DictionaryTests.cs

public class Foo
{
    readonly int bar;

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

    public override bool Equals(object obj)
    {
        return Equals(obj as Foo);
    }

    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);
    }
}