Lambda 连接

Question

我尝试将我的 lambda 字符串与 + 符号一起放入，但它不会让我运行代码，因为它说表达式无效！

代码顶部

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;
using Certifications.Data;
using Certifications.Models;
using Microsoft.EntityFrameworkCore.Internal;

namespace Certifications.Controllers
{
    public class Managerial : Controller
    {
        private readonly CertificationContext _context;

        public Managerial(CertificationContext context)
        {
            _context = context;
        }

过滤器

// Approval Filter
string ApprovalFilterBuild = "";

if (approval == "Approved")
{ 
    ApprovalFilterBuild = ".Where(i => i.Approved == true);"; 
}

if (approval == "Revoked")
{
   ApprovalFilterBuild = ".Where(i => i.Approved == false);"; 
}

if (approval == "ALL")
{ 
    ApprovalFilterBuild = ""; 
}

查询

var certificationContext = _context.INT_CertificationsXREF
            .Include(i => i.INT_CertificationCategories)
            .Include(i => i.INT_Certifications)
            .Include(i => i.INT_CertificationConferred)
            .Include(i => i.RIM_Resource)
            +ApprovalFilterBuild+
            .Where(i => i.RIM_Resource.LAN == i.RIM_Resource.LAN)
            .Where(i => LANlist.Contains(i.RIM_Resource.LAN));


return View(await certificationContext.ToListAsync());

Answer 1

所有这些 Linq 方法都返回 IQueryable<>，在您迭代结果之前，它实际上不会对您的数据库执行。所以你可以简单地做这样的事情：

//Assuming the type name is INT_CertificationsXREF:
IQueryable<INT_CertificationsXREF> certificationContext = _context.INT_CertificationsXREF
    .Include(i => i.INT_CertificationCategories)
    .Include(i => i.INT_Certifications)
    .Include(i => i.INT_CertificationConferred)
    .Include(i => i.RIM_Resource);

if (approval == "Approved")
{
    certificationContext = certificationContext.Where(i => i.Approved == true);
}
else if (approval == "Revoked")
{
    certificationContext = certificationContext.Where(i => i.Approved == false);
}

Answer 2

审批过滤器需要是 lambda 表达式而不是字符串。这就是你构建它的方式。

// Approval Filter
Expression<Func<INT_CertificationsXREF, bool>> ApprovalFilterBuild;

if (approval == "Approved")
{ 
    ApprovalFilterBuild = i => i.Approved == true;
}

if (approval == "Revoked")
{
   ApprovalFilterBuild = i => i.Approved == false;
}

if (approval == "ALL")
{ 
    ApprovalFilterBuild = i => true;
}

这就是您在主查询中使用它的方式。

var certificationContext = _context.INT_CertificationsXREF
    .Include(i => i.INT_CertificationCategories)
    .Include(i => i.INT_Certifications)
    .Include(i => i.INT_CertificationConferred)
    .Include(i => i.RIM_Resource)
    .Where(ApprovalFilterBuild)
    .Where(i => i.RIM_Resource.LAN == i.RIM_Resource.LAN)
    .Where(i => LANlist.Contains(i.RIM_Resource.LAN));

如果批准不能是 3 项检查中的任何一项，则考虑将默认值分配给 i => true

Answer 3

为确保它适用于 IEnumerable 和 IQueryable，您可以使用以下代码：

var certificationContext = _context.INT_CertificationsXREF
        .Include(i => i.INT_CertificationCategories)
        .Include(i => i.INT_Certifications)
        .Include(i => i.INT_CertificationConferred)
        .Include(i => i.RIM_Resource).AsEnumerable();

if (approval == "Approved") 
{
    certificationContext = certificationContext.Where(i => i.Approved).AsEnumerable();
}
if (approval == "Revoked")
{ 
certificationContext = certificationContext.Where(i => !i.Approved).AsEnumerable();
}
var result  = certificationContext.Where(i => i.RIM_Resource.LAN == i.RIM_Resource.LAN).Where(i => LANlist.Contains(i.RIM_Resource.LAN));

“结果”变量将获得您想要的结果。