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What is the best way to define a static property which is defined once per sub-class?

I wrote the following console app to test static properties:
using System;
namespace StaticPropertyTest
{
public abstract class BaseClass
{
public static int MyProperty { get; set; }
}
public class DerivedAlpha : BaseClass
{
}
public class DerivedBeta : BaseClass
{
}
class Program
{
static void Main(string[] args)
{
DerivedBeta.MyProperty = 7;
Console.WriteLine(DerivedAlpha.MyProperty); // outputs 7
}
}
}
As this console app demonstrates, the MyProperty property exists once for all instances of BaseClass. Is there a pattern to use which would allow me to define a static property which will have allocated storage for each sub-class type?
Given the above example, I would like all instances of DerivedAlpha to share the same static property, and all instances of DerivedBeta to share another instance of the static property.
Why am I trying to do this?
I am lazily initializing a collection of class property names with certain attributes (via reflection). The property names will be identical for each derived class instance, so it seems wasteful to store this in each class instance. I can't make it static in the base class, because different sub-classes will have different properties.
I don't want to replicate the code which populates the collection (via reflection) in each derived class. I know that one possible solution is to define the method to populate the collection in the base class, and call it from each derived class, but this is not the most elegant solution.
Update - Example of what I'm doing
At Jon's request, here's an example of what I'm trying to do. Basically, I can optionally decorate properties in my classes with the [SalesRelationship(SalesRelationshipRule.DoNotInclude)] attribute (there are other attributes, this is just a simplified example).
public class BaseEntity
{
// I want this property to be static but exist once per derived class.
public List<string> PropertiesWithDoNotInclude { get; set; }
public BaseEntity()
{
// Code here will populate PropertiesWithDoNotInclude with
// all properties in class marked with
// SalesRelationshipRule.DoNotInclude.
//
// I want this code to populate this property to run once per
// derived class type, and be stored statically but per class type.
}
}
public class FooEntity : BaseEntity
{
[SalesRelationship(SalesRelationshipRule.DoNotInclude)]
public int? Property_A { get; set; }
public int? Property_B { get; set; }
[SalesRelationship(SalesRelationshipRule.DoNotInclude)]
public int? Property_C { get; set; }
}
public class BarEntity : BaseEntity
{
public int? Property_D { get; set; }
[SalesRelationship(SalesRelationshipRule.DoNotInclude)]
public int? Property_E { get; set; }
public int? Property_F { get; set; }
}
Desired end result
Accessing FooEntity.PropertiesWithDoNotInclude returns a List<string> of:
{
"Property_A",
"Property_C"
}
Accessing BarEntity.PropertiesWithDoNotInclude returns a List<string> of:
{
"Property_E"
}

Two possible approaches:
Use attributes; decorate each subclass with an attribute, e.g.
[MyProperty(5)]
public class DerivedAlpha
{
}
[MyProperty(10)]
public class DerivedBeta
{
}
That only works when they're effectively constants, of course.
Use a dictionary:
var properties = new Dictionary<Type, int>
{
{ typeof(DerivedAlpha), 5) },
{ typeof(DerivedBeta), 10) },
};
EDIT: Now that we have more context, Ben's answer is a really good one, using the way that generics work in C#. It's like the dictionary example, but with laziness, thread-safety and simple global access all built in.

Jon has a good solution as usual, although I don't see what good attributes do here, since they have to be explicitly added to every subtype and they don't act like properties.
The Dictionary approach can definitely work. Here's another way to do that, which explicitly declares that there will be one variable per subclass of BaseEntity:
class FilteredProperties<T> where T : BaseEntity
{
static public List<string> Values { get; private set; }
// or static public readonly List<string> Values = new List<string>();
static FilteredProperties()
{
// logic to populate the list goes here
}
}
The drawback of this is that it's rather difficult to pair with a GetType() call such as you might use in methods of BaseEntity. A Dictionary, or wrapper thereto which implements lazy population, is better for that usage.

I just recently needed this same thing and came across this question. I think Jon's and Fried's ideas to use a Dictionary are on the right track but don't quite hit what I was looking for so I thought I'd show my own complete and very easy to extend implementation.
public class TypeStaticProperty<T>
{
T _defaultValue;
Dictionary<Type, T> _values = new Dictionary<Type, T>();
public TypeStaticProperty(T defalutValue = default)
{
_defaultValue = defalutValue;
}
public T Get(object caller)
{
lock (_values)
{
if (_values.TryGetValue(caller?.GetType(), out T val))
return val;
else
return _defaultValue;
}
}
public void Set(object caller, T val)
{
lock (_values)
_values[caller?.GetType()] = val;
}
}
And to demonstrate:
class TestBaseClass
{
static TypeStaticProperty<int> _property = new TypeStaticProperty<int>();
public int Property
{
get => _property.Get(this);
set => _property.Set(this, value);
}
}
class TestClass1 : TestBaseClass
{
}
class TestClass2 : TestBaseClass
{
}
class Program
{
static void Main(string[] args)
{
TestClass1 test1a = new TestClass1();
TestClass1 test1b = new TestClass1();
test1a.Property = 1;
test1b.Property = 2;
TestClass2 test2a = new TestClass2();
TestClass2 test2b = new TestClass2();
test2a.Property = 3;
test2b.Property = 4;
Console.WriteLine($"test1a.Property = {test1a.Property}");
Console.WriteLine($"test1b.Property = {test1b.Property}");
Console.WriteLine($"test2a.Property = {test2a.Property}");
Console.WriteLine($"test2b.Property = {test2b.Property}");
}
}
Output:
test1a.Property = 2
test1b.Property = 2
test2a.Property = 4
test2b.Property = 4
So while you still need a class instance to access and set the property, the value will always be the same across all instances of that precise type. (This includes generics too; Foo<int> will be seen as a different type than Foo<string>). This has the huge advantage over Fried's example in that you don't need to know at compile time the precise type whose "static" value you're looking for when accessing or setting.
PS - For full disclosure, this was heavily inspired by the WPF source code, which uses a very similar pattern for DependencyProperty's and all kinds of other internal bells and whistles designed to improve performance and reduce memory footprint.

Get custom attribute from SubClass in BaseClass (C# .NET 4.0)

Ok, edited the code for clarification:
Question: How can I access the attribute [MyAttr("...")] in TestClassOne/Two from BaseClass.TheAttribute...?
All classes except TestClassOne/Two will be compiled in to my "core" and delivered as a dev-platform to a customer.
The TestClassOne/Two is developed by the customer, so there can be no knowledge of the TestClassOne/Two in the "core".
Code below is compiled into "core" and delivered to customer as dll.
[TestMethod()]
public void AttrTest()
{
var one = new TestClassOne();
var attrOne = one.MyTestProperty.TheAttribute;
var two = new TestClassTwo();
var attrTwo = two.MyTestProperty.TheAttribute;
}
public class MyAttr : Attribute
{
private string _test;
public MyAttr(string test)
{
this._test = test;
}
}
public class BaseClass
{
public string TheAttribute
{
get {
// Here I would like to get the "[MyAttr("...")]" from the classes in the bottom
return null;
}
}
}
public class SubClass : BaseClass
{
}
Code below is developed by customer (using my dll's)
public class TestClassOne
{
[MyAttr("Attribute one")]
public SubClass MyTestProperty = new SubClass();
}
public class TestClassTwo
{
[MyAttr("Attribute two")]
public SubClass MyTestProperty = new SubClass();
}

You can get directly from type Test:
var result = typeof (Test)
.GetField("MyTest", BindingFlags.Public | BindingFlags.Instance)
.GetCustomAttribute<MyAttr>();

Edit 3:
You can walk the call stack, looking for a relevant attribute in a relevant member in a relevant class. Try this:
public class MyAttr : Attribute
{
private string _test;
public MyAttr(string test)
{
this._test = test;
}
public string getAttr()
{
return _test;
}
}
public class BaseClass
{
private string theString;
public BaseClass()
{
StackTrace callStack = new StackTrace();
for ( int i = 0; i < callStack.FrameCount; i++ )
{
Type t = callStack.GetFrame(i).GetMethod().DeclaringType;
foreach ( MemberInfo m in t.GetMembers().Where(x => typeof(BaseClass).IsAssignableFrom(x.Type)) )
{
foreach ( var z in m.GetCustomAttributes(typeof(MyAttr)) )
{
MyAttr theAttr = z as MyAttr;
if ( z!= null )
{
theString = z.getAttr();
return;
}
}
}
}
}
public string Test
{
get {
return theString;
}
}
}
This requires that your customer always initializes the SubClass member inside the class that declares it. If they start deriving TestClassOne or have it and TestClassTwo derive from a common class that initializes the member, this code will break.
With clever use of reflection, you can expand the above code to cover more use cases, but that's beyond the scope of this question.
Edit 2:
No. I'm sorry, but what you're trying to do isn't possible. There's no "normal" way for an instance of SubClass to know if it's being declared in a member field of some other object, or in an element in an array or in a temporary variable in the stack, or whatever. As such, there's no way for that instance to access the attributes of the member field that's declaring it.
(I suppose you might want to try to access the garbage collector to find out where in memory the this object lives, but that's probably way beyond the scope of this problem, and in any case, not something I know how to do.)
I suspect your problem lies elsewhere entirely. Maybe you need to require your customer to make TestClassOne and TestClassTwo derive from a common abstract class. Maybe they need to derive from BaseClass themselves. Maybe you need to add parameters to the constructor. Maybe you need to provide a different interface altogether. We can't know unless you provide more information on your specific business requirements.
Edit:
To access the attributes declared on the MyTest member, try something along these lines:
public class BaseClass
{
public string Test
{
get {
var attr = typeof(Test).GetMembers().Where(x => x.Type == this.GetType()).First().GetCustomAttributes(true);
return null;
}
}
}
This will search class Test for a member with the same type as this and look for attributes on that member.
(I don't have my Visual Studio here, to check the exact Where syntax, but it should be pretty close to that...)
Original Answer:
Your attribute is declared on the MyTest member of class Test. But, you're doing GetCustomAttributes on class SubClass itself.
Try this:
[MyAttr("apa")]
public class SubClass : BaseClass
{
}
public class Test
{
public SubClass MyTest = new SubClass();
}
Should get you what you want.

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't define static abstract string property

I've run into an interesting problem and am looking for some suggestions on how best to handle this...
I have an abstract class that contains a static method that accepts a static string that I would like to define as an abstract property. Problem is that C# doesn't doesn't support the following (see the ConfigurationSectionName and Current properties):
public abstract class ProviderConfiguration : ConfigurationSection
{
private const string _defaultProviderPropertyName = "defaultProvider";
private const string _providersPropertyName = "providers";
protected static string ConfigurationSectionName { get; }
public static Configuration Current
{
get { return Configuration)ConfigurationManager.GetSection(ConfigurationSectionName); }
}
}
I suppose one way to handle this would be to make ConfigurationSectionName NOT abstract and then create a new definition of ConfigurationSectionName in the derived classes, but that feels pretty hackish. Any suggestions would be most welcome.
Gratias!!!

Static members do not have polymorphism, so they can't be abstract. :(
If that's what you need, consider making a Singleton object, and reading the property off that object.

Just use new to override a static method in a derived class. Nothing that makes new a bad thing to do for virtual methods and properties applies since the type name must be supplied:
public class BaseClass
{
public static int Max { get { return 0; } }
}
public class InteriorClass : BaseClass
{
}
public class DerivedClass : InteriorClass
{
public new static int Max { get { return BaseClass.Max + 1; } }
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine("BaseClass.Max = {0}", BaseClass.Max);
Console.WriteLine("InteriorClass.Max = {0}", InteriorClass.Max);
Console.WriteLine("DerivedClass.Max = {0}", DerivedClass.Max);
Console.ReadKey();
}
}

Ok, this is not exactly to create an static abstract property, but you can achieve the desired effect.
You can get this by using generics:
public abstract class MyAbstractClass<T>
{
public static string MyAbstractString{ get; set; }
public static string GetMyAbstracString()
{
return "Who are you? " + MyAbstractString;
}
}
public class MyDerivedClass : MyAbstractClass<MyDerivedClass>
{
public static new string MyAbstractString
{
get
{
return MyAbstractClass<MyDerivedClass>.MyAbstractString;
}
set
{
MyAbstractClass<MyDerivedClass>.MyAbstractString = value;
}
}
}
public class MyDerivedClassTwo : MyAbstractClass<MyDerivedClassTwo>
{
public static new string MyAbstractString
{
get
{
return MyAbstractClass<MyDerivedClassTwo>.MyAbstractString;
}
set
{
MyAbstractClass<MyDerivedClassTwo>.MyAbstractString = value;
}
}
}
public class Test
{
public void Test()
{
MyDerivedClass.MyAbstractString = "I am MyDerivedClass";
MyDerivedClassTwo.MyAbstractString = "I am MyDerivedClassTwo";
Debug.Print(MyDerivedClass.GetMyAbstracString());
Debug.Print(MyDerivedClassTwo.GetMyAbstracString());
}
}
So, calling the test class you will get:
"Who are you? I am MyDerivedClass"
"Who are you? I am MyDerivedClassTwo"
So, you have an static method in an abstract class but the abstract value is different for each derived class, nice :D
Ok, so, what's going here? The trick is the generic tag, the compiler is generating a different abstract class for each derived type.
As I said it's not an abstract property, but you get all benefits of abstract static properties, which are programming static functions on your abstract class but using different static parameters per type.

Elsewhere on this page, #Gusman proposes the nice solution distilled here:
abstract class AbstractBase { };
abstract class AbstractBase<T> : AbstractBase
{
public static String AbstractStaticProp { get; set; }
};
class Derived1 : AbstractBase<Derived1>
{
public static new String AbstractStaticProp
{
get => AbstractBase<Derived1>.AbstractStaticProp;
set => AbstractBase<Derived1>.AbstractStaticProp = value;
}
};
class Derived2 : AbstractBase<Derived2>
{
public static new String AbstractStaticProp
{
get => AbstractBase<Derived2>.AbstractStaticProp;
set => AbstractBase<Derived2>.AbstractStaticProp = value;
}
};
Moving the static property from a non-generic to generic class means there is no longer necessarily a single global instance. There will be a unique AbstractStaticProp for each distinct type T, so the idea is that specifying the type of the derived class(es) themselves for T guarantees each of them generates a unique static for themselves. There are a few hazards to note with this, however.
If for some reason it is not acceptable for AbstractBaseClass to be generic, then you've only moved the problem elsewhere (albeit more clearly distilled), because you still have to figure out how to statically call from AbstractBase to AbstractBase<T>.
Mainly, there is nothing to enforce or require that any/every given derived class actually does "implement" the (psudo-) "overridden" static property;
Related to this, since there is no compiler (polymorphic) unification going on here, correct signatures (method name, parameter arity, typing, etc.) for the "overridden" methods aren't enforced either.
Although the generic parameter is intended to be "TSelf" of a derived class, in reality T is unconstrained and essentially arbitrary. This opportunizes two new classes of bug: if base class specification Y : AbstractBase<...> mistakenly references a different AbstractBase‑der­ived class X, the values of the "abstract static property" for X and Y will be incorrectly conflated -- and/or -- any usage call-site AbstractBase<T>.AbstractStaticProp with a mistaken type argument (such as DateTime) will spontaneously--and silently--demand a fresh new "instance" of the static property.
The last bullet point can be somewhat mitigated by adding a constraint on the generic base:
/// v---- constraint added
abstract class AbstractBase<TSelf> where TSelf : AbstractBase<TSelf>
{
public static String AbstractStaticProp { get; set; }
};
This eliminates the possibility of class Derived2 : AbstractBase<DateTime> { /*...*/ }, but not the error class Derived2 : AbstractBase<Derived1> { /*...*/ }. This is due to a recurring conundrum that foils all attempts at constraining a generic type to some exact branch of the type-inheritance hierarchy:
The "TSelf problem"
Generic constraints are always at the mercy of the type arguments that are supplied, which seems to entail that it's impossible to construct a generic constraint that guarantees that some particular TArg within its scope refers to a type that is derived from itself, that is, the immediate type being defined.
The error in this case is an example of this; while the constraint on AbstractBase<TSelf> rules out incompatible disjoint types, it can't rule out the unintended usage Derived2 : AbstractBase​<Derived1>. As far as AbstractBase is concerned, the supplied type argument Derived1 satisfies its constraint just fine, regardless of which of its subtypes is deriving itself (im-)properly. I've tried everything, for years, to solve TSelf; if anyone knows a trick I've missed, please let me know!
Anyway, there are still a couple other points to mention. For example, unless you can immediately spot the problem in the following code, you'll have to agree that it's a bit dangerous:
public static new String AbstractStaticProp
{
get => AbstractBase<Derived1>.AbstractStaticProp;
set => AbstractBase<Derived2>.AbstractStaticProp = value;
}
Ideally, you want to get the compiler to do what it's meant to, namely, understand that all AbstractStaticProp property instances are related and thus somehow enforce their unification. Since that's not possible for static methods, the only remaining option is to eliminate the extra versions, effectively reducing the problem to the unification of just one, a vacuous operation, obviously.
It turns out that the original code is being too elaborate; the generic-base class approach wants to collapse on the simpler solution all by itself without having to explicitly request it, such as those new-marked properties seem to be doing with the qualification in AbstractBase<Derived1>.​AbstractStaticProp".
You can already refer to each respective independent copy of the static property by qualifying with the derived class name instead (in fact, #Gusman's test harness shows this), so the end result is that the property declarations in the derived class aren't necessary at all. Without further ado, here is the complete simplified version:
abstract class AbstractBase { };
abstract class AbstractBase<TSelf> : AbstractBase
where TSelf : AbstractBase<TSelf>
{
public static String AbstractStaticProp { get; set; }
};
class Derived1 : AbstractBase<Derived1> { };
class Derived2 : AbstractBase<Derived2> { };
This works identically to the code at the top. The test harness gives the same results as before.
static void Test()
{
Derived1.AbstractStaticProp = "I am Derived1";
Derived2.AbstractStaticProp = "I am Derived2";
Debug.Print(Derived1.AbstractStaticProp); // --> I am Derived1
Debug.Print(Derived2.AbstractStaticProp); // --> I am Derived2
}

What you're trying to do is impossible, as others have mentioned.
I'd try something like this
public abstract class ProviderConfiguration : ConfigurationSection
{
public string ConfigurationSectionName { get; set; }
public static ProviderConfiguration Provider { get; set; }
public static Configuration Current
{
get { return (Configuration)ConfigurationManager.GetSection(Provider.ConfigurationSectionName); }
}
}
Then in practice:
public void DoStuff()
{
var provider = new DerivedProviderConfiguration();
ProviderConfiguration.Provider = provider;
}

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
{
}