当条目为零/空时，如何处理一个对多(或零)实体的迭代? [英] How do I deal with iterating over a one to many (or zero) entity when there are zero/null entry?

问题描述

我不断得到NullReferenceException尝试遍历一行中的空条目.

public class Person
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public Address Address { get; set; }
        public int AddressID { get; set; }
    }

public class Address
    {
        public int ID { get; set; }
        public string FirstLine { get; set; }
        public string SecondLine { get; set; }
    }

使用默认支架模板_context.Person.Include(c => c.Address);

无论是否有地址，视图都可以正常工作.

但是，我现在将默认索引页面替换为更新的网格，该网格通过具有过滤和分页机制的AJAX加载.为此，我需要将数据转换为我的ViewModel

我希望能够以文本形式在线显示地址.我尝试了以下方法:

        var tmp = _context.Person.Include(x => x.Address).ToList();

        tmp.ForEach(x => vm.List.Add(new IndexListItem()
        {
            Name = x.Name,
            Address = x.Address.FirstLine + " " + x.Address.SecondLine,
            ID = x.ID

        }));

但是，在调试时，即使行上有数据，地址也似乎总是为空.

我很确定我可以在标准的foreach循环上执行此操作，并执行if空检查，但是，我忍不住觉得这很简单，而且已经使它复杂化了./p>

有没有更简单的方法来返回我需要的地址详细信息?

解决方案

正如我在评论中所说，您应该在一条语句中这样做:

vm.List = _context.Person
    .Select(p => new IndexListItem
    {
        Name = p.Name,
        Address = p.Address.FirstLine + " " + p.Address.SecondLine,
        ID = p.ID
    }).ToList();

这有两个优点:

1. 它已翻译成SQL，因此仅从数据库中提取了四个必填字段(这与更宽的记录有很大的区别).
2. EF将生成将空值考虑在内的SQL.

如果您不希望使用" "，则可以...

Address = (p.Address.FirstLine + " " + p.Address.SecondLine).Trim()

...或...

Address = p.Address.FirstLine != null ? p.Address.FirstLine + " " : "")
    + p.Address.SecondLine

但是我认为查看数据并不重要.

I keep getting NullReferenceException trying to iterate over a null entry on a row.

public class Person
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public Address Address { get; set; }
        public int AddressID { get; set; }
    }

public class Address
    {
        public int ID { get; set; }
        public string FirstLine { get; set; }
        public string SecondLine { get; set; }
    }

With the default scaffold template, _context.Person.Include(c => c.Address);

The view works fine if there is an address or not.

However, I'm now replacing the default index page with a newer grid that loads via AJAX with filtering and paging mechanism. For this, I need to convert the data to my ViewModel

I want to be able to display the address in line as text. I've tried the following:

        var tmp = _context.Person.Include(x => x.Address).ToList();

        tmp.ForEach(x => vm.List.Add(new IndexListItem()
        {
            Name = x.Name,
            Address = x.Address.FirstLine + " " + x.Address.SecondLine,
            ID = x.ID

        }));

But, when debugging, Address seems to always be null, even if there is data on the row.

I'm pretty sure I can do this on a standard foreach loop, and doing an if null check, but, I can't help but feel this is something that is quite simple and I'm over complicating it.

Is there a simpler way to return the address details I need?

解决方案

As I said in a comment, you should do this in one statement:

vm.List = _context.Person
    .Select(p => new IndexListItem
    {
        Name = p.Name,
        Address = p.Address.FirstLine + " " + p.Address.SecondLine,
        ID = p.ID
    }).ToList();

This has two advantages:

1. It's translated into SQL, so only the four required fields are pulled from the database (which makes a considerable difference with wider records).
2. EF will generate SQL that takes null values into account.

If you don't want " " as result you could do...

Address = (p.Address.FirstLine + " " + p.Address.SecondLine).Trim()

...or...

Address = p.Address.FirstLine != null ? p.Address.FirstLine + " " : "")
    + p.Address.SecondLine

But I don't think it matters much for viewing data.

这篇关于当条目为零/空时，如何处理一个对多(或零)实体的迭代?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！