Lambda表达式动态组装查询条件

时间:2023-03-09 07:20:48
Lambda表达式动态组装查询条件

最近比较闲,年底了,项目也进入尾声;每天就是维护一下系统,整理整理文档,整理知识点,这样才觉得有点意思;

问题

在使用Linq的where()查询的时候,不知道大家是怎么动态组装多个查询条件时,是怎么做的?我是这样做的,请看下面代码;

方法一:

1.1 Expression的扩展类

  public static class PredicateExtensions
{
public static Expression<Func<T, bool>> True<T>() { return f => true; } public static Expression<Func<T, bool>> False<T>() { return f => false; } public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>()); return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
} public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{ var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());//将一个委托或lambda表达式应用于参数表达式列表。 return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
}
}

1.2 实例化代码

            List<string> strList = new List<string>() { "郑州","上海户口", "鲁山揽锅菜", "南京酸菜鱼" };
//传统写法
Func<string, bool> func = (t) => t.Length > 3&&t.Contains("鱼");
var listA=strList.Where(func).ToList();
//使用扩展写法
var expression=PredicateExtensions.True<string>();
expression = expression.And(t => t.Length > 3);
expression = expression.And(t => t.Contains("鱼"));
var predicate = expression.Compile();
var listB = strList.Where(predicate).ToList();

  

方法二:合并两个Expression表达式

2.1 表达式参数扩展类

    public class MyExpressionVisitor:ExpressionVisitor
{
private readonly ParameterExpression _parameter; public MyExpressionVisitor(ParameterExpression parameter)
{
_parameter = parameter;
} public ParameterExpression Parameter
{
get { return _parameter; }
} public Expression Nodify(Expression exp)
{
Expression e = this.Visit(exp);
return e;
} protected override Expression VisitParameter(ParameterExpression node)
{
return _parameter;
} }

 2.2 实例化代码

int[] numbers = { 19, 25, 6, 8, 49, 7, 8, 0, 1, 47, 35, 30,29 };
//表达式一
ParameterExpression leftPara = Expression.Parameter(typeof(int), "n");
Expression a_con = Expression.Constant(13);
BinaryExpression a_binary = Expression.GreaterThan(leftPara, a_con);
var a_lambda = Expression.Lambda<Func<int, bool>>(a_binary,leftPara);
var a_result = a_lambda.Compile();
var a_list = numbers.Where(a_result).ToList();
//表达式二
ParameterExpression b_leftPara = Expression.Parameter(typeof(int), "n");
Expression b_con = Expression.Constant(30);
BinaryExpression b_binary = Expression.LessThanOrEqual(b_leftPara, b_con);
var b_lambda = Expression.Lambda<Func<int, bool>>(b_binary, b_leftPara);
var b_result = b_lambda.Compile();
var b_list = numbers.Where(b_result).ToList(); //合并两个表达式
var vistor = new MyExpressionVisitor(leftPara);
Expression c_1 = vistor.Nodify(a_lambda.Body);
Expression c_2 = vistor.Nodify(b_lambda.Body);
BinaryExpression c_binary = Expression.AndAlso(c_1, c_2);
var c_lambda = Expression.Lambda<Func<int, bool>>(c_binary, leftPara);
var c_result = c_lambda.Compile();//编译表达式
var c_list = numbers.Where(c_result).ToList(); 

方法二有点啰嗦,当时是用表达式树来创建的;大家可以用匿名表达式简单一点。但是在合并表达式的逻辑是,重新更改了一下表达式一和表达式二的参数,这样在编译的时候就可以通过了。

写的不咋样,日日精进吧。。。。