Casting Down from Base class

Question

I know casting is not inherently bad practice, but is it bad in this situation?

I have the following class hierarchy, and the cast at the end.

 public class A<T> { public A(string name, T value) { Name = name; Value = value; } public string Name { get; } public T Value { get; } } public class B : A<double> { public B(string name, double value, string expression) : base(name, value) { Expression = expression; } public string Expression { get; } } public class C { public C(A<double> a) { _a = a; } public string Name { get { return _a.Name; } } public double Value { get { return _a.Value; } } protected A<double> _a; } public class D : C { public D(B b) : base(b) { } public string Expression { get { return ((B)_a).Expression; } } }

The line in question: public string Expression { get { return ((B)_a).Expression; } }

Your constructor of D can take an instance of B and pass that to base constructor because B is a subclass of A<double> so id say s — Dave
– Dave, Commented Jun 14, 2018 at 22:23
@Dave The question is hidden in the first sentence: "Is this bad practice" — BradleyDotNET
– BradleyDotNET, Commented Jun 14, 2018 at 22:27

BradleyDotNET · Accepted Answer · 2018-06-14 22:23:12Z

Yes this is bad practice; like all downcasting (sometimes necessary to be sure; but that doesn't make it good).

The following code will generate a runtime exception:

A<double> myA = new A<double>("Test", 1.0d); D test = new D(myA); var boom = test.Expression; //InvalidCastException

Whereas with a different structure that wouldn't even compile. For example, modifying D to take a B instead of an A<double>

Dave · Accepted Answer · 2018-06-14 22:36:28Z

Try this, make the C class generic

public class C<T> where T : A<double> { public C(T thing) { _thing = thing; } protected T _thing; }

Then D can be an instance of C with a generic argument of B

public D : C<B> { Public string Expression { get {return _thing.Expression;}} }

I'm on my phone so please forgive.sny formatting or typo issues

This makes it all look a bit nicer imo but with your edit to your code, I'd say it's fine with the cast and your ctor ensures that the cast with succeed

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