I would like to use different methods to implement the respective get and set case, like this:
public int this[int i]
{
get { return i + 1; }
}
public string this[int i]
{
set { }
}
This leads to error CS0111: Type 'Foo' already defines a member called 'this' with the same parameter types.
It seems this functionality cannot be achieved in the naive way. Is there a workaround?
I would like to use it like this:
class It {
string SomeProperty;
}
class Bar {
Action this[string key] {
set {
// ...
}
}
string this[string key] {
get {
return new It ();
}
}
}
Bar ["key"] = () => {};
Bar ["key"].SomeProperty = 5;
The indexer overload is a special property which accepts arguments. In VB.NET, due to the way VB handles collections, the property is given the name Item(...). If you look at the interface for IList<T> you'll notice it's called Item there too.
As a result, it has to follow the same rules as properties and method overloading. The return type of a method is not considered part of its calling signature, so overload resolution (how the compiler decides which version of a method to call) cannot distinguish a difference between your indexer implementations.
The intent of an indexer is to provide access to the values stored in a collection-like object. If you can get and set the value associated with a given key or index, the expectation is that you should get back the same value you set.
Your examples are trying to achieve a kind of type duality that isn't isn't the intent of an indexer and isn't achievable in the .NET type system. A type cannot simultaneously be an Action and a string. It works against fundamental object-oriented principals to try and make something be two things.
If you want to associate an action and a string, you should create a type that does just that:
public class NamedAction
{
private readonly Action _action;
public string Name { get; }
public NamedAction(Action action, string name)
{
_action = action;
Name = name;
}
public void Invoke()
{
_action.Invoke();
}
}
Now you can have an indexer that gets and sets NamedAction instances and everything makes a lot more sense.
Related
I'm trying to wrap a type (outside of my control) so that it would seamlessly appear to implement an interface (also outside of my control).
Given these defintions
// External types. Not changable.
class Foo {
public int I { get; set; }
public int J { get; set; }
}
interface IGenerateSignature {
string Generate();
}
I would like to use a Foo instance to call a method with an IGenerateSignature parameter:
void Test() {
var foo = new Foo { I = 1, J = 2 };
GetSignature(foo);
}
void GetSignature(IGenerateSignature sig) {
Console.Write(sig.Generate());
}
I tried creating an intermediary struct like this:
struct FooSignaturizer : IGenerateSignature {
private readonly Foo _foo;
public FooSignaturizer(Foo f) {
_foo = f;
}
public static implicit operator FooSignaturizer(Foo f) {
return new FooSignaturizer(f);
}
public string Generate() {
return _foo.I + ":" + _foo.J;
}
}
But for some reason overload resolution fails to find the conversion from Foo to FooSignaturizer, and I get a "Cannot convert" compiler error. If I manually add a cast, GetSignature((FooSignaturizer) foo), it works. However, I need to also add support for the Bar and Qux types, with BarSignaturizer and QuxSignaturizer, so the cast won't work for those cases.
Is there a way to accomplish this?
As per 7.5.3.1 of the C# spec, only implicit conversions from argument expression to parameter type are considered.
7.5.3.1 Applicable function member
A function member is said to be an applicable function member with respect to an argument list A when all of the following are true:
Each argument in A corresponds to a parameter in the function member declaration as described in §7.5.1.1, and any parameter to which no argument corresponds is an optional parameter.
For each argument in A, the parameter passing mode of the argument (i.e., value, ref, or out) is identical to the parameter passing mode of the corresponding parameter, and
for a value parameter or a parameter array, an implicit conversion (§6.1) exists from the argument to the type of the corresponding parameter, or
for a ref or out parameter, the type of the argument is identical to the type of the corresponding parameter. After all, a ref or out parameter is an alias for the argument passed.
What you have here isn't an implicit conversion from Foo to IGenereateSignature, it's a wrapper.
As an explanation for this behaviour, you can't exect the compiler to go though every implementation of IGenerateSignature in scope to see whether it has an implicit conversion to/from Foo. What if there was more than one?
In terms of how you can achieve this for Foo, Bar and Qux...
What you're trying to achieve, one call to GetSignature(fooOrBarOrQux), isn't possible, because (based on your description of Foo) you can't have one variable that can be a Foo or a Bar or a Qux at compile time - they're unrelated. You'll always need three call sites, so there's no reason not to have three slightly-different conversions (wrapper class or overloaded method call or something) for the three cases.
... unless you use dynamic?
Rawling's answer gives a good explanation of why you have the problem. Since you can't fix this with implicit conversion, you could try extension methods to convert all types to IGenerateSignature like this:
void Test() {
var foo = new Foo { I = 1, J = 2 };
GetSignature(foo.AsIGenerateSignature());
}
void GetSignature(IGenerateSignature sig) {
Console.Write(sig.Generate());
}
public static class GenerateSignatureExtensions
{
public static IGenerateSignature AsIGenerateSignature(this IGenerateSignature me)
{
return me;
}
public static IGenerateSignature AsIGenerateSignature(this Foo me)
{
return new FooSignaturizer(me);
}
public static IGenerateSignature AsIGenerateSignature(this Bar me)
{
return new BarSignaturizer(me);
}
//....
}
Rawling's answer gives a great explanation of the why you are having a problem. As to how to achieve what you want. I might consider something like this:
public interface ISignaturizer
{
IGenerateSignature ToSignaturizer();
}
struct FooSignaturizer : IGenerateSignature, ISignaturizer{
private readonly Foo _foo;
public FooSignaturizer(Foo f) {
_foo = f;
}
public string Generate() {
return _foo.I + ":" + _foo.J;
}
public IGenerateSignature ToSignaturizer()
{
return (IGenerateSignature)this;
}
}
Now BarSignaturizer and QuxSignaturizer can implement the same interface. And then you can do:
GetSignature(((ISignaturizer)fooOrBarOrQux).ToSignaturizer());
Which isn't quite as elegant, but I think should accomplish what you need.
I'd like to use a certain operation for multiple variable types (both native and objects) so I'm using the generic return type as follows.
private Generic Field<Generic>(String field)
{
if (BagOfJunk.Properties.Contains(field))
return (Generic)BagOfJunk[field];
return default(Generic);
}
This works well (and BagOfJunk is just a property of this from which I'm pulling out Object typed values). Now, during run-time, when a field isn't contained in the bag, I get the default value to be null. Hence, in the code, I need to perform a check as follows.
NumericType protoNumber = Field<NumericType>("beep");
int number = protoNumber != null ? protoNumber.Value : -1;
DateType protoDate = Field<DateType>("boop");
DateTime date = protoDate != null ? protoDate.Value : null;
I'd like to make the code more compact, so I tried to design a method that does the above four lines in one swoop, for a generic type. The result is below but, of course, it doesn't compile, because the type GenericIn isn't specific enough to have a property Value.
private GenericOut Field<GenericIn, GenericOut>(String field)
{
GenericIn input = Field<GenericIn>(field);
if (input != null)
return (GenericOut)input.Value;
return default(GenericOut);
}
How can I ensure the computer that my GenericIn isn't general - by promising that whatever stuff I'll shove into it, it'll always have the property Value in it?
Edit
It should be emphasized that the type of Value needs to be generic ( equivalent to GenericOut). I noticed that I didn't stress that strongly enough. So the interface that can be used need to declare a property of general type like the following.
interface ObjectWithValue { public Generic Value { get; } }
You can use an interface and apply a where constraint on the type to implement that interface, like below:
interface IHasPropertyValue<TValue> {
TValue Value { get; }
}
class MyType {
public TValue Method<T, TValue>(T obj) where T : IHasPropertyValue<TValue> {
return obj.Value;
}
}
EDIT: Modified the code above to make it more specific to the comment asked below.
put that property in an interface (or a class) and use the generic constraint "where":
public interface IMyInterface
{
public object Value { get; set; }
}
public class C<T> where T:IMyInterface
To build upon the answers so far, you need to create an interface that will be implemented by your GenericIn that will both guarantee that it has a property Value and that the property is of type GenericOut.
interface IHasValue<TOut>
{
TOut Value { get; }
}
private TOut Field<TIn, TOut>(string field) where TIn : IHasValue<TOut>
{
var input = Field<TIn>(field);
return input == null ? default(TOut) : input.Value;
}
I have a public method in a class. I would like to make this private and use a property to expose the method. The problem is that my method accepts an integer parameter and returns a boolean. In my experience a property has to accept and return the same type. What is the best practice to encapsulate this method and expose it using a property?
You shouldn't be using properties this way. Properties are used to wrap get_ and set_ calls to an appropriate backing field and expose them as a single member. The set_ method that is generated internally is void and accepts an instance of the property type as its only argument.
If what you are trying to achieve requires a method, then expose a method. The only solution you could possibly use otherwise would be to use object.
If the value passed in isn't the same as the value returned, then it's not a property. If you go down this route you'll be creating confusion for anyone who needs to call your code, or maintain it in the future.
The only solution I can think of, and a bad one at that, is to declare the property to be of type object.
What is the best practice to encapsulate this method and expose it using a property?
Does it have to be the same property? Could you have something similar to:
private Type2 _valueThatIsStoredAsAResultOfCallMethod;
private Type2 CallMethod(Type1 value)
{
// Whatever logic is required here to take a value of Type1 and
// get a value of Type2 from it
return value.ToType2();
}
public Type1
{
set
{
// value is of type Type1
_valueThatIsStoredAsAResultOfCallMethod = CallMethod(value);
}
}
public Type2
{
get
{
return _valueThatIsStoredAsAResultOfCallMethod;
}
}
public bool MyProp
{
get;
private set;
}
public int MyProp_AsInt
{
set
{
MyProp = (value > 0) ? true : false;
}
}
Your function's body can replace the above 'set'.
I just put some sample code there.
Make sure you assign the return val of your function to MyProp.
full code
public class MyClass
{
public bool MyProp
{
get;
private set;
}
public int MyProp_AsInt
{
set
{
MyProp = (value > 0) ? true : false;
}
}
}
public class Program
{
static void Main(string[] args)
{
MyClass o = new MyClass();
o.MyProp_AsInt = 1;
System.Console.WriteLine("{0}", o.MyProp);
o.MyProp_AsInt = 0;
System.Console.WriteLine("{0}", o.MyProp);
string line = System.Console.ReadLine();
}
}
Well, generally speaking, it's bad practice to call a method via a property (or do anything complicated in a property) as it's not what the user of the class would expect to happen. The expected behaviour being getting a value.
I would just expose the method.
A method that takes a parameter and returns a value doesn't translate well into a property, regardless of the types it uses.
The method accepts the parameter, then returns the result, while a parameter either accepts a value or returns a value, not both in the same operation. If you would have to use the property by first setting the value, then reading the value, that would be very counter intuitive. It's simply not at all how you normally use a property.
There is no best practice for doing this because it's a bad practice.
If you had two covariant types as your accept/return, you could refer to them by their lowest common ancestor, but I'd still be wary.
I have an object, MySession, that has a hashtable for storing arbitrary properties with arbitrary types. The relevant part of the object definition is:
public class MySession
{
private Hashtable _sessionVars;
///
/// Set and retrieve session variables ala the traditional session managers.
/// So, SessionObject["var1"] can be used to set or retrieve a value for var1.
///
/// Name of the variable to access.
/// An object that was stored in the session under key.
public object this[string key] {
get {
if (_sessionVars.ContainsKey(key)) {
return this._sessionVars[key];
}
return null;
}
set {
if (this._sessionVars.ContainsKey(key)) {
this._sessionVars.Remove(key);
}
this._sessionVars[key] = value;
}
}
}
The annoying thing is that I have to properly cast the properties when I want to use them. For example:
MySession session = new MySession();
if ( (bool)session["valid"] == true ) { /* do something fun */ }
I would rather be able to do:
MySession session = new MySession();
if ( session["valid"] == true ) { /* do something fun */ }
Is it possible to do this in C#? If so, how?
Update: I do not want to use explicit methods for accessing the properties. The point is to be able to access them as simply as possible. Not like session.GetProperty(name, type) or something.
If you think carefully, you will realize that this is inherently impossible.
What if you write session[someTextbox.Text]?
What if you assign two different types to the same identifier?
Compiling such code would involve solving the halting problem to figure out what type each string would have.
Instead, you could make a strongly-typed wrapper class around HttpContext.Current.Session with properties that include casts in their getters.
If you are using .Net Framework 4.0 then you can do it by deriving your MySession class from DynamicObject and overriding the necessary methods.
Here is the code:
public class MySession : DynamicObject
{
//Why not use Dictionary class?
private Hashtable _sessionVars = new Hashtable();
public override bool TrySetMember(SetMemberBinder binder, object value)
{
this[binder.Name] = value;
return true;
}
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
result = this[binder.Name];
return true;
}
//You can make it private so that users do not use strings directly.
public object this[string key]
{
get
{
if (_sessionVars.ContainsKey(key))
{
return this._sessionVars[key];
}
return null;
}
set
{
if (this._sessionVars.ContainsKey(key))
{
this._sessionVars.Remove(key);
}
this._sessionVars[key] = value;
}
}
}
And this how you use it:
dynamic ses = new MySession();
ses.number = 5;
ses.boolean = true;
Console.WriteLine(ses.number > 4);
if (ses.boolean)
{
Console.WriteLine(ses.number - 1);
}
Console.ReadKey();
No need for casting or using string to access the new fields! If you are using Resharper you will get intellisense for existing fields too. If you need more functionality you can override other members too.
I personally end up having to handle the scenario where the session variable hasn't been set yet. Therefore, I end up with a method that looks like this:
public class MySession
{
...
public T GetValue<T>(string key, T defaultValue)
{
return _sessionVars.ContainsKey(key) ? this._sessionVars[key] as T : defaultValue;
}
}
Then T can be inferred. It can then be called like this (no casting required):
if (mySession.GetValue("valid", false))
{
// fun stuff here
}
I'm not really sure is "as T" works. If not, you can cast it to (T) done that before. "as T" would be nice if you've got inherited classes and such.
I typically derive off a class like mySession and call base.GetValue() in property getters I expose off the derived class.
If you're passing string (or any sort of object) keys, then it's impossible to do; the indexer method can only return one specific type, so you couldn't possible have it return a string or a double, for instance.
There are a couple of options: one, if this is a limited-scope class that doesn't need the flexibility of arbitrary keys, then you can just add explicit properties--maybe just for commonly used properties if you want to still be able to fall back on the object-returning indexer.
Or, you could add a generic Get method, like so:
public T GetValue<T>(object key) {
if(_hashSet[key] is T) {
return (T)_hashSet[key];
}
throw new InvalidCastException();
}
That doesn't get you much, though, since you'll still have to specify the type name, you're just moving it from the cast to the generic parameter.
EDIT: Of course, how you want to handle invalid casts is up to you, but throwing the exception mimics the behavior of the direct cast. As someone mentioned in another answer, if you also specify a parameter of type T in the signature, then it will get the correct type from that parameter.
Easy and best way to add session
public static void Add<T>(string key, T value)
{
var current = HttpContext.Current;
if (current == null) return;
current.Session.Add(key, value);
}
Example
public Model User
{
private string searchText
{
get { return SessionHelper.Get<string>("searchText"); }
set { SessionHelper.Add("searchText", value); }
}
}
Why can't you create a generic indexer in .NET?
the following code throws a compiler error:
public T this<T>[string key]
{
get => /* Return generic type T. */
}
Does this mean you can't create a generic indexer for a generic member collection?
Here's a place where this would be useful. Say you have a strongly-typed OptionKey<T> for declaring options.
public static class DefaultOptions
{
public static OptionKey<bool> SomeBooleanOption { get; }
public static OptionKey<int> SomeIntegerOption { get; }
}
Where options are exposed through the IOptions interface:
public interface IOptions
{
/* since options have a default value that can be returned if nothing's
* been set for the key, it'd be nice to use the property instead of the
* pair of methods.
*/
T this<T>[OptionKey<T> key]
{
get;
set;
}
T GetOptionValue<T>(OptionKey<T> key);
void SetOptionValue<T>(OptionKey<T> key, T value);
}
Code could then use the generic indexer as a nice strongly-typed options store:
void Foo()
{
IOptions o = ...;
o[DefaultOptions.SomeBooleanOption] = true;
int integerValue = o[DefaultOptions.SomeIntegerOption];
}
I don't know why, but indexers are just syntactic sugar. Write a generic method instead and you'll get the same functionality. For example:
public T GetItem<T>(string key)
{
/* Return generic type T. */
}
Properties can't be generic in C#2.0/3.0 so therefore you can't have a generic indexer.
You can; just drop the <T> part from your declaration and it will work fine. i.e.
public T this[string key]
{
get { /* Return generic type T. */ }
}
(Assuming your class is generic with a type parameter named T).
The only thing I can think of this can be used is something along these lines:
var settings = ConfigurationSection.AppSettings;
var connectionString = settings<string>["connectionString"];
var timeout = settings<int>["timeout"];
But this doesn't actually buy you anything. You've just replaced round parentheses (as in (int)settings["timeout"]) with angle brackets, but received no additional type safety as you can freely do
var timeout = settings<int>["connectionString"];
If you have something that's strongly but not statically typed, you might want to wait until C# 4.0 with its dynamic keyword.
I like the ability to have an indexer without handing out
a direct reference to the "indexed" item. I wrote a simple
"call back" Indexer class below ...
R = the returned type from the indexer
P = the passed type into the indexer
All the indexer really does is pass the operations to
the deployer and allow them to manage what actually occurs
and gets returned.
public class GeneralIndexer<R,P>
{
// Delegates
public delegate R gen_get(P parm);
public delegate void gen_set(P parm, R value);
public delegate P[] key_get();
// Events
public event gen_get GetEvent;
public event gen_set SetEvent;
public event key_get KeyRequest;
public R this[P parm]
{
get { return GetEvent.Invoke(parm); }
set { SetEvent.Invoke(parm, value); }
}
public P[] Keys
{
get
{
return KeyRequest.Invoke();
}
}
}
To use it in a program or class:
private GeneralIndexer<TimeSpan, string> TimeIndex = new GeneralIndexer<TimeSpan,string>();
{
TimeIndex.GetEvent += new GeneralIndexer<TimeSpan, string>.gen_get(TimeIndex_GetEvent);
TimeIndex.SetEvent += new GeneralIndexer<TimeSpan, string>.gen_set(TimeIndex_SetEvent);
TimeIndex.KeyRequest += new GeneralIndexer<TimeSpan, string>.key_get(TimeIndex_KeyRequest);
}
works like a champ especially if you want to monitor access to
your list or do any special operations when something is accessed.
In recent C-sharp you can declare the return type as "dynamic". This is the same as using "object" except that the C# runtime will allow you to use it in code as if it was the type you think it is and then check at run-time to be sure you were right...