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How can I access a static property of type T in a generic class?

I am trying to accomplish the following scenario that the generic TestClassWrapper will be able to access static properties of classes it is made of (they will all derive from TestClass). Something like:
public class TestClass
{
public static int x = 5;
}
public class TestClassWrapper<T> where T : TestClass
{
public int test()
{
return T.x;
}
}
Gives the error:
'T' is a 'type parameter', which is not valid in the given context.
Any suggestions?

You can't, basically, at least not without reflection.
One option is to put a delegate in your constructor so that whoever creates an instance can specify how to get at it:
var wrapper = new TestClassWrapper<TestClass>(() => TestClass.x);
You could do it with reflection if necessary:
public class TestClassWrapper<T> where T : TestClass
{
private static readonly FieldInfo field = typeof(T).GetField("x");
public int test()
{
return (int) field.GetValue(null);
}
}
(Add appropriate binding flags if necessary.)
This isn't great, but at least you only need to look up the field once...

Surely you can just write this:
public int test()
{
return TestClass.x;
}
Even in a nontrivial example, you can't override a static field so will always call it from your known base class.

Why not just return TestClass.x?

Generics do not support anything related to static members, so that won't work. My advice would be: don't make it static. Assuming the field genuinely relates to the specific T, you could also use reflection:
return (int) typeof(T).GetField("x").GetValue(null);
but I don't recommend it.

Another solution is to simply not make it static, and work with the new() constraint on T to instantiate the object. Then you can work with an interface, and the wrapper can get the property out of any class that implements that interface:
public interface XExposer
{
Int32 X { get; }
}
public class TestClass : XExposer
{
public Int32 X { get { return 5;} }
}
public class XExposerWrapper<T> where T : XExposer, new()
{
public Int32 X
{
get { return new T().X; }
}
}
In fact, you can change that to public static Int32 X on the TestClassWrapper and simply get it out as Int32 fetchedX = XExposerWrapper<TestClass>.X;
Though since whatever code calls this will have to give the parameter T those same constraints, the wrapper class is pretty unnecessary at this point, since that calling code itself could also just execute new T().X and not bother with the wrapper.
Still, there are some interesting inheritance models where this kind of structure is useful. For example, an abstract class SuperClass<T> where T : SuperClass<T>, new() can both instantiate and return type T in its static functions, effectively allowing you to make inheritable static functions that adapt to the child classes (which would then need to be defined as class ChildClass : SuperClass<ChildClass>). By defining protected abstract functions / properties on the superclass, you can make functions that apply the same logic on any inherited object, but customized to that subclass according to its implementations of these abstracts. I use this for database classes where the table name and fetch query are implemented by the child class. Since the properties are protected, they are never exposed, either.
For example, on database classes, where the actual fetching logic is put in one central abstract class:
public abstract class DbClass<T> where T : DbClass<T>, new()
{
protected abstract String FetchQuery { get; }
protected abstract void Initialize(DatabaseRecord row);
public static T FetchObject(DatabaseSession dbSession, Int32 key)
{
T obj = new T();
DatabaseRecord record = dbSession.RetrieveRecord(obj.FetchQuery, key);
obj.Initialize(record);
return obj;
}
}
And the implementation:
public class User : DbClass<User>
{
public Int32 Key { get; private set;}
public String FirstName { get; set;}
public String LastName { get; set;}
protected override String FetchQuery
{ get { return "SELECT * FROM USER WHERE KEY = {0}";} }
protected override void Initialize(DatabaseRecord row)
{
this.Key = DbTools.SafeGetInt(row.GetField("KEY"));
this.FirstName = DbTools.SafeGetString(row.GetField("FIRST_NAME"));
this.LastName = DbTools.SafeGetString(row.GetField("LAST_NAME"));
}
}
This can be used as:
User usr = User.FetchObject(dbSession, userKey);
This is a rather simplified example, but as you see, this system allows a static function from the parent class to be called on the child class, to return an object of the child class.

T is a type, not parameter or variable so you cannot pick any value from any members. Here is a sample code.
public class UrlRecordService
{
public virtual void SaveSlug<T>(T entity) where T : ISlugSupport
{
if (entity == null)
throw new ArgumentNullException("entity");
int entityId = entity.Id;
string entityName = typeof(T).Name;
}
}
public interface ISlugSupport
{
int Id { get; set; }
}

cjk and Haris Hasan have the most-correct answers to the question as asked. However in this comment the OP implies that he is after something else not quite possible in C#: a way to define a contract for a static member in a derived class.
There isn't a way to strictly define this, but it is possible to set up a pattern that may be implied by a base class (or interface); e.g.:
public class TestClass
{
private static int x;
public virtual int StaticX => x;
}
or if not intended to be used directly
public abstract class AbstractTestClass
{
public abstract int StaticX {get;}
}
or (my preference in this contrived example)
public interface ITest
{
int StaticX {get;}
}
Elsewhere, this pattern of a StaticXxx member may be (loosely) associated with implementations that should back the member with static fields (as in TestClass above).
What's kind of fun is that this can be (re)exposed as static by the generic wrapper, because generic statics are isolated to each type used.
public class TestClassWrapper<T> where T : ITest, new()
{
private readonly static T testInstance = new T();
public static int test() => testInstance.x;
}
This uses a new() condition, but an associated static, generic factory pattern for creating ITest (or TestClass or AbstractTestClass) instances may also be used.
However this may not be feasible if you can't have long-lived instances of the class.

In this situation you assume that T is a subclass of TestClass. Subclasses of TestClass will not have the static int x.

Can I force a subclass to declare a constant?

I want to force subclasses to define a constant value.
Like
const string SomeConstantEverySubclassMustDefine = "abc";
I need that because I need to have it tied to the Type, rather than to the instance and you can't override static Methods/Properties iirc.
I'd really like to have a compile-time check for those constants.
Let me explain in more detail:
Some classes in our Domain-Model are special, you can take certain actions for them, depending on the type. Thus the logic is tied to the type. The action to be taken requires a string tied to the type. I sure could create an instance everytime as a workaround and declare an abstract property, but that's not what I want. I want to enforce the declaration of the string at compile-time, just to be sure.

No, you can't. I would suggest you make your base class abstract, with an abstract property which you can fetch when you want. Each child class can then implement the property just by returning a constant if it wants. The downside is that you can't use this within static methods in the base class - but those aren't associated with the child classes anyway.
(It also allows child classes to customise the property per instance as well, if necessary... but that's rarely an actual problem.)
If this doesn't do enough for you, you might want to consider a parallel type hierarchy. Basically polymorphism simply doesn't happen in a type-specific way in .NET; only in an instance-specific way.
If you still want to do this and fetch it with reflection, I suggest you just write unit tests to ensure that the relevant constants are defined. When you get beyond what the type system can describe, that's often the best you can do.

Make an abstract property with only a get. That's what I think you could do to enforce a class has a value. Then you can just return a constant in the property.
Example:
Base class:
public abstract string MyConst { get; }
Derived class:
public override string MyConst {
get { return "constant"; }
}

Here is how I made mine work. I used Attribute as others have suggested.
public class ObjectAttribute : Attribute
{
public int ObjectSize { get; set; }
public ObjectAttribute(int objectSize)
{
this.ObjectSize = objectSize;
}
}
public abstract class BaseObject
{
public static int GetObjectSize<T>() where T : IPacket
{
ObjectAttribute[] attributes = (ObjectAttribute[])typeof(T).GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
}
[ObjectAttribute(15)]
public class AObject : BaseObject
{
public string Code { get; set; }
public int Height { get; set; }
}
[ObjectAttribute(25)]
public class BObject : BaseObject
{
public string Code { get; set; }
public int Weight { get; set; }
}
If you would like instance access to the attribute just add it to the base abstract class.
public abstract class BaseObject
{
public static int GetObjectSize<T>() where T : IPacket
{
ObjectAttribute[] attributes = (ObjectAttribute[])typeof(T).GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
public int ObjectSize
{
get
{
ObjectAttribute[] attributes = (ObjectAttribute[])GetType().GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
}
}
Usage of the constants
int constantValueA = AObject.GetObjectSize<AObject>();
int constantValueB = BObject.GetObjectSize<BObject>();
AObject aInstance = new AObject();
int instanceValueA = aInstance.ObjectSize;

New idea
Here's a sort of weird idea: instead of using inheritance directly, you create a separate class to provide a constant value for every type deriving from some type T. The constructor for this type uses reflection to verify that every derived type has indeed been supplied a value.
public abstract class Constant<T, TConstant>
{
private Dictionary<Type, TConstant> _constants;
protected Constant()
{
_constants = new Dictionary<Type, TConstant>();
// Here any class deriving from Constant<T, TConstant>
// should put a value in the dictionary for every type
// deriving from T, using the DefineConstant method below.
DefineConstants();
EnsureConstantsDefinedForAllTypes();
}
protected abstract void DefineConstants();
protected void DefineConstant<U>(TConstant constant) where U : T
{
_constants[typeof(U)] = constant;
}
private void EnsureConstantsDefinedForAllTypes()
{
Type baseType = typeof(T);
// Here we discover all types deriving from T
// and verify that each has a key present in the
// dictionary.
var appDomain = AppDomain.CurrentDomain;
var assemblies = appDomain.GetAssemblies();
var types = assemblies
.SelectMany(a => a.GetTypes())
.Where(t => baseType.IsAssignableFrom(t));
foreach (Type t in types)
{
if (!_constants.ContainsKey(t))
{
throw new Exception(
string.Format("No constant defined for type '{0}'.", t)
);
}
}
}
public TConstant GetValue<U>() where U : T
{
return _constants[typeof(U)];
}
}
Basic example:
public class BaseType
{
public static Constant<BaseType, string> Description { get; private set; }
static BaseType()
{
Description = new BaseTypeDescription();
}
}
public class DerivedType : BaseType
{ }
internal sealed class BaseTypeDescription : Constant<BaseType, string>
{
public BaseTypeDescription() : base()
{ }
protected override DefineConstants()
{
DefineConstant<BaseType>("A base type");
DefineConstant<DerivedType>("A derived type");
}
}
Now I have code that allows me to do this:
var description = BaseType.Description;
// returns "A base type"
string baseTypeDescription = description.GetValue<BaseType>();
// returns "A derived type"
string derivedTypeDescription = description.GetValue<DerivedType>();
Original answer
You may not like it, but the closest way to accomplish this is by declaring an abstract read-only (no set) property.
If you've got an instance of your subclass, then this can work just as well as a constant, even though it is technically instance-level (it will just be the same for all instances of the given class).
Consider, for instance, IList.IsReadOnly. In most cases this is actually a property that tells you about the underlying class implementation, as opposed to any state specific to a particular instance. (It may be an interface member as opposed to an abstract class member, but it's the same idea.)
If you are trying to access it statically, well... then you're out of luck. But in this case I fail to see how you'd obtain the value without using reflection anyway. Maybe that's your intention; I don't know.

You could have a static method in the base class called, for instance "Register", that is passed a Type and a constant value, with the intention being that it is called by the class constructors of the subtypes. Then, add a check in all of your base class constructors that the object being constructed is of a registered type.
abstract class Base
{
private static Dictionary<Type, string> _registry = new Dictionary<Type, string>();
protected static void Register(Type t, string constVal)
{
_registry.Add(t, constVal);
}
protected Base()
{
if(!_registry.ContainsKey(this.GetType()))
throw new NotSupportedException("Type must have a registered constant");
}
public string TypeConstant
{
get
{
return _registry[this.GetType()];
}
}
}
class GoodSubtype : Base
{
static GoodSubtype()
{
Base.Register(typeof(GoodSubtype), "Good");
}
public GoodSubtype()
: base()
{
}
}
class Badsubtype : Base
{
public Badsubtype()
: base()
{
}
}
And then elsewhere, you can construct GoodSubtype instances, but trying to construct a Badsubtype gets an exception. I think a runtime error at construction is the soonest you can get an error with this type of scheme.
(You'd want to use ConcurrentDictionary for your registry if threading is involved)

There's one other method that hasn't been covered and it uses the new modifier to hide consts values in the base class. In a way, it's similar to Nap's solution, but doesn't allow per-instance access and therefore doesn't allow for polymorphic access within the base class. This solution is only useful if you want to have constant value defined but wish to have the option of changing it to different values in different subclasses.
static void Main(string[] args)
{
Console.WriteLine("BaseClass.MyConst = {0}, ClassA.MyConst = {1}, ClassB.MyConst = {2}", BaseClass.MyConst, ClassA.MyConst, ClassB.MyConst);
Console.ReadKey();
}
class BaseClass
{
public const int MyConst = 1;
}
class ClassA : BaseClass
{
public new const int MyConst = 2;
}
class ClassB : BaseClass
{
}

Overloading bitwise operator while using interface-based development

Using C# 3.0, .NET 3.5
I have a need for an enumeration that is a little smarter than a simple number. I have worked around this with static classes and properties, but one thing I am missing is the ability to use the enumeration like a bit flag without accessing a specfic properity.
For example this works:
public interface isomething
{
int value { get; }
}
public class something : isomething
{
public int value {get; private set;}
public somthing(int value)
{
this.value = value
}
public void DoSomething()
{
throw new NotImplementedException();
}
}
public static class SuperEnum
{
public static isomething first = new something(1);
public static isomething second= new something(2);
}
Then execute something like this:
Assert.IsTrue((SuperEnum.first.value | SuperEnum.second.value) == 3);
And you get a true. But if I add this to the something class:
public static int operator | (something leftSide, isomething rightside)
{
return leftside.value | rightside.value
}
Then execute something like this:
Assert.IsTrue((SuperEnum.first | SuperEnum.second) == 3);
I get a message that | operator can not used on the isomething type which sucks.
If I go with concrete types, I am fine, but that is going to cause problems in the future.
Anyone got any ideas?
Thanks, mark

I am not sure what you're trying to achieve. I think there is probably a better solution to the problem, but if the operator is important you can do it using an abstract class instead of interface:
public abstract class SomethingBase
{
protected int value;
protected SomethingBase(int value)
{
this.value = value;
}
public static int operator |(SomethingBase leftSide, SomethingBase rightside)
{
return leftSide.value | rightside.value;
}
}
public class Something : SomethingBase
{
public int Value
{
get { return value; }
}
public Something(int value) : base(value)
{
}
public void DoSomething()
{
throw new NotImplementedException();
}
}

Your issue comes from the fact that the ISomething interface doesn't declare the operator, and I'm not sure that you can make an operator a part of an interface contract. What sort of problems do you expect to be caused by using the concrete class?

If you are doing binary math, it sounds like you are talking about a value. If you are talking about a value, an immutable concrete type would be a good choice (perhaps a struct, depending on the size). And if you have a concrete type, you can add a bespoke operator.
Alternatively you could add an Or method to the interface (or add an extension method to do the same), but this may involve the first operand choosing the concrete type... but then, I'm not sure what the interface is adding anyway.

C# splitting property get and set between classes. Why doesn't it work?

I'm trying to provide two classes to my users - one that is read-only and one that is writable. The r/o will only have getters, while the writable will inherit from it and add the setters.
I was under the impression that C# should be able to handle it, but the compiler disagreed.
Why doesn't this work? Any workarounds?
class A
{
protected int m_val;
public int Val
{
get { return m_val; }
}
}
class B : A
{
public int Val
{
set { m_val = value; }
}
}
class Test
{
static void Main(string[] args)
{
B b = new B();
b.Val++; // <-- WHY DOESN'T THIS WORK?!
}
}
P.S. the protected variable in the example above is artificial. My class actually wraps some native resources and the getting/setting happens on either const or mutable native pointer.

partial applies to a single type - not 2 types (A and B). You would need something more like below, ideally keeping the field private:
class A
{
private int m_val;
public int Val
{
get { return m_val; }
protected set { m_val = value; }
}
}
class B : A
{
public new int Val
{
get { return base.Val;}
set { base.Val = value; }
}
}

I'm not sure about why you need this, but a possibly better design would be to have two interfaces rather than two classes, and a single class that implements both. Then you could hand your client code whichever interface you'd like them to use, with the added bonus of being able to use the values set on a writable interface and hand it over to someone else as a read-only interface.

Mark the setters as protected in the parent, and expose public setters in the child.

Overriding fields or properties in subclasses

I have an abstract base class and I want to declare a field or a property that will have a different value in each class that inherits from this parent class.
I want to define it in the baseclass so I can reference it in a base class method - for example overriding ToString to say "This object is of type property/field".
I have got three ways that I can see of doing this, but I was wondering - what is the best or accepted way of doing this? Newbie question, sorry.
Option 1:
Use an abstract Property and override it on the inherited classes. This benefits from being enforced (you have to override it) and it is clean. But, it feels slightly wrong to return a hard-code value rather than encapsulate a field and it is a few lines of code instead of just. I also have to declare a body for "set" but that is less important (and there is probably a way to avoid that which I am not aware of).
abstract class Father
{
abstract public int MyInt { get; set;}
}
class Son : Father
{
public override int MyInt
{
get { return 1; }
set { }
}
}
Option 2
I can declare a public field (or a protected field) and explicitly override it in the inherited class. The example below will give me a warning to use "new" and I can probably do that, but it feels wrong and it breaks the polymorphism, which was the whole point. Doesn't seem like a good idea...
abstract class Mother
{
public int MyInt = 0;
}
class Daughter : Mother
{
public int MyInt = 1;
}
Option 3
I can use a protected field and set the value in the constructor. This seems pretty tidy but relies on me ensuring the constructor always sets this and with multiple overloaded constructors there is always a chance some code path won't set the value.
abstract class Aunt
{
protected int MyInt;
}
class Niece : Aunt
{
public Niece()
{
MyInt = 1;
}
}
It's a bit of a theoretical question and I guess the answer has to be option 1 as it is the only safe option but I am just getting to grips with C# and wanted to ask this of people with more experience.

Of the three solutions only Option 1 is polymorphic.
Fields by themselves cannot be overridden. Which is exactly why Option 2 returns the new keyword warning.
The solution to the warning is not to append the “new” keyword, but to implement Option 1.
If you need your field to be polymorphic you need to wrap it in a Property.
Option 3 is OK if you don’t need polymorphic behavior. You should remember though, that when at runtime the property MyInt is accessed, the derived class has no control on the value returned. The base class by itself is capable of returning this value.
This is how a truly polymorphic implementation of your property might look, allowing the derived classes to be in control.
abstract class Parent
{
abstract public int MyInt { get; }
}
class Father : Parent
{
public override int MyInt
{
get { /* Apply formula "X" and return a value */ }
}
}
class Mother : Parent
{
public override int MyInt
{
get { /* Apply formula "Y" and return a value */ }
}
}

Option 2 is a non-starter - you can't override fields, you can only hide them.
Personally, I'd go for option 1 every time. I try to keep fields private at all times. That's if you really need to be able to override the property at all, of course. Another option is to have a read-only property in the base class which is set from a constructor parameter:
abstract class Mother
{
private readonly int myInt;
public int MyInt { get { return myInt; } }
protected Mother(int myInt)
{
this.myInt = myInt;
}
}
class Daughter : Mother
{
public Daughter() : base(1)
{
}
}
That's probably the most appropriate approach if the value doesn't change over the lifetime of the instance.

You could do this
class x
{
private int _myInt;
public virtual int myInt { get { return _myInt; } set { _myInt = value; } }
}
class y : x
{
private int _myYInt;
public override int myInt { get { return _myYInt; } set { _myYInt = value; } }
}
virtual lets you get a property a body that does something and still lets sub-classes override it.

option 2 is a bad idea. It will result in something called shadowing; Basically you have two different "MyInt" members, one in the mother, and the other in the daughter. The problem with this, is that methods that are implemented in the mother will reference the mother's "MyInt" while methods implemented in the daughter will reference the daughter's "MyInt". this can cause some serious readability issues, and confusion later down the line.
Personally, I think the best option is 3; because it provides a clear centralized value, and can be referenced internally by children without the hassle of defining their own fields -- which is the problem with option 1.

You could define something like this:
abstract class Father
{
//Do you need it public?
protected readonly int MyInt;
}
class Son : Father
{
public Son()
{
MyInt = 1;
}
}
By setting the value as readonly, it ensures that the value for that class remains unchanged for the lifetime of the object.
I suppose the next question is: why do you need it?

If you are building a class and you want there to be a base value for the property, then use the virtual keyword in the base class. This allows you to optionally override the property.
Using your example above:
//you may want to also use interfaces.
interface IFather
{
int MyInt { get; set; }
}
public class Father : IFather
{
//defaulting the value of this property to 1
private int myInt = 1;
public virtual int MyInt
{
get { return myInt; }
set { myInt = value; }
}
}
public class Son : Father
{
public override int MyInt
{
get {
//demonstrating that you can access base.properties
//this will return 1 from the base class
int baseInt = base.MyInt;
//add 1 and return new value
return baseInt + 1;
}
set
{
//sets the value of the property
base.MyInt = value;
}
}
}
In a program:
Son son = new Son();
//son.MyInt will equal 2

You can go with option 3 if you modify your abstract base class to require the property value in the constructor, you won't miss any paths. I'd really consider this option.
abstract class Aunt
{
protected int MyInt;
protected Aunt(int myInt)
{
MyInt = myInt;
}
}
Of course, you then still have the option of making the field private and then, depending on the need, exposing a protected or public property getter.

I'd go with option 3, but have an abstract setMyInt method that subclasses are forced to implement. This way you won't have the problem of a derived class forgetting to set it in the constructor.
abstract class Base
{
protected int myInt;
protected abstract void setMyInt();
}
class Derived : Base
{
override protected void setMyInt()
{
myInt = 3;
}
}
By the way, with option one, if you don't specify set; in your abstract base class property, the derived class won't have to implement it.
abstract class Father
{
abstract public int MyInt { get; }
}
class Son : Father
{
public override int MyInt
{
get { return 1; }
}
}

I did this...
namespace Core.Text.Menus
{
public abstract class AbstractBaseClass
{
public string SELECT_MODEL;
public string BROWSE_RECORDS;
public string SETUP;
}
}
namespace Core.Text.Menus
{
public class English : AbstractBaseClass
{
public English()
{
base.SELECT_MODEL = "Select Model";
base.BROWSE_RECORDS = "Browse Measurements";
base.SETUP = "Setup Instrument";
}
}
}
This way you can still use fields.

The example implementation when you want to have an abstract class with implementation. Subclasses must:
Parameterize the implementation of an abstract class.
Fully inherit the implementation of the abstract class;
Have your own implementation.
In this case, the properties that are necessary for the implementation should not be available for use except for the abstract class and its own subclass.
internal abstract class AbstractClass
{
//Properties for parameterization from concrete class
protected abstract string Param1 { get; }
protected abstract string Param2 { get; }
//Internal fields need for manage state of object
private string var1;
private string var2;
internal AbstractClass(string _var1, string _var2)
{
this.var1 = _var1;
this.var2 = _var2;
}
internal void CalcResult()
{
//The result calculation uses Param1, Param2, var1, var2;
}
}
internal class ConcreteClassFirst : AbstractClass
{
private string param1;
private string param2;
protected override string Param1 { get { return param1; } }
protected override string Param2 { get { return param2; } }
public ConcreteClassFirst(string _var1, string _var2) : base(_var1, _var2) { }
internal void CalcParams()
{
//The calculation param1 and param2
}
}
internal class ConcreteClassSecond : AbstractClass
{
private string param1;
private string param2;
protected override string Param1 { get { return param1; } }
protected override string Param2 { get { return param2; } }
public ConcreteClassSecond(string _var1, string _var2) : base(_var1, _var2) { }
internal void CalcParams()
{
//The calculation param1 and param2
}
}
static void Main(string[] args)
{
string var1_1 = "val1_1";
string var1_2 = "val1_2";
ConcreteClassFirst concreteClassFirst = new ConcreteClassFirst(var1_1, var1_2);
concreteClassFirst.CalcParams();
concreteClassFirst.CalcResult();
string var2_1 = "val2_1";
string var2_2 = "val2_2";
ConcreteClassSecond concreteClassSecond = new ConcreteClassSecond(var2_1, var2_2);
concreteClassSecond.CalcParams();
concreteClassSecond.CalcResult();
//Param1 and Param2 are not visible in main method
}