I'm using entity framework database first and I faced a problem that I cannot solve elegantly. Let's say in my autogenerated entity I have some proprties that I don't want to use (or I don't want to allow somebody to use them). So I came up with a good idea to write an interface which provide only a part of properties from the entity and I would like to use interface reference rather than exact entity class. For example let's say that I have an Entity table in database and this is the generated code and my interface (I don't want to even know about RedundantString column in database):
public interface IEntity
{
int Id { get; }
Type NavigationProperty { get; }
}
internal class Entity : IEntity
{
public int Id { get; set; }
public string RedundantString { get; set; }
public virtual Type NavigationProperty { get; set; }
}
Everything is ok, however there is one problem. In my DbContext I have a property:
public virtual DbSet<Entity> Entities { get; set; }
As I said I do not want to have direct access to Entity type, I would rather use IEntity. So I want to pack my context in antother interface that would provide only property:
IDbSet<IEntity> Entities { get; }
Here is the problem. I can't implement that property using IDbSet<Entity> because IDbSet is not declared to be covariant, so common cast:
IDbSet<IEntity> IMyDbContext.Entities
{
get { return (IDbSet<IEntity>)Entities; }
}
is compiling fine but throws a runtime exception.
What can I do to solve that problem?
I have library with class in it looks like next:
[DataContract]
class MyBaseClass
{
public string Name { get; set; }
}
Also, I have another project with ref of this library and class in it looks like:
[DataContract]
class MyDeliveredClass : MyBaseClass
{
public string SecondName { get; set; }
}
I wonder how could I serialize list<MyBaseClass> which could contain objects of MyBaseClassand MyDeliveredClass ?
May be I should use [KnownType(MyDeliveredClass)]... but I have no reference to MyDeliveredClass in library...
If you declare both MyBaseClass and MyDeliveredClass as Service known types on the service contract interface then that should do the trick:
[ServiceContract(SessionMode = SessionMode.Required, ...)]
[ServiceKnownType(typeof(MyBaseClass ))]
[ServiceKnownType(typeof(MyDeliveredClass ))]
public interface IMySerivceContract {
...
}
Generally you have to choice between delclaring as KnownTypes on the class declaration or as ServerKnownTypes on the service interface. However sometimes the KnownType route is not an option for various reasons - one of which is your situation where you dont have access to a class declaration. Another case where you are forced to use ServiceKnownTypes is if your contract uses interfaces instead of the base class i.e if your contract had List<IMyBaseInterface>.
I have some classes with common properties, however, I cannot make them derive from a base type (LINQ-to-SQL limitations).
I would like to treat them as if they had a base type, but not by using Reflection (performance is critical).
For example:
public class User
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle
{
public int Id { get; set; }
public string Label { get; set; }
}
In this case I would be happy if I had the Id property available, regardless of the type I'm holding.
Is there any way in C# to to something similar to this:
public static int GetId<T>(T entity) where T // has an int property 'Id'
{
return entity.Id;
}
I guess I could have used dynamic, however, I'm looking for a way to restrict the code in compile time from using this method for an object that has no Id property.
You can use interfaces:
public interface IHasId
{
int Id { get; }
}
public class User : IHasId { ... }
public class Vehicle : IHasId { ... }
public static int GetId<T>(T entity) where T : IHasId
{
return entity.Id;
}
However, if you are not able to modify the classes to add the interface, you won't be able to do this. No compile-time checks will verify that a property exists on T. You'd have to use reflection - which is slow and obviously not ideal.
There is no way to guarantee a type has a given member without constraining to a common base type or interface. One way to work around this limitation is to use a lambda to access the value
public static int Use<T>(T value, Func<T, int> getIdFunc) {
int id = getIdFunc(value);
...
}
Use(new User(), u => u.Id);
Use(new Vehicle(), v => v.Id);
You can create an interface with the common properties and make your classes implement it:
public interface IEntity
{
int Id { get; set; }
}
public class User : IEntity
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle : IEntity
{
public int Id { get; set; }
public string Label { get; set; }
}
public static int GetId<T>(T entity) where T : IEntity
{
return entity.Id;
}
You could simplify GetId like this:
public static int GetId(IEntity entity)
{
return entity.Id;
}
The other answers mentioning the interface approach are certainly good, but I want to tailor the response to your situation involving Linq-to-SQL.
But first, to address the question title as asked
Can C# constraints be used without a base type?
Generally, the answer is no. Specifically, you can use struct, class, or new() as constraints, and those are not technically base types, and they do give some guidance on how the type can be used. That doesn't quite rise to the level of what you wish to do, which is to limit a method to types that have a certain property. For that, you will need to constrain to a specific interface or base class.
For your specific use case, you mention Linq-to-SQL. If you are working from models that are generated for you, then you should have options to modify those classes without modifying the generated model class files directly.
You probably have something like
// code generated by tool
// Customer.cs
public partial class Customer // : EntityBaseClasses, interfaces, etc
{
public int ID
{
get { /* implementation */ }
set { /* implementation */ }
}
}
And other similar files for things such as Accounts or Orders or things of that nature. If you are writing code that wishes to take advantage of the commonly available ID property, you can take utilize the partial in the partial class to define a second class file to introduce a common interface type to these models.
public interface IIdentifiableEntity
{
int ID { get; }
}
And the beauty here is that using it is easy, because the implementation already exists in your generated models. You just have to declare it, and you can declare it in another file.
public partial class Customer : IIdentifiableEntity { }
public partial class Account : IIdentifiableEntity { }
// etc.
This approach has proven valuable for me when using a repository pattern, and wishing to define a general GetById method without having to repeat the same boilerplate in repository after repository. I can constrain the method/class to the interface, and get GetById for "free."
Either you need to make both classes implement an interface with the properties you need, and use that in the generic constraint, or you write separate methods for each type. That's the only way you'll get compile-time safety.
We have the following classes and WCF service (using protobuf-net for serialization):
[DataContract]
[KnownType(typeof(NamedViewModel))]
public class NamedViewModel<TKey> : IViewModel
{
[DataMember]
public virtual TKey Id { get; set; }
[DataMember]
public virtual string Name { get; set; }
}
[DataContract]
[KnownType(typeof(ScheduleTemplateViewModel))]
public class NamedViewModel : NamedViewModel<int>
{
}
[DataContract]
public class ScheduleTemplateViewModel : NamedViewModel
{
[DataMember]
public string Comment { get; set; }
}
[DataContract]
public class Container
{
[DataMember]
public IEnumerable<ScheduleTemplateViewModel> Templates { get; set; }
}
[ServiceContract]
public interface IService
{
[OperationContract]
Container Get();
}
public class Service : IService
{
public IEnumerable<Container> Get()
{
return new Container { Templates = Enumerable.Range(1, 10)
.Select(i => CreateTemplate()).ToArray() };
}
private void ScheduleTemplateViewModel CreateTemplate()
{
var instance = WindsorContainer.Resolve<ScheduleTemplateViewModel>();
// populate instance
return instance;
}
}
We have two problems:
We get an exception during serialization that the Castle DynamicProxy type for ScheduleTemplateViewModel is unexpected. We noticed that there is custom code in protobuf-net to handle NHibernate and EntityFramework proxies...but not Castle DynamicProxies. We worked around this by adding an additional case statement in the protobuf-net source code to check for Castle's IProxyTargetAccessor type...but it would be nice if there were a way of handling this without modifying the protobuf-net source code...
Members on ScheduleTemplateViewModel (namely Comment) are serialized correctly...but base class Members are not. We already have the InferTagFromNameDefault set to true on RuntimeTypeModel.Default.
I can add that; can you tell me the full name (including namespace) of that interface?
From the example you give, none of those values should serialize, as none of them include the necessary numeric field-number information. Since you say some do serialize, I'm going to assume that this is an omission in the copy/paste. Protobuf-net will try to use the Order=n information from [DataMember(...)] if nothing better is available. However, if must be emphasized that protobuf-net cannot use [KnownType(...)], and inheritance again needs some explicit numeric field-number information. This is most easily added via [ProtoInclude(...)], but can also be provided at runtime
This is similar to another topic I recently posted, but perhaps this might be simpler and clearer:
I want to accomplish the following (or something very similar)...
IManageableEntryDao<IManageableEntry> dao = new CompanyNameDao();
... with the following classes:
public interface IManageableEntry {
string Name { get; set; }
}
public class CompanyName : IManageableEntry {
public string Name { get; set; }
}
public interface IManageableEntryDao<T> where T : IManageableEntry {
}
public class CompanyNameDao : IManageableEntryDao<CompanyName> {
}
If I try to do a cast as IManageableEntryDao<IManageableEntry>, I get a null.
I believe you need covariance for this to work. This feature is only available in C# 4.0. What you need to do:
public interface IManageableEntryDao<out T> where T : IManageableEntry { }
See Variance in Generic Interfaces. Change the interface to IManageableEntryDao<out T> and it should work (unless the interface uses it in a way which makes this invalid).