问题描述
目前,我有这样的方法来比较两个数字
专用功能ETForGreaterThan(BYVAL查询作为的IQueryable(Of T)已, BYVAL为PropertyValue作为对象,BYVAL的PropertyInfo作为的PropertyInfo)作为的IQueryable(Of T)已
尺寸E上ParameterExpression = Expression.Parameter(的GetType(T),E)
昏暗米作为MemberExpression = Expression.MakeMemberAccess(即的PropertyInfo)
尺寸C作为常量表达式= Expression.Constant(为PropertyValue,propertyValue.GetType())
尺寸b以BinaryExpression = Expression.GreaterThan(M,C)
尺寸的lambda作为一种表达(中Func键(中T,布尔))= Expression.Lambda(中Func键(中T,布尔))(b,E)
返回query.Where(拉姆达)
端功能
它工作正常,并以这种方式
$ b $消耗b 查询= ETForGreaterThan(查询,价值的PropertyInfo)
正如你所看到的,我给它一个IQueryable收集和它where子句的属性和值添加到它,基地。 Y能够构建每种不超过,LessOrEqualThan等等同物System.Linq.Expressions.Expression有这个运营商预定义的。
¿我如何改造这个代码做琴弦一样吗? System.Linq.Expressions.Expression不给我一个预定义运算符,如包含或startwith,我真的表达式树的小白。
谢谢,并请在发布C#/ VB你的答案。 。选择一个让你感觉更加舒适。
使用系统;
使用System.Linq的;使用System.Linq.Expressions
;
使用的System.Reflection;
命名空间WindowsFormsApplication1
{
静态类节目
{
[STAThread]
静态无效的主要()
{
使用(VAR上下文=新NorthwindEntities())
{
的PropertyInfo的PropertyInfo = typeof运算(客户).GetProperty(客户id);
&IQueryable的LT;客户>查询= context.Customers;
=查询 - ETForStartsWith LT;客户>(查询,A,的PropertyInfo);
无功名单= query.ToList();
}
}
静态的IQueryable< T> ETForStartsWith< T>(IQueryable的< T>查询字符串为PropertyValue,的PropertyInfo的PropertyInfo)
{
ParameterExpression E = Expression.Parameter(typeof运算(T),E);
MemberExpression M = Expression.MakeMemberAccess(即的PropertyInfo);
常量表达式C = Expression.Constant(为PropertyValue,typeof运算(字符串));
MethodInfo的MI = typeof运算(字符串).GetMethod(StartsWith,新类型[] {typeof运算(字符串)});
表达的呼叫= Expression.Call(男,MI,C);
表达式来; Func键< T,BOOL>>波长= Expression.Lambda<&Func键LT; T,BOOL>>(打电话,发);
返回query.Where(拉姆达);
}
}
}
Currently, I have this method to compare two numbers
Private Function ETForGreaterThan(ByVal query As IQueryable(Of T), ByVal propertyValue As Object, ByVal propertyInfo As PropertyInfo) As IQueryable(Of T)
Dim e As ParameterExpression = Expression.Parameter(GetType(T), "e")
Dim m As MemberExpression = Expression.MakeMemberAccess(e, propertyInfo)
Dim c As ConstantExpression = Expression.Constant(propertyValue, propertyValue.GetType())
Dim b As BinaryExpression = Expression.GreaterThan(m, c)
Dim lambda As Expression(Of Func(Of T, Boolean)) = Expression.Lambda(Of Func(Of T, Boolean))(b, e)
Return query.Where(lambda)
End Function
It works fine and is consumed in this way
query = ETForGreaterThan(query, Value, propertyInfo)
As you can see, I give it an IQueryable collection and it add a where clause to it, base on a property and a value. Y can construct Lessthan, LessOrEqualThan etc equivalents as System.Linq.Expressions.Expression has this operators predefined.
¿How can I transform this code to do the same with strings? System.Linq.Expressions.Expression don't give me a predefined operator like "contains" or "startwith" and I'm really noob with Expression trees.
Thanks, and please Post your answer in C#/VB. Choose the one that make you feel more confortable.
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
namespace WindowsFormsApplication1
{
static class Program
{
[STAThread]
static void Main()
{
using (var context = new NorthwindEntities())
{
PropertyInfo propertyInfo = typeof(Customer).GetProperty("CustomerID");
IQueryable<Customer> query = context.Customers;
query = ETForStartsWith<Customer>(query, "A", propertyInfo);
var list = query.ToList();
}
}
static IQueryable<T> ETForStartsWith<T>(IQueryable<T> query, string propertyValue, PropertyInfo propertyInfo)
{
ParameterExpression e = Expression.Parameter(typeof(T), "e");
MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
ConstantExpression c = Expression.Constant(propertyValue, typeof(string));
MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
Expression call = Expression.Call(m, mi, c);
Expression<Func<T, bool>> lambda = Expression.Lambda<Func<T, bool>>(call, e);
return query.Where(lambda);
}
}
}
这篇关于如何创建一个表达式树做同样的" StartsWith"的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!