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Is there a good way to handle private static fields in a generic type in C#?

I am trying to figure out a way I can make use of private static fields in a generic class. This is the obvious way to do it (fiddle). It won't compile because Field is not accessible in BaseChild, and ideally I wouldn't want it to be accessible there:
public class Base<T>
{
private static readonly string Field = "field";
public Base()
{
Console.WriteLine(Field);
}
}
public class BaseChild : Base<string>
{
public BaseChild()
{
Console.WriteLine(Field);
}
}
The problem with this solution is that there is a different Field for each generic type, instead of being shared across them.
I have seen this answer where it says that JetBrains recommends a solution for static fields across generic types:
If you need to have a static field shared between instances with different generic arguments, define a non-generic base class to store your static members, then set your generic type to inherit from this type.
This makes sense for the case where you have public or protected static fields in the base class that you want to share across any child class like this example (fiddle):
public abstract class Base
{
protected static readonly string Field = "field";
}
public class Base<T> : Base
{
public Base()
{
Console.WriteLine(Field);
}
}
public class BaseChild : Base<string>
{
public BaseChild()
{
Console.WriteLine(Field);
}
}
However, what about the case where you want to use a private static field? I would guess that this is not possible since private means only accessible to the class it's declared in and I think that since the generic class is really just a template to create a class, that any private field could only ever be shared by each class, not across all the classes created by the template.
Do I have to just put the private field in the generic class (example 1) and accept it as at least a workable solution for what I want, or is there another way I can accomplish this?

First off -- private is doing exactly what it's made to do: to restrict access to only the type it was declared in. Keep in mind that instantiations of a generic type are all distinct types. You shouldn't be wanting to work around this.
If I understand your question correctly, you can accomplish what you want by using protected with an extra level of inheritance:
class EvenMoreBase
{
protected static readonly string Field = "field";
}
class Base<T> : EvenMoreBase
{
public Base()
{
Console.WriteLine(Field);
}
}
class BaseChild : Base<string>
{
public BaseChild()
{
Console.WriteLine(Field);
}
}
Now each of your Base<T> will share the same instance of Field.

You're correct in your thoughts on private within the base class. Whether it is static or not makes no difference.
Here's a little example:
using System;
public class Program
{
public static void Main()
{
Bar b = new Bar(); // Prints "Foo"
// Console.WriteLine(Foo.BaseField); // Compile error
}
}
public class Foo
{
protected static readonly string BaseeField = "Foo";
}
public class Bar : Foo
{
public Bar()
{
Console.WriteLine(Foo.BaseeField);
}
}
Marking it protected is useful, if you'd like only your children to be able to access it. And leaving it static is how you'd keep only one instance around for all children of the base Foo class.

This is something I came up with that I think actually does what I want better than the initial example I put in my question. It shares a single static field across all the generic types, and it is inaccessible from children of the Base generic class.
public static class Base
{
private static string Field = "field";
public class Base2<T>
{
public Base2()
{
// Field is accessible here, but is the same across all generic classes
Console.WriteLine(Field);
}
}
}
public class BaseChild : Base.Base2<string>
{
public BaseChild()
{
//Field is not accessible here, and I don't really want it to be
//Console.WriteLine(Field);
}
}

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.

How to get value of abstract "const" property using reflection?

I've got a class defined like this:
public abstract class Uniform<T>
{
public abstract string GlslType { get; }
...
}
And then a subclass defined like this:
public class UniformInt : Uniform<int>
{
public override string GlslType
{
get { return "int"; }
}
}
And then a method somewhere else that looks like this:
public static string GetCode<T>()
{
var sb = new StringBuilder();
var type = typeof(T);
sb.AppendFormat("struct {0} {{\n", type.Name);
var fields = type.GetFields(BindingFlags.Public | BindingFlags.Instance);
foreach(var f in fields)
{
sb.AppendFormat(" {0} {1};\n", f.FieldType.GetProperty("GlslType").GetValue(???), f.Name);
}
...
}
I'm having trouble filling in the ???s. I believe GetValue expects an instance of the object, but I don't really care what instance it is because they all return the same value. And AFAIK there's no such thing as a public abstract static readonly value, so I have to use properties.
So what can I put in place of those ???s to get back "int" (assuming one the fields was a UniformInt).
As a side: How can I limit fields to only field types that inherit Uniform<>?

You need an instance of UniformInt in order to get the value of a non-static property:
UniformInt someUniformInt = ...
f.FieldType.GetProperty("GlslType").GetValue(someUniformInt, null)
As a side: How can I limit fields to only field types that inherit Uniform?
bool isDerivesFromUniformOfInt = typeof(Uniform<int>)
.IsAssignableFrom(f.FieldType);
or if you don't know the type of T in advance:
bool isDerivesFromUniformOfT = typeof(Uniform<>)
.MakeGenericType(typeof(T))
.IsAssignableFrom(f.FieldType);

The problem is that since your property is not static the compiler doesn't know that they all return the same value. Since your UniformInt is not sealed, another user could inherit from it and override GlslType to return something else. Then UniformInt and all derived classes could be used for your GetCode<T>() method.
A static method would really be the best option. To make sure that you implement them on all classes (something you can't force because static methods can't be abstract) I would write a simple unit test that uses reflection to load all classes that inherit from Uniform<T> and check if they have the static property defined.
UPDATE
When thinking about how Attributes could help and after some experimenting I came up with the following. It definitely won't win a beauty contest but as a learning exercise it was helpful ;)
using System;
using System.Linq;
namespace StackOverflow
{
internal class StackOverflowTest
{
private static void Main()
{
string sInt = UniformInt.GlslType;
string sDouble = UniformDouble.GlslType;
}
}
public abstract class Uniform<B, T> // Curiously recurring template pattern
where B : Uniform<B, T>
{
public static string GlslType
{
get
{
var attribute = typeof(B).GetCustomAttributes(typeof(GlslTypeAttribute), true);
if (!attribute.Any())
{
throw new InvalidOperationException(
"The GslType cannot be determined. Make sure the GslTypeAttribute is added to all derived classes.");
}
return ((GlslTypeAttribute)attribute[0]).GlslType;
}
}
}
[AttributeUsage(AttributeTargets.Class, Inherited = true, AllowMultiple = false)]
internal sealed class GlslTypeAttribute : Attribute
{
public string GlslType { get; private set; }
public GlslTypeAttribute(string glslType)
{
GlslType = glslType;
}
}
[GlslType("int")]
public class UniformInt : Uniform<UniformInt, int> // Curiously recurring template pattern
{
}
[GlslType("double")]
public class UniformDouble : Uniform<UniformDouble, double> // Curiously recurring template pattern
{
}
}

The GlslType is not static, so you need an object reference before you can access it's value. The subject of static properties in abstract classes has been covered extensively already, ie:
C#, implement 'static abstract' like methods
Can't define static abstract string property

Solution 1
Add static methods to all derived classes that return the GlslType. Nothing needs to be added to the base class. Can use unit testing + reflection to check for missing implementation. Suggested by Wouter de Kort.
Solution 2
Change Uniform<T> to make GlslType static:
public abstract class Uniform<T>
{
public static string GlslType { get { throw new NotImplementedException("Please override with \"new\" in derived class."); } }
...
}
Change UniformInt to "override" GlslType, keeping the static modifier:
public class UniformInt : Uniform<int>
{
public new static string GlslType
{
get { return "int"; }
}
}
Fill ??? with null, null:
sb.AppendFormat(" {0} {1};\n", f.FieldType.GetProperty("GlslType").GetValue(null,null), f.Name);
Solution 3
Use attributes instead. Something like:
[GlslType("int")]
public class UniformInt : Uniform<int>
{
}
Conclusion
All 3 of these solutions are pretty similar and seem to have the same drawbacks (can't enforce derived class to implement it). Throwing an exception via method 1 or 2 will help find errors quickly, or with 3 I can just skip over classes that don't have the attribute by modifying my fields condition.

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
{
}