延迟赋值主要有两点:

1.一个参数可能或可能没被赋值．

2.一个参数在一个函数中每次使用时可能被赋值．

如下面的这种情况：


int Add(int x, int y) {     return (2 + 1) + (1); }


使用Func<T>，我们轻松实现，看代码：

/// <summary> /// LazyExpression /// </summary> /// <typeparam name="T">T</typeparam> public class LazyExpression<T> {     Func<T> thunk;     public LazyExpression(Func<T> Thunk)     {         thunk = Thunk;     }     public T Evaluate()     {         return thunk();     } }  /// <summary> /// LazyBoolExpression /// </summary> public static class LazyBoolExpression {     public static bool And(LazyExpression<bool> LHS, LazyExpression<bool> RHS)     {         return LHS.Evaluate() && RHS.Evaluate();     }     public static bool Or(LazyExpression<bool> LHS, LazyExpression<bool> RHS)     {         return LHS.Evaluate() == true ? true : RHS.Evaluate();     } }  /// <summary> /// LazyMemoizedExpression /// </summary> /// <typeparam name="T"></typeparam> public class LazyMemoizedExpression<T> {     bool thunked;     T value;     Func<T> thunk;     public LazyMemoizedExpression(Func<T> Thunk)     {         thunked = false;         thunk = Thunk;     }     public T Evaluate()     {         if (!thunked)         {             value = thunk();             thunked = true;         }         return value;     } }

LazyExpression<T>类实现了基本的延迟赋值，LazyMemoizedExpression<T>类实现了一次赋值，多次使用．

LazyBoolExpression实现逻辑表达式． 

看UnitTest，一切就明白了．

/// <summary>
     /// Lazies the expression test.
     /// </summary>
     [TestCase]
     public void LazyExpressionTest()
     {
         var lme1 = new LazyExpression<int>(() => 2 + 1);
         var lme2 = new LazyExpression<int>(() => 1);
         Assert.AreEqual(4, Add(lme1, lme2));   
     }

     /// <summary>
     /// Adds the specified x.
     /// </summary>
     /// <param name="x">The x.</param>
     /// <param name="y">The y.</param>
     /// <returns>result</returns>
     private int Add(LazyExpression<int> x, LazyExpression<int> y)
     {
         return x.Evaluate() + y.Evaluate();
     }

     /// <summary>
     /// Lazies the expression with logic.
     /// </summary>
     [TestCase]
     public void LazyExpressionWithLogic()
     {
        var exp1 = new LazyExpression<bool>(() => true);
         var exp2 = new LazyExpression<bool>(() => true || false);
         if (LazyBoolExpression.And(exp1, exp2))
         {
             Console.WriteLine("lazy and");
         }
         if (LazyBoolExpression.Or(exp1, exp2))
         {
             Console.WriteLine("lazy or");
         }
     }

     /// <summary>
     /// Lazies the memoized expression test.
     /// </summary>
     /// <remarks>Author Petter Liu javascript:void(0) </remarks>
     [TestCase]
     public void LazyMemoizedExpressionTest()
     {
         var lme1 = new LazyMemoizedExpression<int>(() => GetIntResult());
         Assert.AreEqual(943, lme1.Evaluate());
         Assert.AreEqual(943, lme1.Evaluate());
         //output:
         //1 passed, 0 failed, 0 skipped, took 2.80 seconds (NUnit 2.5).
     }

     /// <summary>
     /// Compares to lazy expression test.
     /// </summary>
/// <remarks>Author Petter Liu javascript:void(0) </remarks> 
     [TestCase]
     public void CompareToLazyExpressionTest()
     {
         var lme1 = new LazyExpression<int>(() => GetIntResult());
         Assert.AreEqual(943, lme1.Evaluate());
         Assert.AreEqual(943, lme1.Evaluate());
         //output:
         //1 passed, 0 failed, 0 skipped, took 4.80 seconds (NUnit 2.5).
     }

     /// <summary>
     /// Gets the int result.
     /// </summary>
     /// <returns></returns>
     private int GetIntResult()
     {
         //current thread sleep two second.
         System.Threading.Thread.Sleep(2000);
         return 943;
     }


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