Using generic types within a foreach loop

Question

I really cannot figure out how to use generic types with IEnumerable so that I can iterate through values contained by a given generic value.

Consider the following class (note that the classes here are only for example purposes):

public class Parameter<T> : IParameter<T> where T : IEnumerable<T>
{
  public List<UInt64> output = new List<UInt64>();

   private T _value;
   public T Value
   {
       get => ...;
       set
       {

           // I want to be able to apply special treat to the value
           // Value can be of any type: int, int[], bool, bool[]
           foreach (var v in value)
           {
               output.Add(Convert.UInt64(v) + 5);
           }

           ...

       }
   }
}

public interface IParameter<T> where T : IEnumerable<T>
{
   T Value { get; set; }
}

I then have a test module that instantiate some parameters as per, but I cannot even compile here. I have even tried to replace bool[] to IEnumerable here below, but the compiler does not like it either.

public class TestModule : ModuleBase, ITestModule
{
    public IParameter<bool[]> Test1 { get; set; } = new Parameter<bool[]>();
    public IParameter<uint[]> Test2 { get; set; } = new Parameter<uint[]>();
    ...
    public IParameter<int> Test3 { get; set; } = new Parameter<int>();
}

I did consider using overload for the Parameter() class, but I thought it to be overkill to create a class per supported type (considering it is only for the Value property).

In the foreach loop, what type do you want v to be of? In other words, what does Convert.UInt64 accept? — Sweeper
– Sweeper, Commented May 28, 2020 at 13:25
Also "ANY TYPE" means you fall back to a generic object. There is not exactly anything else that is identical between bool and bool[]. You are better at least to have a couple of methods - one for arrays, one not for. arrays. — TomTom
– TomTom, Commented May 28, 2020 at 13:26
TBH, if you've got type-specific behaviour, generics probably isn't the right choice here. — spender
– spender, Commented May 28, 2020 at 13:26
It looks like you need more than one generic type because T : IEnumerable<T> doesn't make much sense. Something like Parameter<T, U> : IParameter<T, U> where T : IEnumerable<U> maybe? Note that bool[] does not match that constraint because it's a IEnumerable<bool> and not an IEnumreable<bool[]> And if you need to handle single items and collections, then you should separate those as you cannot deal with them in a generic way (unless you just force single items into a collection). — juharr
– juharr, Commented May 28, 2020 at 13:26

kha · Accepted Answer · 2020-05-28 14:10:00Z

1

Your issue is that your generic parameter is specified incorrectly.

public class Parameter<T> : IParameter<T> where T : IEnumerable<T>

implies that whatever comes in of type T is an enumerable of the same type, meaning for instance a T of type bool[] should be an IEnumerable<bool[]> which is clearly incorrect.

One way to get it to compile is this:

    public class Parameter<TEnumerable, TType> : IParameter<TEnumerable, TType> where TEnumerable : IEnumerable<TType>
    {
        public List<ulong> output = new List<ulong>();

        private TEnumerable _value;
        public TEnumerable Value
        {
            get => { return null; }
            set
            {

                // I want to be able to apply special treat to the value
                // Value can be of any type: int, int[], bool, bool[]
                foreach (Q v in value)
                {
                    output.Add(Convert.ToUInt64(v) + 5);
                }
            }
        }
    }

    public interface IParameter<TEnumerable, TType> where TEnumerable : IEnumerable<TType>
    {
        TEnumerable Value { get; set; }
    }

    public class TestModule
    {
        public IParameter<bool[], bool> Test1 { get; set; } = new Parameter<bool[], bool>();
        public IParameter<uint[], uint> Test2 { get; set; } = new Parameter<uint[], uint>();
        public IParameter<int[], int> Test3 { get; set; } = new Parameter<int[], int>();
    }

As for your additional comment, no, there's no way you can avoid having to specify the two types since IEnumerable is not a T in the form you've formulated your code. You have 2 separate parameters here and as such, you will have to use 2 generic parameters if you must do it the way you've done it.

A much simpler solution to your problem would be something like this which serves the same purpose more or less, although I don't really know your requirements so this may or may not suffice (interface omitted for clarity):

    public class Parameter<TType>
    {
        public List<ulong> output = new List<ulong>();

        private IEnumerable<TType> _value;
        public IEnumerable<TType> Value
        {
            get => { return null; }
            set
            {

                // I want to be able to apply special treat to the value
                // Value can be of any type: int, int[], bool, bool[]
                foreach (TType v in value)
                {
                    output.Add(Convert.ToUInt64(v) + 5);
                }
            }
        }
    }

    public class TestModule
    {
        public Parameter<bool> Test1 { get; set; } = new Parameter<bool>();
        public Parameter<uint> Test2 { get; set; } = new Parameter<uint>();
        public Parameter<int> Test3 { get; set; } = new Parameter<int>();
    }

edited May 28, 2020 at 14:10

answered May 28, 2020 at 13:36

kha

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8 Comments

stackMeUp Over a year ago

Thanks, is there anyway I could have a single "argument" in "Parameter<TEnumerable, TType>". ie. Can we somehow combine TEnumerable and TType so that I don't need to declare each param using both, eg. <bool[], bool>

kha Over a year ago

Added an update. I don't really know what you're trying to achieve so I just gave it my best shot given the original code.

stackMeUp Over a year ago

Oh, but will that work for both array and non-array types?

kha Over a year ago

It works for anything. the type is unconstrained. but the Value property will always be an enumerable of type T

stackMeUp Over a year ago

So, you are saying that I either need option 1 (to support both array and non-array types) or 2 separate implementations of that class (one for arrays and one for non-array types)?

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