// ==++==
// 
//   Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// ==--==
 
    using System;
 
namespace 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;
 
    // 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;
            this.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)
                 }
        
        [System.Runtime.InteropServices.ComVisible(true)]
    public static class Nullable
    {
    [System.Runtime.InteropServices.ComVisible(true)]
        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;
            }            
        
        [System.Runtime.InteropServices.ComVisible(true)]
        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("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)
    }        
}