[英]Converting Linq query to expression tree
I have a lambda expression: 我有一个lambda表达式:
query = query.Include(e => e.Product);
How do I represent e => e.Product
via an Expression tree so that i can make it dynamically at runtime? 如何通过表达式树表示
e => e.Product
,以便可以在运行时动态创建它?
You can dynamically create such expression like this: 您可以像这样动态创建这样的表达式:
// Declare input parameter of dynamically generated lambda expression
var parameterExpr = Expression.Parameter(typeof(e), "e");
// Select desired navigation property that should be included in a query result
var propertyExpr = Expression.Property(parameterExpr, "Product");
// Generate expression to cast value of this property to type Object. This is
// not strictly needed (if you know type of included property at compile-time).
var convertExpr = Expression.Convert(propertyExpr, typeof(Object));
// Create lambda expression. Thanks to previous "Expression.Convert",
// we can cast lambda to Expression<Func<e, Object>> instead of
// Expression<Func<e, Product>> (because since you want to create expression
// dynamically, type of Product property is not necesarilly known at compile-time)
var lambdaExpr = (Expression<Func<e, Object>>)Expression.Lambda(convertExpr, parameterExpr);
// Use generated expression in a query
query = query.Include(lambdaExpr);
BTW: EF Core
(and possibly EF6
too, but I don't have experience with it) has overload of the .Include()
method that takes property name as an argument, so you actually don't need to generate expression tree at all: 顺便说一句:
EF Core
(可能还有EF6
,但我没有经验)具有以属性名作为参数的.Include()
方法的重载,因此您实际上根本不需要生成表达式树:
query = query.Include("Product");
As you didn't provide your types I created my own, so your expression will look a little different. 由于您没有提供您的类型,因此我创建了自己的类型,因此您的表情看起来会有些不同。
using System;
using System.Linq;
using System.Linq.Expressions;
using Microsoft.EntityFrameworkCore;
namespace ConsoleApp2
{
class Product
{
}
class Shop
{
}
class SomeEntity
{
public Product Product { get; set; }
public Shop Shop { get; set; }
}
class MyDbContext : DbContext
{
public DbSet<SomeEntity> Entities { get; set; }
}
class Program
{
private static Expression<Func<SomeEntity, Object>> GetExpression()
{
// a stupid random strategy
if (DateTime.Now.Ticks % 2 == 0)
{
Expression<Func<SomeEntity, Object>> expression = e => e.Shop;
return expression;
}
else
{
Expression<Func<SomeEntity, Object>> expression = e => e.Product;
return expression;
}
}
static void Main(string[] args)
{
var dbContext = new MyDbContext();
IQueryable<SomeEntity> query = dbContext.Entities;
var expression = GetExpression();
query = query.Include(expression);
}
}
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.