I have the following scenario:
/* Attribute to be filled */
public class Flower
{
public FlowerType Type { get; set; }
public string Value { get; set; }
}
public enum FlowerType
{
Rose,
Daisy,
Tulip
}
[AttributeUsage(AttributeTargets.Class)]
public sealed class FlowerAttribute : Attribute
{
public string FlowerValue { get; }
public FlowerAttribute(string flowerValue)
{
FlowerValue = flowerValue;
}
}
What I am trying to achieve is a way to give the class Flower an attribute depending on runtime, meaning that if the flower will be instantiated as a Rose type, then the attribute on top of Flower will have the FlowerValue set to "Rose". Can I achieve this somehow? Thanks as always in advance!
Can I achieve this somehow?
No. Attribute arguments are always decided at compile-time - that's why they have to be compile-time constants. The values are baked into the IL.
It's not clear what the bigger goal is here, but anything dynamic is unlikely to be a good fit for attributes.
Related
Lets say we have this example hierarchy:
abstract class Shape { }
class Circle : Shape
{
public int Radius { get; set; }
}
class Rectangle : Shape
{
public int A { get; set; }
public int B { get; set; }
}
I am wondering if it's possible to use pattern matching in C# on a variable of type Shape to determine if it has property Radius e.g.
// Imaginary syntax
if (myShape is { int Radius} )
{
}
I have not found that this is possible, so I assume the answer is no, but probably someone knows better.
Alternatives that I can think of:
Enumerate all the known scenarios (i.e. of Type Circle, etc.), downside is that an introduction of a new type in the future will break it
Introducing an interface, which might be cumbersome depending on the scenario
Using reflection (not too nice)
I have a class Thing that is deserialized from a JSON file using DataContractJsonSerializer:
[DataContract]
class Thing
{
[DataMember]
public int Id { get; set; }
[DataMember]
public string Name { get; set; }
// and so on...
}
To make this work, all properties need to have public or at least internal setters.
Two other modules (assemblies) need to access this class:
ThingView should have read-only access to most attributes.
ThingEditor needs full access to all attributes.
I could make the setters internal but then the ThingEditor would not get full access.
To restrict access for the ThingView class, the best idea I could come up with is to create a read-only wrapper of the Thing class. But this would involve a lot of code duplication.
Are there better ways to achieve more encapsulation for in a case like this?
(The actual code contains about 20 classes of this type, some of which form nested structures. Also, there are more than two modules that need either read or full access to the properties.)
There is nothing out of the box in the specs that would achieve this. However, you could do something like this for example
public interface IReadOnlInterface
{
int Id { get; }
string Name { get; }
}
public interface IInterface
{
int Id { get; set; }
string Name { get; set; }
}
internal class Thing : IInterface , IReadOnlInterface
{
public int Id { get; set; }
public string Name { get; set; }
}
However there is nothing stopping coders with dirty little fingers casting to the other interface. If its only a runtime issue, then interfaces should be fine. Otherwise youll have to create a better api design with secured data, and proxy type objects with readonly access, Though maybe someone has a better idea.
I have a question and am getting a little stumped on this one. It's probably easy but I'm just missing it.
If I have a class what contains necessary structures and the structures can inherit. I need a generic method what that an be passed to and then used. For example, let's say I have a structure Test, Test-A which inherits from Test. Then I have a MyTest and MyTest-B both which inherit from Test-A.
How can I create a method using T so that I can supply both MyTest and MyTest-B and I can use either of them in the method?
So:
public class Test
{
public int Hello {get; set; }
}
pubilc class Test-A : Test
{
public string Name { get; set; }
}
public class MyTest : Test-A
{
public string Last { get; set; }
}
public class MYTest-B : Test-A
{
public int Age {get; set; }
}
I need a method like:
private void MyList<T>(List<T> TestList) where T : **{not sure what this would be}**
{
TestList.Age = 10;
**OR**
TestList.Name = "Jane";
}
How or what am I missing (or not understanding) to be able to do that?
Any assistance would be greatly appreciated.
There is no constraint you could use which would match both of those conditions. The only way to handle this would be to use two, overloaded methods, as there is no shared contract.
Generic constraints only work if there is a base class or interface shared which all types implement, and use the same contract (ie: both have Name and Age properties).
How can I create a method using T so that I can supply both MyTest and
MyTest-B and I can use either of them in the method?
**TestList.Age = 10;**
...
How would this work if you passed in an object of type MyTest, given that MyTest doesn't have an Age property?
When I define an interface that contains a write-only property:
public interface IModuleScreenData
{
string Name { set; }
}
and attempt to (naively) implement it explicitly with an intention for the property to also have a publicly available getter:
public class ModuleScreen : IModuleScreenData
{
string IModuleScreenData.Name { get; set; }
}
then I get the following error:
Error 'IModuleScreenData.Name.get' adds an accessor not found in
interface member 'IModuleScreenData.Name'
The error is more or less expected, however, after this alternative syntax:
public class ModuleScreen : IModuleScreenData
{
public string Name { get; IModuleScreenData.set; }
}
has failed to compile, I suppose that what I am trying to do is not really possible. Am I right, or is there some secret sauce syntax after all?
You can do this:
public class ModuleScreen : IModuleScreenData
{
string IModuleScreenData.Name
{
set { Name = value; }
}
public string Name { get; private set; }
}
On a side note, I generally wouldn't recommend set-only properties. A method may work better to express the intention.
You can't change the how the interface is implemented in the inheriting class. That is the whole point.. if you need to do something new with a property you could make a new property that references the inherited properties specific implementation. Interfaces are there so you can conform to a specified standard for object inheritance.
UPDATE:
On second thought.. you should be able to just do this.... this will compile fine:
public class ModuleScreen : IModuleScreenData
{
public string Name { get; set; }
}
This might be a simple one, but my head is refusing to wrap around that, so an outside view is always useful in that case!
I need to design an object hierarchy to implement a Parameter Registration for a patient. This will take place on a certain date and collect a number of different parameters about a patient (bloodpressure, heartrate etc). The values of those Parameter Registrations can be of different types, such as strings, integers, floats or even guids (for lookup lists).
So we have:
public class ParameterRegistration
{
public DateTime RegistrationDate { get; set; }
public IList<ParameterRegistrationValue> ParameterRegistrationValues { get; set; }
}
public class ParameterRegistrationValue
{
public Parameter Parameter { get; set; }
public RegistrationValue RegistrationValue { get; set; } // this needs to accomodate the different possible types of registrations!
}
public class Parameter
{
// some general information about Parameters
}
public class RegistrationValue<T>
{
public RegistrationValue(T value)
{
Value = value;
}
public T Value { get; private set; }
}
UPDATE: Thanks to the suggestions, the model has now morphed to the following:
public class ParameterRegistration
{
public DateTime RegistrationDate { get; set; }
public IList<ParameterRegistrationValue> ParameterRegistrationValues { get; set; }
}
public abstract class ParameterRegistrationValue()
{
public static ParameterRegistrationValue CreateParameterRegistrationValue(ParameterType type)
{
switch(type)
{
case ParameterType.Integer:
return new ParameterRegistrationValue<Int32>();
case ParameterType.String:
return new ParameterRegistrationValue<String>();
case ParameterType.Guid:
return new ParameterRegistrationValue<Guid>();
default: throw new ArgumentOutOfRangeException("Invalid ParameterType: " + type);
}
}
public Parameter Parameter { get; set; }
}
public class ParameterRegistrationValue<T> : ParameterRegistrationValue
{
public T RegistrationValue {get; set; }
}
public enum ParameterType
{
Integer,
Guid,
String
}
public class Parameter
{
public string ParameterName { get; set; }
public ParameterType ParameterType { get; set;}
}
which is indeed a bit simpler, but now I'm wondering, since the IList in ParameterRegistration points to the abstract ParameterRegistrationValue object, how will I be able to get the actual value out (since its stored on the sub-objects)?
Maybe the whole generic thing is indeed not quite the way to go after all :s
If you don't know the final set of parameter and the corresponding type of each parameter then the generics probably won't help - use object as a parameter value type.
Furthermore iterating through the list of parameters will be a pain since you'll have to examine the type of each item in order to determine how to treat the value.
What are you trying to achieve with generics ? Yes, they are cool (and going for boxing/unboxing is probably not a best idea), but in some cases you might want to use object instead (for both simplicity and flexibility).
-- Pavel
What you might want to introduce is an abstract base class for RegistrationValue<T> that is not generic, so that your ParameterRegistrationValue class can hold a non-generic reference, without needing knowledge of the type involved. Alternatively, it may be appropriate to make ParameterRegistrationValue generic also, and then add a non-generic base class for it instead (so that the list of values in ParameterRegistration can be of different types.
1st way:
public abstract class RegistrationValue
{
}
public class RegistrationValue<T> : RegistrationValue
{
public RegistrationValue(T value)
{
Value = value;
}
public T Value { get; private set; }
}
And now your code should compile.
Once you have a non-generic base class, I'd also move any members of the generic class that don't depend on the generic type parameters up into this base class. There aren't any in this example, but if we were instead modifying ParameterRegistrationValue to be generic, I'd move Parameter up into the non-generic base class (because it doesn't depend on the type parameter for RegistrationValue)
May be, you should use public RegistrationValue RegistrationValue, where T - is type, using in generic. For example, T - is String or other class or struct.
Or you should make class ParameterRegistrationValue as generic, to use generic argument in the field RegistrationValue.
I believe you want to have a collection of instances of different RegistrationValues-derived classes and be able to iterate it and for to have different type for each element. That's rather impossible.
You'll still need to cast each element to the type you know it is, because iterating the collection will return references to your base type (ParameterRegistrationValue - this one specified by IList type parameter). So it won't make any real difference from iterating over non-generic object list.
And if you can safely do that casting for each parameter (you know all the types), you probably don't need collection like this at all - it'll be better to have a class that encapsulates all the parameters in one type, so that you can call it with strong types, with IntelliSense etc. like this:
public class ParameterRegistration
{
public DateTime RegistrationDate { get; set; }
public PatientData PatientData { get; set; }
public Guid Identifier { get; set; }
// ...
}