c#3.0实现延迟赋值示例

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：
复制代码代码如下:
int Add(int x, int y)
{
return (2 + 1) + (1);
}

使用Func，我们轻松实现，看代码：
复制代码代码如下:
///

/// Lazyexpression
///

/// T
public class Lazyexpression
{
    Func thunk;
    public Lazyexpression(Func Thunk)
    {
        thunk = Thunk;
    }
    public T evaluate()
    {
        return thunk();
    }
}

///

/// LazyBoolexpression
///

public static class LazyBoolexpression
{
    public static bool And(Lazyexpression LHS, Lazyexpression RHS)
    {
        return LHS.evaluate() && RHS.evaluate();
    }
    public static bool Or(Lazyexpression LHS, Lazyexpression RHS)
    {
        return LHS.evaluate() == true ? true : RHS.evaluate();
    }
}

///

/// LazyMemoizedexpression
///

///
public class LazyMemoizedexpression
{
    bool thunked;
    T value;
    Func thunk;
    public LazyMemoizedexpression(Func Thunk)
    {
        thunked = false;
        thunk = Thunk;
    }
    public T evaluate()
    {
        if (!thunked)
        {
            value = thunk();
            thunked = true;
        }
        return value;
    }
}

Lazyexpression类实现了基本的延迟赋值，LazyMemoizedexpression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
复制代码代码如下:
///

///Laziestheexpressiontest.
///

[TestCase]
publicvoidLazyexpressionTest()
{
varlme1=newLazyexpression(()=>2+1);
varlme2=newLazyexpression(()=>1);
Assert.AreEqual(4,Add(lme1,lme2));
}

///

///Addsthespecifiedx.
///

///Thex.
///They.
///result
privateintAdd(Lazyexpressionx,Lazyexpressiony)
{
returnx.evaluate()+y.evaluate();
}

///

///Laziestheexpressionwithlogic.
///

[TestCase]
publicvoidLazyexpressionWithLogic()
{
varexp1=newLazyexpression(()=>true);
varexp2=newLazyexpression(()=>true||false);
if(LazyBoolexpression.And(exp1,exp2))
{
Console.WriteLine("lazyand");
}
if(LazyBoolexpression.Or(exp1,exp2))
{
Console.WriteLine("lazyor");
}
}

///

///Laziesthememoizedexpressiontest.
///

[TestCase]
publicvoidLazyMemoizedexpressionTest()
{
varlme1=newLazyMemoizedexpression(()=>GetIntResult());
Assert.AreEqual(943,lme1.evaluate());
Assert.AreEqual(943,lme1.evaluate());
//output:
//1passed,0failed,0skipped,took2.80seconds(NUnit2.5).
}

///

///Comparestolazyexpressiontest.
///

[TestCase]
publicvoidCompareToLazyexpressionTest()
{
varlme1=newLazyexpression(()=>GetIntResult());
Assert.AreEqual(943,lme1.evaluate());
Assert.AreEqual(943,lme1.evaluate());
//output:
//1passed,0failed,0skipped,took4.80seconds(NUnit2.5).
}

///

///Getstheintresult.
///

///
privateintGetIntResult()
{
//currentthreadsleeptwosecond.
System.Threading.Thread.Sleep(2000);
return943;
}

c#3.0实现延迟赋值示例

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