---------------------------------------------快速适用
效果：
where name like '%王%' and Age>=35 or Age<10
构建表达式树：（快速式）
1.添加命名空间：
using System.Linq.Expressions;
2.声明参数表达式（比如Book实体）
ParameterExpression param = Expression.Parameter(typeof(Book));
3.
Expression exp = Expression.Constant(true);初始化表达式树；
4.通过反射机制调用string.Contains,构建like;
var methodinfo = typeof(string).GetMethod("Contains");
5.拼接
//Expression.And:表示 and 后面的条件当然还有Or、 >=、<=、=等
//exp:and前面的语句，拼接之前的语句；
//Expression.Call():方法调用表达式；
//Expression.Property(param, "Name")字段或属性表达式
//methodinfo:调用上面Contains方法；
//Expression.Constant(s1):常量表达式；s1=“王”；
exp = Expression.And(exp, Expression.Call(Expression.Property(param, "Name"),methodinfo, Expression.Constant(s1)));
exp = Expression.And(exp, Expression.GreaterThanOrEqual(Expression.Property(param, "Id"),Expression.Constant(35)));
exp = Expression.Or(exp, Expression.LessThan(Expression.Property(param, "Id"), Expression.Constant(10)));
6.描述lambda表达式
var lam = Expression.Lambda<Func<Book, bool>>(exp, param);
7.调用
var lis = books.Where<Book>(lam).ToList();
------------------------------------------------------------详细（转述网友秦迷）
//---------------动态创建表达式目录树---------------//
//名字模糊查询tony
//年龄大于等于20
//年龄小于30
string uname = "tony";
List<Expression> expList = new List<Expression>(2);
ParameterExpression parExp = Expression.Parameter(typeof(UserInfo), "infos");//创建需要查找的参数
//查姓名infos.UserName.Contains("tony")
MemberExpression unameExp = Expression.Property(parExp, "UserName");
MethodInfo containsMethod = typeof(string).GetMethod("Contains");//using System.Reflection;指定实现Contains方法
ConstantExpression unamevalueExp = Expression.Constant(uname, typeof(string));
MethodCallExpression containsExp = Expression.Call(unameExp, containsMethod, unamevalueExp);
expList.Add(containsExp);
//查年龄infos.Age>=20
MemberExpression age1 = Expression.Property(parExp, "Age");
ConstantExpression agevalue1 = Expression.Constant(20, typeof(int));
BinaryExpression gteExp = Expression.GreaterThanOrEqual(age1, agevalue1);
expList.Add(gteExp);
//查年龄infos.Age<30
//if (1 > 2)
//{
MemberExpression age2 = Expression.Property(parExp, "Age");
ConstantExpression agevalue2 = Expression.Constant(30, typeof(int));
BinaryExpression ltExp = Expression.LessThan(age2, agevalue2);
expList.Add(ltExp);
//}
//把上面的表达式连接起来
Expression whereExp = null;
foreach (var item in expList)
{
if (whereExp == null)
{
whereExp = item;
}
else
{
whereExp = Expression.And(whereExp, item);
/*
* 出错信息：
* 没有为类型“System.Boolean”和“System.Boolean”定义二进制运算符 Add。
* 是And，开始我写错成了Add一直找不到原因
*/
}
}
//生成Lambda表达式
//Expression<Func<UserInfo, bool>> lambda = Expression.Lambda<Func<UserInfo, bool>>(whereExp, parExp);
LambdaExpression lambda = Expression.Lambda<Func<UserInfo, bool>>(whereExp, parExp);
Console.WriteLine(lambda);
/*
* 生成结果：
* infos => ((infos.UserName.Contains("tony") And (infos.Age>=20)) And (infos.Age<30))
*/
