有条件地组合函数以创建 Linq 滤波器

Question

我有一个要有条件地查询的测试实体存储库和测试实体过滤器。

测试实体过滤器如下所示：

    public class TestEntityFilter
    {
        public string Name { get; set; } = null!;
        public string Type { get; set; } = null!;
        public Status? Status { get; set; }
        public int Take { get; set; }
        public int PageNumber { get; set; }
    }

我希望查询在Name 、 Type和Status为 null 时返回所有内容，并在它们不是时使用给定值进行过滤。 如果其中一个是 null，它将被忽略，另外两个将用于过滤。 对于这个条件查询，我尝试有条件地创建一个函数并根据值组合它们，但我的存储库方法看起来并不像它不起作用。 解决此类问题的最佳方法是什么？

        public async Task<IEnumerable<TestEntity>> GetTestEntitiesAsync(TestEntityFilter testEntityFilter, CancellationToken cancellationToken)
        {
            var combinedFilter = BuildFilers(testEntityFilter);
            return await _dbContext.TestEntity
                .Where(testEntity => combinedFilter(testEntity))
                .ToListAsync(cancellationToken);
        }
            
            
            
        public Func<TestEntity, bool> BuildFilers(TestEntityFilter testEntityFilter)    
        {   
            Func<TestEntity, bool> mainFilter = testEntityFilter => true;
            Func<TestEntity, bool> filterByName = string.IsNullOrEmpty(testEntityFilter.Name) ? 
                (TestEntity j) => true :
                (TestEntity j) => testEntityFilter.Name == j.Name;

            Func<TestEntity, bool> filterByType = string.IsNullOrEmpty(testEntityFilter.Type) ?
                (TestEntity j) => true :
                (TestEntity j) => testEntityFilter.Type == j.Type; 

            Func<TestEntity, bool> filterByStatus = testEntityFilter.Status is null?
                (TestEntity j) => true :
                (TestEntity j) => testEntityFilter.Status == j.Status;

            Func<TestEntity, bool> combinedFilter = (TestEntity j) => mainFilter(j) && filterByName(j) && filterByType(j) && filterByStatus(j);
        }

使用过滤器运行上述方法会引发以下异常：

'The LINQ expression 'DbSet<TestEntity>()
    .Where(j => Invoke(__combinedFilter_0, j))' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'.

Answer 1

您可以结合Where条件

var res = _db.TestEntity;
if (!string.IsNullOrWhiteSpace(filter.Name))
    res = res.Where(x => x.Name = filter.Name)
        
if (!string.IsNullOrWhiteSpace(filter.Status))
    res = res.Where(x => x.Status = filter.Status)
        
// .. rest
            
return await res.ToListAsync(cancellationToken);

Answer 2

你可以试试这个：

public async Task<IEnumerable<TestEntity>> GetTestEntitiesAsync(TestEntityFilter testEntityFilter, CancellationToken cancellationToken)
    {
        var combinedFilter = BuildFilers(testEntityFilter);

        var data = BuildFilers(_dbContext.TestEntity.AsQueryable(), testEntityFilter);

        return await data.ToListAsync(cancellationToken);
    }



    public IQueryable<TestEntity> BuildFilers(IQueryable<TestEntity> entities, TestEntityFilter testEntityFilter)
    {
        if (string.IsNullOrEmpty(testEntityFilter.Name))
        {
            entities = entities.Where(j => testEntityFilter.Name == j.Name);
        }

        if (string.IsNullOrEmpty(testEntityFilter.Type))
        {
            entities = entities.Where(j => testEntityFilter.Type == j.Type);
        }

        if (testEntityFilter.Status.HasValue)
        {
            entities = entities.Where(j => testEntityFilter.Status == j.Status);
        }

        return entities;
    }