Question

I am trying to design a class that will be flexible enough to chart data about different kinds of data. I’m new to OOP in C#, so I’m fumbling around trying to achieve this using some combination of generics, delegates and classes.

Here’s the class that I’ve written so far:

using System;
using System.Collections.Generic;

namespace Charting
{
    public class DataChart<T>
    {
        public Func<T, object> RowLabel { get; set; }
    }
}

And here’s how I’m trying to call it:

var model = new DataChart<MyClass>()
{
    RowLabel = delegate(MyClass row)
    {
        return String.Format("{0}-hello-{1}", row.SomeColumn, row.AnotherColumn);
    }
};

The problem with this approach is that I’d have to explicitly cast the object emitted by RowLabel. I was hoping I could somehow make the output type a generic and add constraints to it, like so:

    public class DataChart<T>
    {
        // The output of the RowLabel method can only be a value type (e.g. int, decimal, float) or string.
        public Func<T, U> RowLabel where U : struct, string { get; set; }
    }

Is this possible? And if so, how do I do it? Thanks in advance!

Answer 1

You can do some of this.

First, genericizing the output: just add another type parameter to the class.

public class DataChart<T, U>
{
  public Func<T, U> RowLabel  { get; set; }
}

But those type constraints you mentioned do not make sense. Type constraints are “and”-ed, not “or”-ed. A string is not a struct, so you can’t confine it to that specific combination of types. It will still work if you leave it unconstrained, although you lose some compile-time safety.

Edit: Also, it turns out you can’t specify string as a type parameter, anyway. It’s a sealed class! Having a generic that only accepts types of a sealed class would be pointless, and the compiler prevents it.