Question

使用以下

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我收到此错误

无法创建类型为＆＃39; System.Int32 []＆＃39;的null常量值。只要支持实体类型，枚举类型或基元类型这个背景。

我明白了错误，但我无法弄清楚应该如何修复它。 PagedData.Products = from p in db.Products where (from m in p.Manufacturers where model.man.Contains(m.ManufacturerID) select m).Any() where (from s in p.Sizes where model.size.Contains(s.SizeID) select s).Any() where (from c in p.Colors where model.color.Contains(c.ColorID) select c).Any() select p; model.man和model.size是整数数组，也可以为null。

Answer 1

由于所有条件必须为true才能传递任何Product，因此您应首先检查所有数组是否都包含任何内容：

if (model.man != null && model.size != null && model.color != null
      && model.man.Any() && model.size.Any() && model.color.Any())
{
    PagedData.Products = from p in db.Products ...

如果您事先知道它不会返回任何数据，那么您现在不会执行查询。并且它不会抛出异常，因为您从未使用null数组运行查询。

Answer 2

我更希望使用方法语法和Where动态构建if子句，但如果您希望在查询中嵌入条件，则需要确保IEnumerable s您用于Contains条件的标准不是null。这应该发生在外面查询：

var man = model.man ?? Enumerable.Empty<int>();
var size = model.size ?? Enumerable.Empty<int>();
var color = model.color ?? Enumerable.Empty<int>();

PagedData.Products = from p in db.Products
                                     where (from m in p.Manufacturers
                                            where man.Any() && man.Contains(m.ManufacturerID)
                                            select m).Any()
                                     where (from s in p.Sizes
                                            where size.Any() && size.Contains(s.SizeID)
                                            select s).Any()
                                     where (from c in p.Colors
                                            where color.Any() && color.Contains(c.ColorID)
                                            select c).Any()
                                     select p;

请注意filter.Any() && filter.Contains(...)毫无意义，相当于filter.Contans(...)。如果要忽略空过滤器，则应使用!filter.Any() || filter.Contans(...)。

所以IMO你的查询应该是这样的

var man = model.man ?? Enumerable.Empty<int>();
var size = model.size ?? Enumerable.Empty<int>();
var color = model.color ?? Enumerable.Empty<int>();

PagedData.Products = from p in db.Products
                     where (from m in p.Manufacturers
                            where !man.Any() || man.Contains(m.ManufacturerID)
                            select m).Any()
                     where (from s in p.Sizes
                            where !size.Any() || size.Contains(s.SizeID)
                            select s).Any()
                     where (from c in p.Colors
                            where !color.Any() || color.Contains(c.ColorID)
                            select c).Any()
                     select p;

或者

var query = db.Products.AsQueryable();
if (model.man != null && model.man.Length > 0)
    query = query.Where(p => p.Manufacturers.Any(m => model.man.Contains(m.ManufacturerID)));
if (model.size != null && model.size.Length > 0)
    query = query.Where(p => p.Sizes.Any(s => model.size.Contains(s.SizeID)));
if (model.color != null && model.color.Length > 0)
    query = query.Where(p => p.Colors.Any(c => model.color.Contains(c.ColorID)));
PagedData.Products = query;

无法创建类型为＆＃39; System.Int32 []＆＃39;的null常量值。

2 个答案: