比较对象int或dbnull.value linq [英] compare object int or dbnull.value linq

问题描述

我有一个linq查询:

I have a linq query:

var capacityQuery =来自mySQLDataTable.AsEnumerable()中的信息其中Info.Field< object>(位置")== Location.ID按Info.Field< DateTime>("date")

其中Info.Field("location")== Location.ID行是我遇到问题的地方

The line "where Info.Field("location") == Location.ID" is where I am having problems

我的问题是.如何在where语句中进行比较，该语句可能是DBNull.Value或任一侧的int值

My Question is. How do I do a comparison in the where statement that might be either a DBNull.Value or an int value on either side

以下仅供参考:

Location是一个自定义类.ID返回一个对象.

Location is a custom class. The ID returns an object.

    private int? addressID;

    public object ID {
        get
        {
            if (addressID == null)
                return DBNull.Value;
            else
                return addressID;
        }
    }

必须为DBNull的原因是因为它来自数据库.还可以从数据库中填写表".

The reason it has to be DBNull is because it is coming from a database. The "table" is also being filled in from a database.

我知道==在这种情况下是错误的，因为它比较的引用显然是错误的.

I know == in this case is wrong because its comparing the references which is obviously false.

我知道object.equal(var1，var2)应该可以工作，但是我猜不出来是因为它们是2个不同的对象.

I know object.equal(var1, var2) should work but it doesnt which im guessing is because they are 2 different objects.

在表端和类端的值都可以是int或DBNull.

The values could be an int or DBNull both on the table side and the class side.

这是在foreach循环中，因此我使用相同的查询，其中Location的值为int或DBNull的值.

This is in a foreach loop so I use the same query where Location has an int value or when it has a DBNull value.

我的问题是.如何在where语句中进行比较，该语句可能是DBNull.Value或任一侧的int值

Again My Question is. How do I do a comparison in the where statement that might be either a DBNull.Value or an int value on either side

推荐答案

使用

private int? addressID;

public object ID {
    get
    {
        return addressID.HasValue ? addressID.Value as object : DBNull.Value;    
    }
}

相反.这样，如果addressID尚未初始化或故意将其设置为null，则ID属性将返回DBNull.Value.否则，它将返回一个 int，但您必须自己将它从 object 转换为 int 或 int?.

instead. This way, if addressID has not been initialized or if it has been deliberately set to null, the ID property will return a DBNull.Value. Otherwise, it will return an int, but you have to cast it yourself from object to int or int?.

话虽如此，作者将空值的创建称为十亿美元的错误"，但现在我认为它已达到数万亿美元.您应该返回一个int吗?如果可以，请使用数据库中的数据类型，在这种情况下，比较将始终有效.我不确定您使用哪个ORM，但据我所知，Dapper和linq2db可以正确处理此方案.

Having said that, creation of the null has been called "a billion dollar mistake" by the author, but by now I think it is in the trillions. You should return an int? datatype from the database if you can, in which case, the comparison will always work. I am not sure which ORM do you use, but Dapper and linq2db handle this scenario correctly as far as I know.

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