Question

我使用PetaPoco微型ORM和C#4.0。

下面的代码从数据库中取回一行:

var result = db.SingleOrDefault<TdUsers>(getUserQuery);

我想看看结果是否包含任何行,是否是空的。这样做的最佳方式是什么?

Answer 1

if (result == null || result.Count() == 0) {
    // Checks whether the entire result is null OR
    // contains no resulting records.
}

我认为问题不在您对 null 的检查中, 因为 Linq 是懒惰的装入。您的错误在于使用表达式 db. SingleOrDefault< TdUsers> (getUserQuery); 。

.Single<T>(expression) does not return null - it errors if the result returns no values. .SingleOrDefault<T>(expression), however, returns a null value if the expression results in no values - and therefore is best combined with an if (result == null) type check, as you re using here.

Answer 2

var result = db.SingleOrDefault<TdUsers>(getUserQuery);

In above code SingleOrDefault will return null vale or the specified generic type(it s known on runtime).

要检查返回的值是否无效, 您可以简单地使用

if(result!=null)
{
//do your code stuff 
}
else
{
//stuff do be done in case where result==null
}

Answer 3

你可以做到这一点:

result.ToList() // Convert result to a list

if (result.Any()) {
   // result is not null
}

Answer 4

 var v = result.ToList();

现在检查

if (v is not null)
{

}
else if (v.Count()>0)
{


}

友情链接