Question

My question is at the end of the following code in comments:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


namespace ConsoleApplication1
{
class MyObj
{
    int m_id;

    public MyObj(int id)
    {
        this.m_id = id;
    }

    public int GetId()
    {
        return this.m_id;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var m0 = new MyObj(0);
        var m1 = new MyObj(1);
        var m2 = new MyObj(2);
        var m3 = new MyObj(3);
        var m4 = new MyObj(4);


        List<MyObj> refList1 = new List<MyObj>()
            {
                m0, m1, m2, m3
            };

        List<MyObj> refList2 = new List<MyObj>()
            {
                m1, m2, m3, m4
            };


        //How to merge refList2 into refList1 without id repeating,
        //so refList1 must be [m0, m1, m2, m3, m4]
    }
}
}

Answer 1

You would do this using LINQ, with:

var resultList = refList1.Union(refList2).ToList();

However, this requires either that MyObj implements IEquatable<MyObj>, or the other overload of Union is used which takes an IEqualityComparer<T> parameter.

The first solution would need this change:

class MyObj : IEquatable<MyObj>
{
    int m_id;

    public MyObj(int id)
    {
        this.m_id = id;
    }

    public int GetId()
    {
        return this.m_id;
    }

    public bool Equals(MyObj other)
    {
        if (ReferenceEquals(null, other)) {
            return false;
        }
        if (ReferenceEquals(this, other)) {
            return true;
        }
        return other.m_id == this.m_id;
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) {
            return false;
        }
        if (ReferenceEquals(this, obj)) {
            return true;
        }
        if (obj.GetType() != typeof(MyObj)) {
            return false;
        }
        return Equals((MyObj)obj);
    }

    public override int GetHashCode()
    {
        return this.m_id;
    }
}

The second solution would go like:

class MyComparer : IEqualityComparer<MyObj>
{
    public bool Equals(MyObj x, MyObj y)
    {
        return x.GetId() == y.GetId();
    }

    public int GetHashCode(MyObj obj)
    {
        return obj.GetId();
    }
}

var resultList = refList1.Union(refList2, new MyComparer()).ToList();

and would not require changes to class MyObj.