Compile Error with class DerivedClass : BaseClass

Question

2.00/5 (1 vote)

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C#

class BaseClass<T>
{
    public void Reset() { data.Clear(); }
    public void Add(T value) { data.Add(value); }
    public int Count { get { return(data.Count); } }
    public T this[int index]
    {
        get { return (data[index]); }
        set { data[index] = value; }
    }

    List<T> data = new List<T>();
}

class DerivedClass<T> : BaseClass<T>
{
    public DerivedClass()
    {
        this.DoSomething();
    }

    private void DoSomething()
    {
        Add(200);
    }
}

When I do
DerivedClass<int> inst = new DerivedClass<int>();

inst.Add(200) will compile

but Add(200) in method DoSomething() will give a compile error "cannot convert 'int' to 'T'.

Posted 22-Nov-13 9:13am

fheyn

Updated 22-Nov-13 10:04am

RaisKazi

v3

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3 solutions

Solution 1

You need to cast it:

C#

private void DoSomething()
{
    Add((T)200);
}

But be careful, if 200 isn't convertable to T, it will throw an exception at runtime.

Posted 22-Nov-13 9:21am

Ron Beyer

Comments

fheyn 22-Nov-13 15:31pm

that's what I tried first and still get the same error-message

Ron Beyer 22-Nov-13 15:40pm

I apologize, my solution is wrong, I will give a new one.

Solution 3

You can't do Add(200) in the derived class because how do you know that T is a value type?

The compiler is trying to tell you that 200 can't be converted to T because it doesn't satisfy the constraints.

In order to do what you want, you need to either do what Solution 2 has, or there is a lengthy discussion in this CodeProject article[^].

Many have said, having a generic constraint for numeric types was left out of the framework. It would be nice to have but its just not there.

Posted 22-Nov-13 9:45am

Ron Beyer

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Karthik_Mahalingam · Accepted Answer · 2013-11-22T09:24:00

try this code...

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace console_poc
{
    class Program
    {
        static void Main(string[] args)
        {
            DerivedClass inst = new DerivedClass();

            inst.Add(200);
 


        }
    }

    class BaseClass<T>
    {
        public void Reset() { data.Clear(); }
        public void Add(T value) { data.Add(value); }
        public int Count { get { return (data.Count); } }
        public T this[int index]
        {
            get { return (data[index]); }
            set { data[index] = value; }
        }

        List<T> data = new List<T>();
    }

    class DerivedClass : BaseClass <int> 
    {
        public DerivedClass()
        {
            this.DoSomething();
        }

        private void DoSomething()
        {
            Add(200);
        }
    }

}