无法翻译 Linq 表达式错误 Entity Framework Core 3

Question

我在一个项目中遇到了一些数据访问问题。我使用 EF 核心，并且我有这些堆栈跟踪。

执行的第一个代码

public static void Main(){
 EntityServiceBase<Product> manager = new EntityServiceBase(new EFProductDal());//efproductdal for to access database
 Product result = manager.GetByPrimaryKey(5);
}

执行的第二个文件代码

在代码中，IEntity 是一个空接口，用于确定数据库对象
IEntityRepostiory 具有访问数据库的 EF 代码，该代码显示在属于的第三个代码部分PrimaryKeyComparable 具有比较函数来检查对象的主键是否等于给定键

using System;
using System.Collections.Generic;
using ECommercial.Core.DataAccess;
using ECommercial.Core.Entities;

namespace ECommercial.Core.Business
{
    public abstract class EntityServiceBase<TEntity> : IService<TEntity> 
        where TEntity :class,IEntity, new() // IENTITY IS AN EMPTY INTERFACE TO JUST DETERMINE DATABASE ENTITIES
    {
        private IEntityRepository<TEntity> _entityRepository;

        public EntityServiceBase(IEntityRepository<TEntity> entityRepository)
        {
            _entityRepository = entityRepository;
        }

        public TEntity GetByPrimaryKey(Object key)
        {
            PrimaryKeyComparable primaryKeyComparable = new PrimaryKeyComparable();
            return _entityRepository.Get(o=>primaryKeyComparable.comparePrimaryKey<TEntity>(o,key));
        }
    }
}

执行的第三个代码部分

在代码上下文是 EF dbcontext
过滤器被赋予过滤器，即 o=>primaryKeyComparable.comparePrimaryKey(o,key) 第二个代码文件。错误发生在 .SingleOrDefault(filter) 部分。如果我删除它，功能会很好。

public TEntity Get(Expression<Func<TEntity, bool>> filter)
        {
            using (var context=new TContext()){
                TEntity result = context.Set<TEntity>().SingleOrDefault(filter);
                return result;
            }
        }

ComparePrimaryKey 函数

public bool comparePrimaryKey<TEntity>(TEntity entity,Object value)
            where TEntity:class,IEntity,new()
        {
            Type entityType = typeof(TEntity);
            FieldInfo[] fields = entityType.GetFields();
            FieldInfo found =null;
            foreach(var field in fields){
                Attribute attribute=field.GetCustomAttribute(typeof(PrimaryKeyFieldAttribute));
                if(attribute!=null){
                    found=field;
                    break;
                }
            }
            if(found!=null){
                Type foundDeclaringType= found.DeclaringType;
                Type valueDeclaringType = value.GetType().DeclaringType;
                if((foundDeclaringType.IsSubclassOf(valueDeclaringType)) ||valueDeclaringType.IsSubclassOf(foundDeclaringType) || valueDeclaringType==foundDeclaringType)
                {
                    return entity.Equals(value);
                }
                throw new InvalidCastException("Given entity's primary key must have same or child-base related declaration type with key object compared");
            }
            throw new CustomAttributeFormatException("The Entity Has Any field has PrimaryKeyFieldAttribute attribute. Must be PrimaryKeyAttribute on primary key field.");
        }

错误信息（我觉得英文部分就够理解了）

.SingleOrDefault(filter) PART 的第三个文件出现错误

Exception has occurred: CLR/System.InvalidOperationException
'System.InvalidOperationException' türünde özel durum Microsoft.EntityFrameworkCore.dll öğesinde oluştu, fakat kullanıcı kodunda işlenmedi: 'The LINQ expression 'DbSet<Product>
    .Where(p => new PrimaryKeyComparable().comparePrimaryKey<Product>(
        entity: p, 
        value: __key_0))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.'
   konum Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.<VisitMethodCall>g__CheckTranslated|8_0(ShapedQueryExpression translated, <>c__DisplayClass8_0& )
   konum Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   konum Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   konum System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   konum System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   konum Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   konum Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   konum System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   konum System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   konum Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   konum Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   konum Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   konum Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   konum Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   konum Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   konum Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   konum Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   konum System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)
   konum ECommercial.Core.DataAccess.EntitiyFramework.EFIEntityRepositoryBase`2.Get(Expression`1 filter) C:\Users\nihaSWin\Desktop\ECommercial\ECommercial.Core\DataAccess\EntitiyFramework\EFIEntityRepositoryBase.cs içinde: 36. satır
   konum ECommercial.Core.Business.EntityServiceBase`1.GetByPrimaryKey(Object key) C:\Users\nihaSWin\Desktop\ECommercial\ECommercial.Core\Business\EntityServiceBase.cs içinde: 26. satır
   konum ECommercial.MVC.Startup..ctor(IConfiguration configuration) C:\Users\nihaSWin\Desktop\ECommercial\ECommercial.MVC\Startup.cs içinde: 19. satır

Answer 1

所以看起来您正在尝试使用反射来确定主键并基于此进行查询。您不能直接在表达式中使用这样的方法，并期望 EF 解开它的作用并将其转换为 sql。

但是，您可以使用反射来生成表达式并直接使用它。

public Expression<Func<TEntity, bool>> comparePrimaryKey<TEntity>(object value)
{
    var parm = Expression.Parameter(typeof(TEntity), "e");
    var objType = value.GetType();
    return Expression.Lambda<Func<TEntity, bool>>(
        typeof(TEntity)
            .GetFields()
            .Where(f => f.GetCustomAttribute(typeof(PrimaryKeyFieldAttribute)) != null)
            .Select(f => (Expression)Expression.Equal(
                Expression.MakeMemberAccess(parm, f),
                Expression.Constant(
                    (objType == f.FieldType)
                        ? value
                        : objType.GetField(f.Name).GetValue(value),
                    f.FieldType)
            ))
            .Aggregate((l, r) => Expression.AndAlso(l, r)),
        parm);
}

//...

return _entityRepository.Get(primaryKeyComparable.comparePrimaryKey<TEntity>(key));

虽然我建议使用 dbContext.Model 元数据来发现主键，而不是依赖于反射和属性约定。