since there is no way to set it? Conversely, what good would be a property you can write to but can neither read from nor hook into its setter?
C# 6.0 gives you the option of having write-once, read many auto-properties; this is a huge difference as the value can be arbitrarily chosen, allowing you convenient syntax for properties with immutable values.
I have no idea why your question have been down voted. This is interesting observation but please keep in mind that it is not a breaking change - it is just 'new functionality' that is 'leftover' of other functionality - initialization of auto-implemented properties.
That means it had no sense previously, but now it has.
Moreover, I think it has always had sense. E.g. when you have some base class or interface, for example
interface IPerson
{
int Age { get; }
}
Some day you may want to implement null-object pattern where the age is irrelevant. In c#5 you have to write public int Age { get { return 0; } }, while in c#6 you can simply do public int Age { get; } or even transform interface to abstract class changing only its definition from interface to abstract class.
Intro
I'm working with the legacy code which contains two classes:
I have a class which stores its value of System.Object type.
(I named this class as DomainItem)
Its Identifier property refers to
enum which holds information what a type of DomainItem is (in the
context of business domain).
There is also a class which stores these
items as an Enumerable List. (DomainItems)
What's more:
I don't want to change these classes into generic. This code is very sensitive and not covered by tests.
In order to get DomainItem, I must get it from DomainItems.Items collection.
Code
The code for classes is equivalent as below:
public class DomainItem
{
public Identifier Identifier { get; set; } // Readonly in the "real" code
public object Value { get; set; }
}
public class DomainItems
{
public IEnumerable<DomainItem> Items { get; set; }
}
The question is
How can I extend these classes using generics, to resolve type of Value property in the compile time. Is it even possible?
Example case might be as following:
DomainItem price = new DomainItem { Value = 25.20d, Identifier = Identifier.Price };
// ....
double priceValue = price.ProperValue; // generic property of type T
Obviously, above code is conceptual and it shows what I want to achieve. Any suggestions how to resolve that? Is it even possible?
Edit
My idea is to create a new IEnumerable<DomainItem<T>> where the collection is populated from non-generic DomainItem objects. Since the type of DomainItem.Value is known, it should be possible to make such collection somehow.
There's no such thing as a generic property, but you could easily create a generic method:
public T GetValue<T>() { ... }
public void SetValue<T>(T value) { ... }
You could then check typeof(T) within the method to make sure that it was appropriate for your identifier, ideally having made the identifier read-only. (It would be better as a constructor argument - I wouldn't expect it to make any sense to have a domain item whose identifier changed over time.)
Alternatively, you could just make the type of the Value property dynamic instead of object, assuming you're using C# 4+ with .NET 4+. Then your example code would compile - but it would perform an implicit (dynamic) conversion to double at execution time. You wouldn't get much safety there, but it would compile...
Is there any way to access the backing field for a property in order to do validation, change tracking etc.?
Is something like the following possible? If not is there any plans to have it in .NET 4 / C# 4?
public string Name
{
get;
set
{
if (value != <Keyword>)
{
RaiseEvent();
}
<Keyword> = value;
}
}
The main issue I have is that using auto properties doesn't allow for the same flexibility in validation etc. that a property with a explicit backing field does. However an explicit backing field has the disadvantage in some situations of allowing the class it is contained in to access the backing field when it should be accessing and reusing the validation, change tracking etc. of the property just like any other class that may be accessing the property externally.
In the example above access to the backing field would be scoped to the property thus preventing circumvention of the property validation, change tracking etc.
Edit: I've changed < Backing Field > to < Keyword >. I would propose a new keyword similar to value. field would do nicely although I'm sure it's being used in a lot of existing code.
No there isn't. If you want to access the backing field, then don't use auto properties and roll your own.
I agree that it would be great to have a field that was only accessible by the property and not by the rest of the class. I would use that all the time.
As the MSDN states:
"In C# 3.0 and later, auto-implemented
properties make property-declaration
more concise when no additional logic
is required in the property accessors.
They also enable client code to create
objects When you declare a property as
shown in the following example, the
compiler creates a private, anonymous
backing field can only be accessed
through the property's get and set
accessors."
Since you have additional logic in you accessors, the use of auto-implemented properties is not appropriate in your scenario.
While the backing field does exist, it is given a mangled name to stop you referencing it easily - the idea is that you never reference the field directly. For interests sake, you can use Reflector to disassemble your code and discover the field name, but I would recommend you not use the field directly as this name may indeed be volatile, so your code could break at any time.
Having read your comments in Mehrdad's answer, I think I understand your problem a bit better.
It appears that you are concerned about the ability of the developer to access private state in the class they are writing, bypassing your validation logic, etc. This suggests that the state should not be contained in the class at all.
I would suggest the following strategy. Write a generic class that represents a ValidatedValue. This class holds only the backing value and only allows access/mutation via get and set methods. A delegate is passed to the ValidatedValue to represent the validation logic:
public class ValidatedValue< T >
{
private T m_val;
public ValidationFn m_validationFn;
public delegate bool ValidationFn( T fn );
public ValidatedValue( ValidationFn validationFn )
{
m_validationFn = validationFn;
}
public T get()
{
return m_val;
}
public void set(T v)
{
if (m_validationFn(v))
{
m_val = v;
}
}
}
You could, of course, add more delegates as required (eg, to support pre/post change notification).
Your class would now use the ValidatedValue in place of a backing store for your property.
The example below shows a class, MyClass, with an integer that is validated to be less than 100. Note that the logic to throw an exception is in MyClass, not the ValidatedValue. This allows you to do complex validation rules that depend on other state contained in MyClass. Lambda notation was used to construct the validation delegate - you could have bound to a member function instead.
public partial class MyClass
{
private ValidatedValue<int> m_foo;
public MyClass()
{
m_foo = new ValidatedValue<int>(
v =>
{
if (v >= 100) RaiseError();
return true;
}
);
}
private void RaiseError()
{
// Put your logic here....
throw new NotImplementedException();
}
public int Foo
{
get { return m_foo.get(); }
set { m_foo.set(value); }
}
}
Hope that helps - somewhat off the original topic, but I think it's more inline with your actual concerns. What we have done is taken the validation logic away from the property and put it on the data, which is exactly where you wanted it.
No, but you can in a subclass:
public class Base
{
public string Name
{
get;
virtual set;
}
}
public class Subclass : Base
{
// FIXME Unsure as to the exact syntax.
public string Name
{
override set
{
if (value != base.Name)
{
RaiseEvent();
}
base.Name = value;
}
}
}
If you're gonna do so, why you are using auto properties?!
A simple property has done it way back in 1.0. I don't think it makes sense to add complexity to the language for every special case. You either need the property to do plain store/retrieve model or need more than that. In the latter case, a normal property will do.
You can't do this I'm afraid. That's one of the reasons I started writing MoXAML Power Toys, to provide the ability to convert automatic properties into Notify properties.