無法翻譯 Linq 表達式錯誤 Entity Framework Core 3

Question

我在一個項目中，在數據訪問方面遇到了一些問題。 我使用 EF 核心並且我有這些堆棧跟蹤。

執行的第一個代碼

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

執行的第二個文件代碼

在代碼中 IEntity 是一個確定數據庫對象的空接口
IEntityRepostiory 具有訪問數據庫的 EF 代碼，該代碼顯示在屬於
PrimaryKeyComparable 具有比較功能來檢查對象的主鍵是否等於給定的鍵

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));
        }
    }
}

執行的第三個代碼部分

在代碼上下文是 EF dbcontext
過濾器被賦予過濾器，即 o=>primaryKeyComparable.comparePrimaryKey(o,key) 第二個代碼文件。 錯誤發生在 .SingleOrDefault(filter) 部分。 如果我刪除它，功能運行良好。

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

比較主鍵函數

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.");
        }

錯誤信息（我認為英文部分足以理解）

錯誤發生在 .SingleOrDefault(filter) 部分的第三個文件

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

所以看起來你正在嘗試使用反射來確定主鍵和基於它的查詢。 您不能直接在表達式中使用這樣的方法，並期望 EF 解開它的作用並將其轉換為 sql。

但是，您可以使用反射來生成表達式並直接使用它。

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));

雖然我建議使用 dbContext.Model 元數據來發現主鍵，而不是依賴於反射和屬性約定。