// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.


using System;

using System.Globalization;
using System.Reflection;
using System.Collections.Generic;
using System.Runtime;
using System.Runtime.CompilerServices;
using System.Security;
using System.Diagnostics.Contracts;

namespace System
{
    // Warning, don't put System.Runtime.Serialization.On*Serializ*Attribute
    // on this class without first fixing ObjectClone::InvokeVtsCallbacks
    // Also, because we have special type system support that says a a boxed Nullable<T>
    // can be used where a boxed<T> is use, Nullable<T> can not implement any intefaces
    // at all (since T may not).   Do NOT add any interfaces to Nullable!
    // 
    [Serializable]
    [System.Runtime.Versioning.NonVersionable] // This only applies to field layout
    public struct Nullable<T> where T : struct
    {
        private bool hasValue;
        internal T value;

        [System.Runtime.Versioning.NonVersionable]
        public Nullable(T value)
        {
            this.value = value;
            hasValue = true;
        }

        public bool HasValue
        {
            [System.Runtime.Versioning.NonVersionable]
            get
            {
                return hasValue;
            }
        }

        public T Value
        {
            get
            {
                if (!hasValue)
                {
                    ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NoValue);
                }
                return value;
            }
        }

        [System.Runtime.Versioning.NonVersionable]
        public T GetValueOrDefault()
        {
            return value;
        }

        [System.Runtime.Versioning.NonVersionable]
        public T GetValueOrDefault(T defaultValue)
        {
            return hasValue ? value : defaultValue;
        }

        public override bool Equals(object other)
        {
            if (!hasValue) return other == null;
            if (other == null) return false;
            return value.Equals(other);
        }

        public override int GetHashCode()
        {
            return hasValue ? value.GetHashCode() : 0;
        }

        public override string ToString()
        {
            return hasValue ? value.ToString() : "";
        }

        [System.Runtime.Versioning.NonVersionable]
        public static implicit operator Nullable<T>(T value)
        {
            return new Nullable<T>(value);
        }

        [System.Runtime.Versioning.NonVersionable]
        public static explicit operator T(Nullable<T> value)
        {
            return value.Value;
        }

        // The following already obsoleted methods were removed:
        //   public int CompareTo(object other)
        //   public int CompareTo(Nullable<T> other)
        //   public bool Equals(Nullable<T> other)
        //   public static Nullable<T> FromObject(object value)
        //   public object ToObject()
        //   public string ToString(string format)
        //   public string ToString(IFormatProvider provider)
        //   public string ToString(string format, IFormatProvider provider)

        // The following newly obsoleted methods were removed:
        //   string IFormattable.ToString(string format, IFormatProvider provider)
        //   int IComparable.CompareTo(object other)
        //   int IComparable<Nullable<T>>.CompareTo(Nullable<T> other)
        //   bool IEquatable<Nullable<T>>.Equals(Nullable<T> other)
    }

    public static class Nullable
    {
        public static int Compare<T>(Nullable<T> n1, Nullable<T> n2) where T : struct
        {
            if (n1.HasValue)
            {
                if (n2.HasValue) return Comparer<T>.Default.Compare(n1.value, n2.value);
                return 1;
            }
            if (n2.HasValue) return -1;
            return 0;
        }

        public static bool Equals<T>(Nullable<T> n1, Nullable<T> n2) where T : struct
        {
            if (n1.HasValue)
            {
                if (n2.HasValue) return EqualityComparer<T>.Default.Equals(n1.value, n2.value);
                return false;
            }
            if (n2.HasValue) return false;
            return true;
        }

        // If the type provided is not a Nullable Type, return null.
        // Otherwise, returns the underlying type of the Nullable type
        public static Type GetUnderlyingType(Type nullableType)
        {
            if ((object)nullableType == null)
            {
                throw new ArgumentNullException(nameof(nullableType));
            }
            Contract.EndContractBlock();
            Type result = null;
            if (nullableType.IsGenericType && !nullableType.IsGenericTypeDefinition)
            {
                // instantiated generic type only                
                Type genericType = nullableType.GetGenericTypeDefinition();
                if (Object.ReferenceEquals(genericType, typeof(Nullable<>)))
                {
                    result = nullableType.GetGenericArguments()[0];
                }
            }
            return result;
        }

        // The following already obsoleted methods were removed:
        //   public static bool HasValue<T>(Nullable<T> value)
        //   public static T GetValueOrDefault<T>(Nullable<T> value)
        //   public static T GetValueOrDefault<T>(Nullable<T> value, T valueWhenNull)

        // The following newly obsoleted methods were removed:
        //   public static Nullable<T> FromObject<T>(object value)
        //   public static object ToObject<T>(Nullable<T> value)
        //   public static object Wrap(object value, Type type)
        //   public static object Unwrap(object value)
    }
}