Question

.NET 4.0有一个很好的实用程序类，叫做System.Lazy，可以进行惰性对象初始化。我想将这个类用于3.5项目。有一次我在stackoverflow的答案中看到某个实现，但我再也找不到了。有人有Lazy的替代实现吗？它不需要框架4.0版本的所有线程安全功能。

更新

Answers包含非线程安全和线程安全版本。

Answer 1

这是我使用的实现。

/// <summary>
/// Provides support for lazy initialization.
/// </summary>
/// <typeparam name="T">Specifies the type of object that is being lazily initialized.</typeparam>
public sealed class Lazy<T>
{
    private readonly object padlock = new object();
    private readonly Func<T> createValue;
    private bool isValueCreated;
    private T value;

    /// <summary>
    /// Gets the lazily initialized value of the current Lazy{T} instance.
    /// </summary>
    public T Value
    {
        get
        {
            if (!isValueCreated)
            {
                lock (padlock)
                {
                    if (!isValueCreated)
                    {
                        value = createValue();
                        isValueCreated = true;
                    }
                }
            }
            return value;
        }
    }

    /// <summary>
    /// Gets a value that indicates whether a value has been created for this Lazy{T} instance.
    /// </summary>
    public bool IsValueCreated
    {
        get
        {
            lock (padlock)
            {
                return isValueCreated;
            }
        }
    }


    /// <summary>
    /// Initializes a new instance of the Lazy{T} class.
    /// </summary>
    /// <param name="createValue">The delegate that produces the value when it is needed.</param>
    public Lazy(Func<T> createValue)
    {
        if (createValue == null) throw new ArgumentNullException("createValue");

        this.createValue = createValue;
    }


    /// <summary>
    /// Creates and returns a string representation of the Lazy{T}.Value.
    /// </summary>
    /// <returns>The string representation of the Lazy{T}.Value property.</returns>
    public override string ToString()
    {
        return Value.ToString();
    }
}

Answer 2

如果您不需要线程安全，那么将它与工厂方法放在一起非常容易。我使用的非常类似于以下内容：

public class Lazy<T>
{
    private readonly Func<T> initializer;
    private bool isValueCreated;
    private T value;

    public Lazy(Func<T> initializer)
    {
        if (initializer == null)
            throw new ArgumentNullException("initializer");
        this.initializer = initializer;
    }

    public bool IsValueCreated
    {
        get { return isValueCreated; }
    }

    public T Value
    {
        get
        {
            if (!isValueCreated)
            {
                value = initializer();
                isValueCreated = true;
            }
            return value;
        }
    }
}

Answer 3

aaron的简化版

public class Lazy<T> where T : new()
{ 
  private T value; 

  public bool IsValueCreated { get; private set;}

  public T Value 
  { 
    get 
    { 
        if (!IsValueCreated) 
        { 
            value = new T();
            IsValueCreated = true; 
        } 
        return value; 
    } 
  } 
}

Answer 4

可以添加一些有趣（但不是很有用）的东西：来自委托的隐式转换：

public static implicit operator Lazy<T>(Func<T> initializer)
{
    return new Lazy<T>(initializer);
}

使用

private static Lazy<int> Value = new Func<int>(() => 24 * 22);

C＃编译器在执行此转换时遇到一些问题，例如，分配lambda表达式不起作用，但是还有一件事会导致你的同事思考一下：）

实现Lazy <t> for .NET 3.5 </t>

4 个答案: