using System;

using System.Linq;

using System.Collections.Generic;

​

public class A : IEquatable<A> {

    public string Ab {get; set;}

    public string B{get; set;}

    public bool IsD {get; set;}

​

    public bool Equals(A other)

    {

​

        // Check whether the compared object is null.

        if (Object.ReferenceEquals(other, null)) return false;

​

        // Check whether the compared object references the same data.

        if (Object.ReferenceEquals(this, other)) return true;

​

        // Check whether the error's properties are equal.

        return Ab == other.Ab && B == other.B && IsD == other.IsD;

    }

​

    // If Equals() returns true for a pair of objects

    // then GetHashCode() must return the same value for these objects.

    public override int GetHashCode()

    {

        return $"{Ab}-{B}-{IsD}".GetHashCode(); // adjust this as you see fit

    }

    public string ToString() => $"{Ab}-{B}-{IsD}";

​

}

​

public class Program

{

    public static void Main()

    {

        var a = new A { Ab = "a", B = "b", IsD = false };

        var b = new A { Ab = "a", B = "b", IsD = false };

        var c = new A { Ab = "abcd", B = "b", IsD = false };

​

        var list = new List<A>();

        list.Add(a);

        list.Add(b);

        list.Add(c);

        foreach(var o in list) Console.WriteLine(o.ToString());

​

        var query = list.GroupBy(x => x)

              .Where(g => g.Count() > 1)

              .Select(y => y.Key)

              .ToList();

        foreach(var o in query) {

            foreach(var i in list.FindAll(x=>x.Ab == o.Ab && x.B == o.B)) {

                i.IsD = true;

            }

        }

        Console.WriteLine();

        foreach(var o in list) Console.WriteLine(o.ToString());

​

    }

}