Option.cs

using System;
using System.Diagnostics.CodeAnalysis;

public struct Option<T> : IEquatable<Option<T>>
{
    [SuppressMessage("Microsoft.Security", "CA2104:DoNotDeclareReadOnlyMutableReferenceTypes")]
    public static readonly Option<T> None = new Option<T>();
    private readonly T _value;
    private readonly bool _hasValue;

    public T Value
    {
        get
        {
            if (!_hasValue)
                throw new InvalidOperationException("Value is not set");
            return _value;
        }
    }

    public bool HasValue
    {
        get
        {
            return _hasValue;
        }
    }

    public Option(T value)
    {
        if (value != null)
        {
            _value = value;
            _hasValue = true;
        }
        else
        {
            _value = default(T);
            _hasValue = false;
        }
    }

    public static implicit operator Option<T>(T value)
    {
        if (value == null)
            return None;
        return new Option<T>(value);
    }

    public static bool operator ==(Option<T> first, Option<T> second)
    {
        return first.Equals(second);
    }

    public static bool operator !=(Option<T> first, Option<T> second)
    {
        return !(first == second);
    }

    public override int GetHashCode()
    {
        if (_hasValue)
            return Value.GetHashCode();
        return base.GetHashCode();
    }

    public override bool Equals(object obj)
    {
        if (obj is Option<T>)
            return Equals((Option<T>)obj);
        return false;
    }

    public bool Equals(Option<T> other)
    {
        if (!HasValue && !other.HasValue)
            return true;
        if (HasValue && other.HasValue)
            return _value.Equals(other.Value);
        return false;
    }

    bool IEquatable<Option<T>>.Equals(Option<T> other)
    {
        return Equals(other);
    }

    public T GetValueOrDefault(T @default)
    {
        if (!HasValue)
            return @default;
        return _value;
    }

    public T GetValueOrDefault()
    {
        if (!HasValue)
            return default(T);
        return _value;
    }
}