DictionaryTests.cs

    public class Foo : IEquatable<Foo>
    {
        readonly int bar;

        public Foo(int bar)
        {
            this.bar = bar;
        }

        public override bool Equals(object obj)
        {
            return base.Equals(obj);
        }

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

        public override int GetHashCode()
        {
            return bar;
        }
    }

    [TestClass]
    public class DictionaryTests
    {
        [TestMethod]
        public void Dictionary_ContainsKey_ShouldRespectGetHashCodeOverride()
        {
            var input = new Dictionary<Foo, string>
        {
            { new Foo(123), "123" },
            { new Foo(456), "456" },
        };

            var exists = input.ContainsKey(new Foo(123));

            Assert.IsTrue(exists);
        }
    }