// <copyright>
//   Copyright (c) 2012 Codefarts
//   All rights reserved.
//   contact@codefarts.com
//   http://www.codefarts.com
// </copyright>

namespace Codefarts.ObjectPooling
{
    using System;

    /// <summary>
    /// Provides a generic object pooling manager.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class ObjectPoolManager<T> where T : class
    {
        /// <summary>
        /// Holds a reference to a singleton instance.
        /// </summary>
        private static ObjectPoolManager<T> instance;

        /// <summary>
        /// Used to track the number of items that have been pushed to the pool.
        /// </summary>
        private int count;

        /// <summary>
        /// Holds the pooled item references.
        /// </summary>
        private T[] cachedItems = new T[10000];


        /// <summary>
        /// Gets or sets the creation callback.
        /// </summary>
        public Func<T> CreationCallback { get; set; }

        /// <summary>
        /// Gets the number of items in the pool.
        /// </summary>
        public int Count
        {
            get
            {
                return this.count;
            }
        }

        /// <summary>
        /// Pops a item from the pool.
        /// </summary>
        /// <returns>A pooled object reference.</returns>
        /// <exception cref="System.NullReferenceException">'CreationCallback' property must be set if you try to pop a item and there are no items available.</exception>
        public T Pop()
        {
            // lock here to prevent treading conflicts with array manipulation
            lock (this.cachedItems)
            {
                // check if there are any pooled objects
                if (this.count < 1)
                {
                    // check if creation callback is null
                    if (this.CreationCallback == null)
                    {
                        throw new NullReferenceException("'CreationCallback' property must be set if you try to pop a item and there are no items available.");
                    }

                    // there are no available objects so create a new one.
                    return this.CreationCallback();
                }

                // reduce the count
                this.count--;

                // retrieve the item and return it
                return this.cachedItems[this.count];
            }
        }

        /// <summary>
        /// Pushes the specified value.
        /// </summary>
        /// <param name="value">The value to push into the pool.</param>
        public void Push(T value)
        {
            // lock here to prevent treading conflicts with array manipulation
            lock (this.cachedItems)
            {
                // update the count
                this.count++;

                // if we need more room for storage increase the size of the cache array
                if (this.count > this.cachedItems.Length)
                {
                    Array.Resize(ref this.cachedItems, this.cachedItems.Length * 2);
                }

                // store the value 
                this.cachedItems[this.count - 1] = value;
            }
        }

        /// <summary>
        /// Gets the singleton instance of the class.
        /// </summary>
        public static ObjectPoolManager<T> Instance
        {
            get
            {
                return instance ?? (instance = new ObjectPoolManager<T>());
            }
        }
    }
}

And some unit tests to go along with it.

// <copyright>
//   Copyright (c) 2012 Codefarts
//   All rights reserved.
//   contact@codefarts.com
//   http://www.codefarts.com
// </copyright>

namespace Codefarts.Tests.ObjectPooling
{
    using System;

    using Codefarts.ObjectPooling;

    using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;

    [TestClass]
    public class ObjectPoolingManagerTests
    {
        ObjectPoolManager<TestObject> manager;

        [TestInitialize]
        public void Setup()
        {
            this.manager = new ObjectPoolManager<TestObject>();
        }

        [TestCleanup]
        public void Cleanup()
        {
            this.manager = null;
        }

        public class TestObject
        {
            public string stringValue;
            public int intValue;
        }

        [TestMethod]
        public void Pop_With_Empty_Pool_NoCallback()
        {
            try
            {
                var item = this.manager.Pop();
                Assert.IsNotNull(item);
            }
            catch (Exception ex)
            {
                Assert.IsTrue(ex is NullReferenceException);
            }
        }

        [TestMethod]
        public void Pop_With_Empty_Pool_WithCallback()
        {
            try
            {
                this.manager.CreationCallback = this.Callback;
                var item = this.manager.Pop();
                Assert.IsNotNull(item);
                Assert.AreEqual(0, item.intValue);
                Assert.AreEqual(null, item.stringValue);
            }
            catch (Exception ex)
            {
                Assert.IsTrue(ex is NullReferenceException);
            }
        }

        private TestObject Callback()
        {
            return new TestObject();
        }

        [TestMethod]
        public void Push_Object()
        {
            try
            {
                Assert.AreEqual(0, this.manager.Count);
                this.manager.Push(new TestObject());
                Assert.AreEqual(1, this.manager.Count);
            }
            catch (Exception ex)
            {
                Assert.Fail(ex.ToString());
            }
        }

        [TestMethod]
        public void Push_Pop_Objects()
        {
            try
            {
                Assert.AreEqual(0, this.manager.Count);
                for (var i = 0; i < 3; i++)
                {
                    this.manager.Push(new TestObject() { stringValue = "Item" + i, intValue = i });
                }

                Assert.AreEqual(3, this.manager.Count);

                for (var i = 3 - 1; i >= 0; i--)
                {
                    var item = this.manager.Pop();
                    Assert.AreEqual(i, item.intValue);
                    Assert.AreEqual("Item" + i, item.stringValue);
                }

                Assert.AreEqual(0, this.manager.Count);
            }
            catch (Exception ex)
            {
                Assert.Fail(ex.ToString());
            }
        }
    }
}