Override a Property with a Derived Type and Same Name C#

Question

I'm trying to override a property in a base class with a different, but derived type with the same name. I think its possible by covarience or generics but am not sure how to do it?

The following code gets the error:

Error 1 'Sun.Cache': type must be 'OuterSpace.Cache' to match overridden member 'OuterSpace.Cache'

public class OuterSpace {
    public virtual OuterSpaceData Data {get; set;}
    public virtual OuterSpaceAnalysis Analysis {get; set;}
    public virtual OuterSpaceCache Cache {get; set;}


    public class OuterSpaceData {
        //Lots of basic Data Extraction routines eg
        public virtual GetData();
    }
    public class OuterSpaceAnalysis {
        //Lots of Generic Analysis on Data routines eg
        public virtual GetMean();
    }
    public class OuterSpaceCache {
        //Lots of Caches of Past Analysis Results:
        public Dictionary<AnalysisType, List<Result>> ResultCache;
    }
}

public class Sun : OuterSpace {
    public override SunData Data {get; set;}
    public override SunAnalysis Analysis {get; set;}
    public override SunCache Cache {get; set;}

    public SunData : OuterSpaceData {
        //Routines to specific get data from the sun eg
        public override GetData();
    }

    public SunAnalysis : OuterSpaceAnalysis {
        //Routines specific to analyse the sun: eg
        public double ReadTemperature();
    }
    public SunCache : OuterSpaceCache {
        //Any data cache's specific to Sun's Analysis
        public Dictionary<AnalysisType, List<Result>> TempCache;
    }
}

public class Moon : OuterSpace {} etc.

For the end result, when I address the "Data" Object of Sun I don't want there to be two Data Objects (Inherited & Base Class) but when I try override the property it requires the Sun variables to be the same type as base class. Eg:

Sun EarthSun = new Sun()
EarthSun.Analyse()  //The OuterSpace Analysis Saves results to Sun Cache:

//Now try use the result:
EarthSun.Cache[0]...

Very similar to this but with derived type instead of string-array: C# Member variable overrides used by base class method

And this answer didn't make much sense to me: How to override member of base class after inheritance in C++

Or perhaps this means its just not possible? Can I Override with derived types?

Help! :) Any work around?

Answer 1

It is not possible the way you want it, exactly.

You can create a "new" property with the same name:

public new SunData Data
{
  get { return (SunData) base.Data; }
  set { base.Data = value; }
}

It's not quite the same thing, but it's probably as close as you're going to get.

Another possible approach would be:

public SunData SunData { get; set; }

public override OuterSpaceData Data
{
  get { return SunData; }
  set { SunData = (SunData)value; }
}

The advantage of this approach is that it does guarantee that it's impossible to put something in your Data property that is not a SunData. The downside is that your Data property is not strongly typed and you have to use the SunData property if you want it statically typed to SunData.

There's an ugly way to get the "best of both worlds" at the expense of being confusing to code and difficult to understand afterwards - by introducing an intermediate derived class that does a 'sealed override' on the base class Data property and redirects to a different protected property with a new name, and then have the ultimate derived class add a 'new' Data property that then calls the intermediate property. It really is an ugly hack, though, and even though I've done it, I wouldn't recommend it.

Answer 2

Try going about it with generics. The following class provides a generic base class for OuterSpace with constraints on its parameters that enforce inheritance rules on the property types. I've omitted methods in the small types for clarity.

public class OuterSpace<TData, TAnalysis, TCache>
    where TData : OuterSpaceData
    where TAnalysis : OuterSpaceAnalysis
    where TCache : OuterSpaceCache
{
    public virtual TData Data { get; set; }
    public virtual TAnalysis Analysis { get; set; }
    public virtual TCache Cache { get; set; }
}
public class OuterSpaceData { }
public class OuterSpaceAnalysis { }
public class OuterSpaceCache { }

public class Sun : OuterSpace<SunData, SunAnalysis, SunCache>
{
    public override SunData Data { get; set; }
    public override SunAnalysis Analysis { get; set; }
    public override SunCache Cache { get; set; }
}
public class SunData : OuterSpaceData { }
public class SunAnalysis : OuterSpaceAnalysis { }
public class SunCache : OuterSpaceCache { }

The appeal of this approach is that it permits you to return strongly typed properties and enforce a basic inheritance constraint on the types of those properties. The methods are fully overridable; however, be advised that overriding may well be required to avoid casting issues with the base class implementation.

Answer 3

If you're looking for the covariance route, something like this should work:

public class OuterSpace<TCacheType> where TCacheType : OuterSpaceCache {
    public virtual OuterSpaceData Data {get; set;}
    public virtual OuterSpaceAnalysis Analysis {get; set;}
    public virtual TCacheType Cache {get; set;}


    public class OuterSpaceData {
        //Lots of basic Data Extraction routines eg
        public virtual GetData();
    }
    public class OuterSpaceAnalysis {
        //Lots of Generic Analysis on Data routines eg
        public virtual GetMean();
    }
    public class OuterSpaceCache {
        //Lots of Caches of Past Analysis Results:
        public Dictionary<AnalysisType, List<Result>> ResultCache;
    }
}

public class Sun : OuterSpace<SunCache> {
    public override SunData Data {get; set;}
    public override SunAnalysis Analysis {get; set;}

    public SunData : OuterSpaceData {
        //Routines to specific get data from the sun eg
        public override GetData();
    }

    public SunAnalysis : OuterSpaceAnalysis {
        //Routines specific to analyse the sun: eg
        public double ReadTemperature();
    }
    public SunCache : OuterSpaceCache {
        //Any data cache's specific to Sun's Analysis
        public Dictionary<AnalysisType, List<Result>> TempCache;
    }
}

Fair warning, totally uncompiled code and I could be totally wrong :) But it's a stab at what I think you want...

Oh, and I think this is a .Net 4.0 only solution, I don't think you can do this earlier, co-variance was introduced there...

Answer 4

If SunData is assignment compatible with Outerspace Data (inherits from) then you don't need to modify the type of of OuterSpaceData

Answer 5

Not sure about Generics, but you could achieve a limited effect with simple Polymorphism (assuming SunData inherits from Data etc)

public class Sun : OuterSpace 
{
  void SomeInitializationMethod()
  {
    Data = new SunData();
    Analysis = new SunAnalysis(); // etc
  }

    }

// You could also make casting simpler with a generic method on the base
public T GetData<T>()
{
  if (Data is T)
  {
    return Data as T;
  }
  return null;
};