string.IsNullOrEmpty + Entity Framework 5

Question

我已经使用了Entity Framework已经有一段时间了，我正在重新使用EF 5，但不会这个查询：

SELECT 
    c.OrganizationName as CompanyName,
    c.OrganizationKey as CompanyId,
    ISNULL(a.Line1, '') as Line1,
    ISNULL(a.Line2, '') as Line2,
    a._CityStateZip as CityStateZip
FROM Organizations c
JOIN Addresses a ON c.AddressKey = a.AddressKey
WHERE c.OrganizationName LIKE @term + '%'
AND c.IsSuspended = 0
AND c.IsActive = 1

与以下相同：

var results = (from c in adms.Organizations
               where c.OrganizationName.StartsWith(term)
               where !c.IsSuspended
               where c.IsActive
               select new
               {
                 CompanyName = c.OrganizationName,
                 CompanyId = c.OrganizationKey,
                 Line1 = (string.IsNullOrEmpty(c.Address.Line1) ? string.Empty : c.Address.Line1),
                 Line2 = (string.IsNullOrEmpty(c.Address.Line2) ? string.Empty : c.Address.Line2),
                 CityStateZip = c.Address._CityStateZip
               }).ToList();

当我运行LINQ to SQL代码时，我收到以下错误：

Could not translate expression 
'Table(Organization).Where(c => c.OrganizationName
.StartsWith(Invoke(value(System.Func`1[System.String]))))
.Where(c => Not(c.IsSuspended))
.Where(c => c.IsActive)
.Select(c => new <>f__AnonymousType2`5(
CompanyName = c.OrganizationName, 
CompanyId = c.OrganizationKey, 
Line1 = IIF(IsNullOrEmpty(c.Address.Line1), 
Invoke(value(System.Func`1[System.String])), c.Address.Line1), 
Line2 = IIF(IsNullOrEmpty(c.Address.Line2), 
Invoke(value(System.Func`1[System.String])), c.Address.Line2), 
CityStateZip = c.Address._CityStateZip))' 
into SQL and could not treat it as a local expression.

我在这里完全遗漏了什么吗？ 我以为我可以使用string.IsNullOrEmpty和LINQ to SQL。

Answer 1

将string.Empty替换为"" 。 可悲的是，EF不支持string.Empty 。

EF LINQ支持一般非常糟糕。 始终要注意这个问题。 这是EF的悲伤的常见原因。

LINQ to SQL没有常见语言习语的问题。

顺便说一句，你可以更好地重写这个： c.Address.Line1 ?? "" c.Address.Line1 ?? "" 。

Answer 2

(string.IsNullOrEmpty(c.Address.Line1) ? string.Empty : c.Address.Line1)

正在翻译成

IIF(IsNullOrEmpty(c.Address.Line1), Invoke(value(System.Func`1[System.String])), c.Address.Line1)

您正在做的只是将字符串值设置为""如果它已经为null或"" 。

您应该尝试使用Line1 = c.Address.Line1