using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {            

        var attributes = new Attribute[] {

            new Attribute{WhatAttri=AttributeType.maxhp, amount=6 },

            new Attribute{WhatAttri=AttributeType.str, amount=4 },

            new Attribute{WhatAttri=AttributeType.dex, amount=3 },

        };

​

        //Attribute modifier has to be either positive or negative

        var attributesModifier = new Attribute[] {

            new Attribute{WhatAttri=AttributeType.str, amount=-9 },

            new Attribute{WhatAttri=AttributeType.@int, amount=-7 },

            new Attribute{WhatAttri=AttributeType.wis, amount=-5 },

        };

​

​

        var newAttributes = attributes

                                .Concat(attributesModifier)

                                .GroupBy(x => x.WhatAttri)

                                .Select(group => 

                                        new Attribute { 

                                            WhatAttri = group.Key, 

                                            amount = group.Sum(g => g.amount) 

                                        });

        newAttributes.Dump();

    }

    public class Attribute

    {

        public AttributeType WhatAttri { get; set; }

        public float amount { get; set; }

    }

​

    public enum AttributeType

    {

        maxhp, str, dex, @int, wis

    }

}