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Cannot implement interface member using derived type

Lets assume I've got a class Called StreetModel that implements an interface called IStreetModel:
public class StreetModel: IStreetModel {
// Properties go here
}
Said IStreetModel looks like so:
public interface IStreetModel {
// Properties go here
}
Now, say I have another interface called ILocationModel that contains a property of type IStreetModel:
public interface ILocationModel {
IStreetModel Street { get; }
}
And this ILocationModel interface is implemented by a class called LocationModel:
public class LocationModel: ILocationModel {
public StreetModel Street { get; } // This is where my query is
}
My Question: Within LocationModel, why can't I implement StreetModel even though it implements IStreetModel. Why does the compiler want IStreetModel specifically? It's confusing for me because in almost every other scenario within my program (except for collections) I can interchange usage of the two. Why not here?
Error Message:
'LocationModel' does not implement interface member
'ILocationModel.Street'. 'LocationModel.Street' cannot implement
'ILocationModel.Street' because it does not have the matching return
type of 'IStreetModel'.

The answer to your actual question ("why doesn't it compile?") is: The c# language requires that a return type for an implemented interface member matches exactly. That's the way the language was designed.
This was fixed by c# 9 in some cases, but not for interface implementations. See the section following The remainder of the draft specification below proposes a further extension to covariant returns of interface methods to be considered later in this documentation.
In the meantime, a possible workaround is to make your ILocationModel generic like so:
public interface IStreetModel
{
// Properties go here
}
public interface ILocationModel<out T> where T: IStreetModel
{
T Street { get; }
}
public class StreetModel : IStreetModel
{
// Properties go here
}
public class LocationModel : ILocationModel<StreetModel>
{
public StreetModel Street { get; } // This is where my query is
}
Of course the most obvious solution is simply to declare the return type of LocationModel.Street as IStreetModel.
Another alternative is to use explicit interface implementation, like so:
public class LocationModel : ILocationModel
{
public StreetModel Street { get; } // This is where my query is
IStreetModel ILocationModel.Street => Street;
}

Inner class within Interface in C#

The following question shows how to implement an interface that contains a class in java:
inner class within Interface
The code in Java is:
public interface A {
class B {
}
}
I was hoping the same thing was possible in C#, though I haven't yet been able to get anything working.
For reference, I have a class which makes lookups of key values, but the keys aren't named in a way that makes them easy to understand. I'd like to have a compile time lookup for keys, so the interface would be something like:
interface Lookup {
class Keys {
string SomeKey() => "0"
}
}
Which means I suppose I have two questions:
Is it possible to have an interface containing a class?
Is there a better way of having a lookup between two strings (or any other values) that I can reference reliably at compile time?

Simply put no you can't have a class inside an interface.
From your comments you are talking about having a restricted list of available strings for the keys so I'm wondering if you are in fact not looking for a string/string lookup but just want a convenient way of referencing a list of fixed strings. So a class with constants is all you need:
public static class Strings
{
public const string AString = "A";
public const string BString = "B";
public const string CString = "C";
}
Accessed like this:
var s = Strings.AString;

You cannot have an class within an interface in C#. Interfaces are very simple in C#, and only provide a contract of functionality.
If you want to have a mapping between two strings, a Dictionary<string, string> may be of use to you.

An interface can't itself have any instance data. It's implementation however can have any instance data it requires.
For example, a random example that might give you some insight:
public class SomeClass
{
public string Key {get; set;}
}
public interface ISomeInterface
{
string Value { get; set; }
SomeClass SomeClass { get; set;}
}
public class SomeInterfaceImplementation : ISomeInterface
{
public SomeInterfaceImplementation()
{
SomeClass = new SomeClass()
{
Key = "ABC"
};
}
public string Value { get; set; }
public SomeClass SomeClass { get; set; }
}
public class Program
{
public static void Main()
{
var example = new SomeInterfaceImplementation()
{
Value = "A value",
} as ISomeInterface;
Console.WriteLine($"{example.SomeClass.Key} has value '{example.Value}'");
}
}
In the example, the default constructor "generates" a key of ABC. We could implement this any way your logic requires. But you also have a contract that requires "SomeClass" and it's key is present.
Anywhere you want to use the contract, just accept the Interface and not an implementation class.
Additionally, feel free to play with the fiddle:

Most of the answers under this question are no longer true.
Since C# 8.0, when default interface methods were added, it is possible for an interface to have member declaration that declare for example nested type.
The following code is correct for C# 8.0:
public interface IA
{
class B
{
}
}

Can C# constraints be used without a base type?

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.

What is the best way to design this class hierarchy?

I have a database table which contains an ID column and a Name column. I am tasked with designing a program that accepts one of the IDs as an argument to Main().
Bold is edit 2
I need to use that ID which must exist in the database, to correspond to some code to run. Each row in the table corresponds to slightly different code, but a lot of them share a lot of code. I need a design that will minimize code duplication.
So far what I've developed is an abstract base class that has an abstract Int32 field ID to enforce derived classes having their corresponding ID in the database. That way I can reflect over the derived classes to find the one whose ID matches the Main() argument and instantiate that class. Then I just call the virtual methods from Main() which runs the most derived code that has been defined.
public abstract class Base {
public abstract Int32 Id { get; }
public void Foo() {
// Do something
}
}
public class Derived {
public override Int32 Id { get { return 42; } }
public void Foo() {
// Do something more specific
}
}
Does anyone have any better ideas how to achieve what I want? I like the idea of keeping the ID right in the class definition, but I'm open to changing that if it makes sense.
Thanks!
EDIT:
One thing I don't like about this is that I have to reflect over each derived type and instantiate that type to check the ID. Does anyone have a better idea on how to do that?

Instead of using a property to define the ID of the class, use a custom attribute. That way, you don't have to instantiate the object to check what its ID is.
When your program runs, it can scan the assembly for all classes with that attribute tag, and find the one with the matching ID, instantiate that class, and then run it's Foo method. If you perform this kind of lookup multiple times per application run, you could instatiate all the classes with your custom attribute and then put them into a Dictionary to provide quick lookups by ID.
Your code might look something like this:
[AttributeUsage(AttributeTargets.Class)]
public class CommandAttribute {
public CommandAttribute(int id) {
ID = id;
}
public int ID { get; private set; }
}
public abstract class Command {
public abstract void Execute();
}
[Command(2)]
public class MyCommand : Command {
public override void Execute() {
//Do something useful
}
}
The other advantage of using a custom attribute is that you have to explicitly tag everything that is a candidate for being instantiated and executed by ID, rather than assuming than anything derived from your base class is a candidate. If you are sharing code between the classes, you might want to make a common base class for them that derives from your base class, but should not be instantiated or executed on its own.
One thing I don't understand is, what is the point of the "Name" field if the class you want to run is identified by the ID? If you can decide what the name of each ID is, then you could use the name field as the fully qualified type name of the class you want to execute, which then avoid having to scan through all the types in your assembly (or application domain, depending upon the scope of your search). That setup is a bit more prone to typos, however.

It sounds like you need to implement a factory pattern.
I would define an interface:
public interface IWidget
{
void Foo();
}
Then the base class:
public abstract class WidgetBase : IWidget
{
public void Foo()
{
this.Bar()
}
protected virtual void Bar()
{
// Base implementation
}
}
The factory:
public static WidgetFactory
{
public static IWidget Create(int id)
{
// Get class name from id, probably use the name in your database.
// Get Type from class name
// Get constructor for Type
// Create instance using constructor and return it.
}
}
A derived class:
public class DerivedWidget : WidgetBase
{
protected override void Bar()
{
// call base implementation
base.Bar();
// derived implementation
}
}
In your main:
public void Main(int id)
{
var widget = WidgetBase.Create(id);
widget.Foo();
}

I like #Xint0's idea of using a Factory for this kind of task, but I thought I'd still contribute another answer.
A better way to implement your original design would be to pass the ID to the base constructor as follows:
public abstract class Base {
public Int32 Id { get; private set; }
protected Base(Int32 id) {
this.Id = id;
}
public void Foo() {
// Do something
}
}
public class Derived : Base {
public Derived : base(42) {}
public void Foo() {
// Do something more specific
}
}

c# - cast generic class to its base non-generic class

I have following classes:
public abstract class CustomerBase
{
public long CustomerNumber { get; set; }
public string Name { get; set; }
}
public abstract class CustomerWithChildern<T> : CustomerBase
where T: CustomerBase
{
public IList<T> Childern { get; private set; }
public CustomerWithChildern()
{
Childern = new List<T>();
}
}
public class SalesOffice : CustomerWithChildern<NationalNegotiation>
{
}
The SalesOffice is just one of few classes which represent different levels of customer hierarchy. Now I need to walk through this hierarchy from some point (CustomerBase). I can't figure out how to implement without using reflection. I'd like to implement something like:
public void WalkHierarchy(CustomerBase start)
{
Print(start.CustomerNumber);
if (start is CustomerWithChildern<>)
{
foreach(ch in start.Childern)
{
WalkHierarchy(ch);
}
}
}
Is there any chance I could get something like this working?
The solution based on suggested has-childern interface I implemented:
public interface ICustomerWithChildern
{
IEnumerable ChildernEnum { get; }
}
public abstract class CustomerWithChildern<T> : CustomerBase, ICustomerWithChildern
where T: CustomerBase
{
public IEnumerable ChildernEnum { get { return Childern; } }
public IList<T> Childern { get; private set; }
public CustomerWithChildern()
{
Childern = new List<T>();
}
}
public void WalkHierarchy(CustomerBase start)
{
var x = start.CustomerNumber;
var c = start as ICustomerWithChildern;
if (c != null)
{
foreach(var ch in c.ChildernEnum)
{
WalkHierarchy((CustomerBase)ch);
}
}
}

You could move the WalkHierarchy method to the base class and make it virtual. The base class implementation would only process the current node. For the CustomerWithChildern<T> class, the override would do an actual walk.

Try this:
if(start.GetType().GetGenericTypeDefinition() == typeof(CustomerWithChildern<>))

I believe that you want to make the lookup for the determination of doing to the walk an interface.
So maybe add an "IWalkable" interface that exposes the information needed to do the walk, then you can create your method checking to see if the passed object implements the interface.

"Is" and "As" only work on fully qualified generic types.
See this MSDN discussion for details including workarounds.
The most common workaround I've seen is to add an interface to the mix that your CustomerWithChildren could implement, and check for that interface.

I think everyone hits this "issue" when first working with generic classes.
Your first problem is hinted at in your question phrasing: an open generic type is NOT the base class to a closed one. There is no OO relationship here, at all. The real base class is CustomerBase. An "open" generic type is like a half-completed class; specifying type arguments "closes" it, making it complete.
While you can do:
Type t = typeof(CustomerWithChildern<>)
the condition
typeof(CustomerWithChildern<>).IsAssignableFrom(CustomerWithChildern<Foo>)
will always be False.
-Oisin

Explicitly with that method, no. However you can achieve the same functionality with an interface. In fact, you could just have your generic class implement IEnumerable. It's also worth noting that your class should also have "where T : CustomerBase" in order to ensure type safety.