使用EF Core和条件WHERE子句从数据库读取行时出现问题

Question

我想在我的ASP .Net Core 3.0 Web API中查询MySql数据库，并有条件地应用一些WHERE筛选器。所以我在我的控制器动作之一中有这个：

[HttpGet]
public async Task<IEnumerable<Customer>> GetCustomers([FromQuery] bool? isActive, [FromQuery] int? typeId, [FromQuery] bool? isProcessed)
{
    var customers = _context.Customers.Where(c => c.IsDeleted == false);

    if (isActive.HasValue)
        customers = customers.Where(c => c.IsActive == isActive.Value);

    if (typeId.HasValue)
        customers = customers.Where(c => c.TypeId == typeId.Value);

    if (isProcessed.HasValue)
        customers = customers.Where(c => c.IsProcessed == isProcessed.Value);

    return await customers.ToListAsync();
}

这完美地工作了，因为在第一行中有一个Where子句：

var customers = _context.Customers.Where(c => c.IsDeleted == false);

但实际上我不想放入Where子句。我只想要这样：

[HttpGet]
public async Task<IEnumerable<Customer>> GetCustomers([FromQuery] bool? isActive, [FromQuery] int? typeId, [FromQuery] bool? isProcessed)
{
    var customers = _context.Customers;

    if (isActive.HasValue)
        customers = customers.Where(c => c.IsActive == isActive.Value);

    if (typeId.HasValue)
        customers = customers.Where(c => c.TypeId == typeId.Value);

    if (isProcessed.HasValue)
        customers = customers.Where(c => c.IsProcessed == isProcessed.Value);

    return await customers.ToListAsync();
}

但是，一旦我删除了第一个Where子句，我就会得到以下异常：

错误CS0266无法将类型' System.Linq.IQueryable<PropWorx.API.Models.Customer>' 隐式转换为' Microsoft.EntityFrameworkCore.DbSet<PropWorx.API.Models.Customer>'。存在显式转换（您是否缺少演员表？）

有任何想法吗？

Answer 1

没有的原始代码var看起来像

DbSet<Customer> customers = _context.Customers;

if (isActive.HasValue)
    customers = customers.Where(c => c.IsActive == isActive.Value);
    //Where returns IQuaryable<Customer>, hence the error

在这种情况下，var您可能无法理解正在编写的代码。

使用AsQueryable()扩展名获得所需的行为

var customers = _context.Customers.AsQueryable(); //IQueryable<Customer>

//...

或明确说明使用的类型

IQueryable<Customer> customers = _context.Customers;

//...