c# - Lambda 表达式的位置在延迟初始化中是否重要？

Question

我尝试了延迟初始化 sample code来自MSDN。我尝试使用 lambda 表达式通过静态 Func< T > 委托(delegate)来模拟静态 InitLargeObject() 方法，因为 Lazy< T > 构造函数可以接受它。

static Lazy<LargeObject> lazyLargeObject = new Lazy<LargeObject>(InitWithLambda);

static Func<LargeObject> InitWithLambda = () =>
{
    LargeObject large = new LargeObject(Thread.CurrentThread.ManagedThreadId);
    // Perform additional initialization here. 
    return large;
};


static LargeObject InitLargeObject()
{
    LargeObject large = new LargeObject(Thread.CurrentThread.ManagedThreadId);
    // Perform additional initialization here. 
    return large;
}

static void Main()
{

    //lazyLargeObject = new Lazy<LargeObject>(InitLargeObject); // <---- This one use the static method.
    lazyLargeObject = new Lazy<LargeObject>(InitWithLambda); // <---- Thsi one uses the lambda expression.


    Console.WriteLine(
        "\r\nLargeObject is not created until you access the Value property of the lazy" +
        "\r\ninitializer. Press Enter to create LargeObject.");
    Console.ReadLine();

    // Create and start 3 threads, each of which uses LargeObject.
    Thread[] threads = new Thread[3];
    for (int i = 0; i < 3; i++)
    {
        threads[i] = new Thread(ThreadProc);
        threads[i].Start();
    }

    // Wait for all 3 threads to finish.  
    foreach (Thread t in threads)
    {
        t.Join();
    }

    Console.WriteLine("\r\nPress Enter to end the program");
    Console.ReadLine();
}

如果我将 InitWithLambda 委托(delegate)放在 lazyLargeObject 声明之前，一切都很好。

如果我将 InitWithLambda 委托(delegate)放在 lazyLargeObject 声明之后，我会收到此错误:

Unhandled Exception: System.TypeInitializationException: The type initializer for 'Program' threw an exception. ---> System.ArgumentNullException: Value cannot be null. Parameter name: valueFactory at System.Lazy1..ctor(Func1 valueFactory, LazyThreadSafetyMode mode) at System.Lazy1..ctor(Func1 valueFactory) at Program..cctor() in E:\myCode\Misc\LazyWithLambda\LazyWithLambda\Class1.cs:line 10 --- End of inner exception stack trace --- at Program.Main()

lambda 表达式似乎未能分配给 valueFactory 参数。

但似乎位置不影响 InitLargeObject() 方法，它没有使用 Lambda 表达式。

为什么？

更新1

根据 Billy ONeal ，我用更简单的代码解决了这个问题:

这个没问题:

class FieldInitInOrder
{        
    static string s1 = "abc";
    static Int32 s1_length = s1.Length;

    static void Main()
    {
        Console.WriteLine(s1_length);
    }
}

这个抛出相同的 NullReference 异常:

class FieldInitInOrder
{
    static Int32 s1_length = s1.Length;  // Order switched
    static string s1 = "abc";  // Order switched

    static void Main()
    {
        Console.WriteLine(s1_length);
    }
}

我不知道为什么C#编译器是这样设计的。它可能会导致非常微妙的错误。有没有设计上的考虑？

Answer 1

C# 按照声明的顺序初始化成员。