EF4.0、4.3创建表达式树状动态查询总结

---------------------------------------------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))

*/