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<T>，我们轻松实现，看代码：
复制代码 代码如下:
/// <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
复制代码 代码如下:
///<summary>
///Laziestheexpressiontest.
///</summary>
[TestCase]
publicvoidLazyExpressionTest()
{
varlme1=newLazyExpression<int>(()=>2+1);
varlme2=newLazyExpression<int>(()=>1);
Assert.AreEqual(4,Add(lme1,lme2));
}

///<summary>
///Addsthespecifiedx.
///</summary>
///<paramname="x">Thex.</param>
///<paramname="y">They.</param>
///<returns>result</returns>
privateintAdd(LazyExpression<int>x,LazyExpression<int>y)
{
returnx.Evaluate()+y.Evaluate();
}

///<summary>
///Laziestheexpressionwithlogic.
///</summary>
[TestCase]
publicvoidLazyExpressionWithLogic()
{
varexp1=newLazyExpression<bool>(()=>true);
varexp2=newLazyExpression<bool>(()=>true||false);
if(LazyBoolExpression.And(exp1,exp2))
{
Console.WriteLine("lazyand");
}
if(LazyBoolExpression.Or(exp1,exp2))
{
Console.WriteLine("lazyor");
}
}

///<summary>
///Laziesthememoizedexpressiontest.
///</summary>
[TestCase]
publicvoidLazyMemoizedExpressionTest()
{
varlme1=newLazyMemoizedExpression<int>(()=>GetIntResult());
Assert.AreEqual(943,lme1.Evaluate());
Assert.AreEqual(943,lme1.Evaluate());
//output:
//1passed,0failed,0skipped,took2.80seconds(NUnit2.5).
}

///<summary>
///Comparestolazyexpressiontest.
///</summary>
[TestCase]
publicvoidCompareToLazyExpressionTest()
{
varlme1=newLazyExpression<int>(()=>GetIntResult());
Assert.AreEqual(943,lme1.Evaluate());
Assert.AreEqual(943,lme1.Evaluate());
//output:
//1passed,0failed,0skipped,took4.80seconds(NUnit2.5).
}

///<summary>
///Getstheintresult.
///</summary>
///<returns></returns>
privateintGetIntResult()
{
//currentthreadsleeptwosecond.
System.Threading.Thread.Sleep(2000);
return943;
}