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CSharp · Console

/Program.cs

using System;
using System.Linq;

    public class Program
    {
        public static void Main(string[] args)
        {
            var tests1 = new[] { new TestWithOverride(1), new TestWithOverride(2), new TestWithOverride(3) };

            var contains = tests1.Contains(new TestWithOverride(2));

            Console.WriteLine(contains);

            var tests2 = new[] { new TestWithoutOverride(1), new TestWithoutOverride(2), new TestWithoutOverride(3) };

            contains = tests2.Contains(new TestWithoutOverride(2));

            Console.WriteLine(contains);
        }
    }

    internal class TestWithOverride : IEquatable<TestWithOverride>
    {
        public TestWithOverride(int id)
        {
            Id = id;
        }

        public int Id { get;}

        public bool Equals(TestWithOverride other) => this.Id == other.Id;

        public override bool Equals(object obj)
        {
            var test = obj as TestWithOverride;

            return test == null ? obj.Equals(this) : Equals(test);
        }
    }

    internal class TestWithoutOverride : IEquatable<TestWithoutOverride>
    {
        public TestWithoutOverride(int id)
        {
            Id = id;
        }

        public int Id { get;}

        public bool Equals(TestWithoutOverride other) => this.Id == other.Id;
    }