I have two interfaces:
public interface IDbModel {}
public interface IDmModel {}
And classes derived from this:
public class DbModel : IDbModel {}
public class DmModel : IDmModel {}
public class Middle { }
Also I have two interfaces with restrictions:
public interface IRule { }
public interface IRule<in TInput, out TOutput> : IRule
where TInput : IDmModel
where TOutput : IDbModel
{
TOutput Apply(TInput elem);
}
And one abstract class derived from this interface:
public abstract class Rule<TDmModel, TMiddle, TDb> : IRule<TDmModel, TDb>
where TDmModel : IDmModel
where TDb : IDbModel
{
private readonly Func<TDmModel, TMiddle> _rule;
protected Rule(Func<TDmModel, TMiddle> rule) { _rule = rule; }
protected abstract TDb Apply(TMiddle transformedMessage);
public TDb Apply(TDmModel elem) { ... }
}
After this I created two classes derived from this abstract class:
public class RuleA : Rule<DmModel, Middle, DbModel>
{
public RuleA(Func<DmModel, Middle> rule) : base(rule) {}
protected override DbMode Apply(Middle transformedMessage) { ... }
}
public class RuleB : RuleA
{
public RuleB() : base((dm) => new Middle()) {}
}
RuleB : RuleA : Rule< DmModel,Middle,DbModel > : IRule< IDmModel,IDbModel > : IRule
And when I try to cast object of RuleB
to IRule<IDmModel, IDbModel>
occours unhandled exception
Unable to cast object of type 'ParsreCombinators.RuleB' to type 'ParsreCombinators.IRule`2[ParsreCombinators.IDmModel,ParsreCombinators.IDbModel]'.
var ruleB = (IRule<IDmModel, IDbModel>)new RuleB(); // Exception
IDbModel dbModel = ruleB.Apply(new DmModel());
What wrong with this
To make the example less confusing I simplify it:
EDIT:
After the answers I understood, what is the problem and to make the example less confusing I simplify it:
public interface IDbModel {}
public interface IDmModel {}
public class DbModel : IDbModel {}
public class DmModel : IDmModel {}
public interface IRule<in TInput, out TOutput>
where TInput : IDmModel
where TOutput : IDbModel
{
TOutput Apply(TInput elem);
}
public class RuleA : IRule<DmModel, DbModel>
{
public DbModel Apply(DmModel elem) { ... }
}
var ruleA = (IRule<IDmModel, IDbModel>)new RuleA(); // Exception
That's a lot of levels of indirection you got there...
Here's the issue:
public abstract class Rule<TDmModel, TMiddle, TDb> : IRule<TDmModel, TDb>
where TDmModel : IDmModel
where TDb : IDbModel
public class RuleA : Rule<DmModel, Middle, DbMode>
public class RuleB : RuleA
...
var ruleB = (IRule<IDmModel, IDbModel>)new RuleB();
RuleB implements IRule<DmModel, DbMode>
This cannot be cast to IRule<IDmModel, IDbModel>. C# does not support this type of casting. For the same reason, you cannot do List<object> b = (List<object>)new List<string>();
(Gives "Cannot convert type 'System.Collections.Generic.List<string> to System.Collections.Generic.List<object>.")
This is an issue with covariance.
Here is some more information from Microsoft on the subject: https://docs.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance
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