Lazy.cs

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//
// Lazy.cs
//
// Authors:
//  Zoltan Varga (vargaz@gmail.com)
//  Marek Safar (marek.safar@gmail.com)
//
// Copyright (C) 2009 Novell
//
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

using System;
using System.Runtime.Serialization;
using System.Runtime.InteropServices;
using System.Security.Permissions;
using System.Threading;
using System.Diagnostics;

namespace OpenSim.Framework
{
    public enum LazyThreadSafetyMode
    {
        None,
        PublicationOnly,
        ExecutionAndPublication
    }

    [SerializableAttribute]
    [ComVisibleAttribute(false)]
    [HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, ExternalThreading = true)]
    public class Lazy<T>
    {
        T value;
        bool inited;
        LazyThreadSafetyMode mode;
        Func<T> factory;
        object monitor;
        Exception exception;

        public Lazy()
            : this(LazyThreadSafetyMode.ExecutionAndPublication)
        {
        }

        public Lazy(Func<T> valueFactory)
            : this(valueFactory, LazyThreadSafetyMode.ExecutionAndPublication)
        {
        }

        public Lazy(bool isThreadSafe)
            : this(() => Activator.CreateInstance<T>(), isThreadSafe ? LazyThreadSafetyMode.ExecutionAndPublication : LazyThreadSafetyMode.None)
        {
        }

        public Lazy(Func<T> valueFactory, bool isThreadSafe)
            : this(valueFactory, isThreadSafe ? LazyThreadSafetyMode.ExecutionAndPublication : LazyThreadSafetyMode.None)
        {
        }

        public Lazy(LazyThreadSafetyMode mode)
            : this(() => Activator.CreateInstance<T>(), mode)
        {
        }

        public Lazy(Func<T> valueFactory, LazyThreadSafetyMode mode)
        {
            if (valueFactory == null)
                throw new ArgumentNullException("valueFactory");
            this.factory = valueFactory;
            if (mode != LazyThreadSafetyMode.None)
                monitor = new object();
            this.mode = mode;
        }

        // Don't trigger expensive initialization
        [DebuggerBrowsable(DebuggerBrowsableState.Never)]
        public T Value
        {
            get
            {
                if (inited)
                    return value;
                if (exception != null)
                    throw exception;

                return InitValue();
            }
        }

        T InitValue()
        {
            switch (mode)
            {
                case LazyThreadSafetyMode.None:
                    {
                        var init_factory = factory;
                        if (init_factory == null)
                            throw exception = new InvalidOperationException("The initialization function tries to access Value on this instance");
                        try
                        {
                            factory = null;
                            T v = init_factory();
                            value = v;
                            Thread.MemoryBarrier();
                            inited = true;
                        }
                        catch (Exception ex)
                        {
                            exception = ex;
                            throw;
                        }
                        break;
                    }
                case LazyThreadSafetyMode.PublicationOnly:
                    {
                        var init_factory = factory;
                        T v;

                        //exceptions are ignored
                        if (init_factory != null)
                            v = init_factory();
                        else
                            v = default(T);

                        lock (monitor)
                        {
                            if (inited)
                                return value;
                            value = v;
                            Thread.MemoryBarrier();
                            inited = true;
                            factory = null;
                        }
                        break;
                    }
                case LazyThreadSafetyMode.ExecutionAndPublication:
                    {
                        lock (monitor)
                        {
                            if (inited)
                                return value;

                            if (factory == null)
                                throw exception = new InvalidOperationException("The initialization function tries to access Value on this instance");

                            var init_factory = factory;
                            try
                            {
                                factory = null;
                                T v = init_factory();
                                value = v;
                                Thread.MemoryBarrier();
                                inited = true;
                            }
                            catch (Exception ex)
                            {
                                exception = ex;
                                throw;
                            }
                        }
                        break;
                    }
                default:
                    throw new InvalidOperationException("Invalid LazyThreadSafetyMode " + mode);
            }

            if (monitor == null)
            {
                value = factory();
                inited = true;
            }
            else
            {
                lock (monitor)
                {
                    if (inited)
                        return value;

                    if (factory == null)
                        throw new InvalidOperationException("The initialization function tries to access Value on this instance");

                    var init_factory = factory;
                    try
                    {
                        factory = null;
                        T v = init_factory();
                        value = v;
                        Thread.MemoryBarrier();
                        inited = true;
                    }
                    catch
                    {
                        factory = init_factory;
                        throw;
                    }
                }
            }

            return value;
        }

        public bool IsValueCreated
        {
            get
            {
                return inited;
            }
        }

        public override string ToString()
        {
            if (inited)
                return value.ToString();
            else
                return "Value is not created";
        }
    }
}