记录Linq中lambda动态表达式的使用方式

项目中有的时候我们会用到动态表达式的方式去查询数据，这里简单记录下个人的使用方式，方便使用↓

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//构建参数表达式
ParameterExpression parameter = Expression.Parameter(typeof(SimpleResult));
Expression conditionValue = Expression.Equal(Expression.Property(parameter, groupFileName), Expression.Constant(value, typeof(Object)));
var lambdaValue = Expression.Lambda<Func<SimpleResult, bool>>(
   conditionValue,
   parameter);
var groupCount = resultData.AsQueryable().Count(lambdaValue);
var item = new GroupModel { GroupName = groupFileName, GroupValue = value?.ToString(), GroupCount = groupCount };
listObj.Add(item);



/*

 1、创建表达式
 2、创建表达式参数
 3、查询

 */

// 表达式传过去的参数
// ParameterExpression parameter1 = Expression.Parameter(typeof(SimpleResult), "o");
// 使用的字段
//MemberExpression member1 = Expression.PropertyOrField(parameter1, groupFileName);
// 要比较的值
//ConstantExpression constant1 = Expression.Constant(value, typeof(Object));
// query
//var query = Expression.Equal(member1, constant1);//Equal等于；GreaterThanOrEqual大于；LessThanOrEqual小于

// var lambda = Expression.Lambda<Func<SimpleResult, bool>>(query, parameter1);

// var groupCount2 = resultData.AsQueryable().Count(lambda);

还有另外一种动态表达式的使用方式，通过安装DynamicExpresso.CoreNuget包

参考地址：https://github.com/davideicardi/DynamicExpresso

==============================（Install - Package DynamicExpresso.Core）======================================

var whereExpression = $"m.{queryField}=="{queryValue}"";

string whereExpression = "customer.Age > 18 && customer.Gender == 'F'";

var interpreter = new Interpreter();
Func<Customer, bool> dynamicWhere = interpreter.ParseAsDelegate<Func<Customer, bool>>(whereExpression, "customer");

Assert.AreEqual(1, customers.Where(dynamicWhere).Count());

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string whereExpression = "customer.Age > 18 && customer.Gender == 'F'";

var interpreter = new Interpreter();
Expression<Func<Customer, bool>> expression = interpreter.ParseAsExpression<Func<Customer, bool>>(whereExpression, "customer");

Assert.AreEqual(1, customers.Where(expression).Count());

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