Lambda表达式树

1、常量表达式树

Func<int> compile = (() => 2 + 3);

使用表达式树的方式

ConstantExpression a = Expression.Constant(2);
ConstantExpression b = Expression.Constant(3);
BinaryExpression add = Expression.Add(a, b);

Expression<Func<int>> lambda= Expression.Lambda<Func<int>>(add);
Func<int> compile2 = lambda.Compile();

调用

Console.WriteLine(compile());
Console.WriteLine(compile2());

输出结果

 要点：

1、ConstantExpression 是常量表达式，通过Expression.Constant(object value)方法创建一个常量表达式

2、BinaryExpression 二元表达式 Expression.Add(Expression left,Expression right)这是加法二元表达式，同理还有Subtract、Multiply和Divide分别对应减、乘和除二元表达式

3、Expression.Lambda<Func<int>>(add)转换成lambda表达式树

4、lambda.Compile()将lambda表达式树转化成委托

2、参数表达式树

lambda表达式

Func<int, int> compile = x=>x+2;

lambda表达式树

ConstantExpression a = Expression.Constant(2);
ParameterExpression b = Expression.Parameter(typeof(int),"b");
BinaryExpression add = Expression.Add(a, b);

Expression<Func<int,int>> lambda= Expression.Lambda<Func<int,int>>(add,b);
Func<int,int> compile2 = lambda.Compile();

注意使用Expression.Lambda方法时，不需要提供常量表达式树

调用

Console.WriteLine(compile(3));
Console.WriteLine(compile2(3));

 当left和right都是参数表达式树时

Func<int, int,int> compile = (x,y)=>x+y;


ParameterExpression a = Expression.Parameter(typeof(int), "a");
ParameterExpression b = Expression.Parameter(typeof(int),"b");
BinaryExpression add = Expression.Add(a, b);

Expression<Func<int,int,int>> lambda= Expression.Lambda<Func<int,int,int>>(add,a,b);
Func<int,int, int> compile2 = lambda.Compile();

Console.WriteLine(compile(2,3));
Console.WriteLine(compile2(2,3));