我正在使用Entity Framework Core来存储和检索一些数据。我正在尝试编写一种通用方法，该方法可在任何DbSet<T>上使用，以避免代码重复。此方法针对该集合运行LINQ查询，为此它需要知道“键”列（即表的主键）。
为了帮助这一点，我定义了一个接口，该接口返回代表键列的属性的名称。实体随后实现此接口。因此，我有这样的事情：
interface IEntityWithKey
{
string KeyPropertyName { get; }
}
class FooEntity : IEntityWithKey
{
[Key] public string FooId { get; set; }
[NotMapped] public string KeyPropertyName => nameof(FooId);
}
class BarEntity : IEntityWithKey
{
[Key] public string BarId { get; set; }
[NotMapped] public string KeyPropertyName => nameof(BarId);
}
我尝试编写的方法具有此签名：
static List<TKey> GetMatchingKeys<TEntity, TKey>(DbSet<TEntity> dbSet, List<TKey> keysToFind)
where TEntity : class, IEntityWithKey
基本上，给定一个DbSet包含类型为[[TEntity的实体和类型为TKey的键的列表，该方法应返回数据库中相关表中当前存在的键的列表。] >查询看起来像这样：
dbSet.Where(BuildWhereExpression()).Select(BuildSelectExpression()).ToList()
在BuildWhereExpression中，我试图创建一个适当的Expression<Func<TEntity, bool>>，在BuildSelectExpression中，我试图创建一个适当的Expression<Func<TEntity, TKey>>。但是，我只是在创建Select（）表达式而苦苦挣扎，这是两者中比较容易的。这是我到目前为止的内容：
Expression<Func<TEntity, TKey>> BuildSelectExpression()
{
// for a FooEntity, would be: x => x.FooId
// for a BarEntity, would be: x => x.BarId
ParameterExpression parameter = Expression.Parameter(typeof(TEntity));
MemberExpression property1 = Expression.Property(parameter, nameof(IEntityWithKey.KeyPropertyName));
MemberExpression property2 = Expression.Property(parameter, property1.Member as PropertyInfo);
UnaryExpression result = Expression.Convert(property2, typeof(TKey));
return Expression.Lambda<Func<TEntity, TKey>>(result, parameter);
}
这将运行，并且传递给数据库的查询看起来正确，但是我得到的只是键属性名称的列表。例如，这样调用：
List<string> keys = GetMatchingKeys(context.Foos, new List<string> { "foo3", "foo2" });
它生成此查询，看起来不错（注意：尚无Where（）实现）：
SELECT "f"."FooId"
FROM "Foos" AS "f"
但是查询仅返回包含“ FooId”的列表，而不是存储在数据库中的实际ID。
我觉得我已经接近解决方案了，但是我只是略微绕过表达式的东西，以前没有做太多事情。如果有人可以帮助使用Select（）表达式，那将是一个开始。
这里是完整代码：
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
namespace StackOverflow
{
interface IEntityWithKey
{
string KeyPropertyName { get; }
}
class FooEntity : IEntityWithKey
{
[Key] public string FooId { get; set; }
[NotMapped] public string KeyPropertyName => nameof(FooId);
}
class BarEntity : IEntityWithKey
{
[Key] public string BarId { get; set; }
[NotMapped] public string KeyPropertyName => nameof(BarId);
}
class TestContext : DbContext
{
public TestContext(DbContextOptions options) : base(options) { }
public DbSet<FooEntity> Foos { get; set; }
public DbSet<BarEntity> Bars { get; set; }
}
class Program
{
static async Task Main()
{
IServiceCollection services = new ServiceCollection();
services.AddDbContext<TestContext>(
options => options.UseSqlite("Data Source=./test.db"),
contextLifetime: ServiceLifetime.Scoped,
optionsLifetime: ServiceLifetime.Singleton);
services.AddLogging(
builder =>
{
builder.AddConsole(c => c.IncludeScopes = true);
builder.AddFilter(DbLoggerCategory.Infrastructure.Name, LogLevel.Error);
});
IServiceProvider serviceProvider = services.BuildServiceProvider();
var context = serviceProvider.GetService<TestContext>();
context.Database.EnsureDeleted();
context.Database.EnsureCreated();
context.Foos.AddRange(new FooEntity { FooId = "foo1" }, new FooEntity { FooId = "foo2" });
context.Bars.Add(new BarEntity { BarId = "bar1" });
await context.SaveChangesAsync();
List<string> keys = GetMatchingKeys(context.Foos, new List<string> { "foo3", "foo2" });
Console.WriteLine(string.Join(", ", keys));
Console.WriteLine("DONE");
Console.ReadKey(intercept: true);
}
static List<TKey> GetMatchingKeys<TEntity, TKey>(DbSet<TEntity> dbSet, List<TKey> keysToFind)
where TEntity : class, IEntityWithKey
{
return dbSet
//.Where(BuildWhereExpression()) // commented out because not working yet
.Select(BuildSelectExpression()).ToList();
Expression<Func<TEntity, bool>> BuildWhereExpression()
{
// for a FooEntity, would be: x => keysToFind.Contains(x.FooId)
// for a BarEntity, would be: x => keysToFind.Contains(x.BarId)
throw new NotImplementedException();
}
Expression<Func<TEntity, TKey>> BuildSelectExpression()
{
// for a FooEntity, would be: x => x.FooId
// for a BarEntity, would be: x => x.BarId
ParameterExpression parameter = Expression.Parameter(typeof(TEntity));
MemberExpression property1 = Expression.Property(parameter, nameof(IEntityWithKey.KeyPropertyName));
MemberExpression property2 = Expression.Property(parameter, property1.Member as PropertyInfo);
UnaryExpression result = Expression.Convert(property2, typeof(TKey));
return Expression.Lambda<Func<TEntity, TKey>>(result, parameter);
}
}
}
}
这将使用以下NuGet软件包：
，以避免代码重复。此方法运行...
IEntityWithKey接口是冗余的。要从KeyPropertyName方法访问BuildSelectExpression值，您需要具有实体实例，但只有Type对象。