Null 和 System.DBNull. Value 之间的区别是什么？

Null 和 System.DBNull. Value 之间有什么区别吗? 如果有，是什么？

我现在注意到这种行为

while (rdr.Read())
{
if (rdr["Id"] != null) //if (rdr["Id"] != System.DBNull.Value)
{
int x = Convert.ToInt32(rdr["Id"]);
}
}

虽然我使用 sql 数据读取器从数据库中检索数据，但是没有返回值 if(rdr["Id"] != null)返回 true，并且最终抛出了将 null 转换为整数的异常。

但是，如果我使用 if (rdr["Id"] != System.DBNull.Value)，这将返回 false。

Null 和 System.DBNull. Value 之间的区别是什么？

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DBNull.Value is what the .NET Database providers return to represent a null entry in the database. DBNull.Value is not null and comparissons to null for column values retrieved from a database row will not work, you should always compare to DBNull.Value.

http://msdn.microsoft.com/en-us/library/system.dbnull.value.aspx

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最佳答案

Well, null is not an instance of any type. Rather, it is an invalid reference.

However, System.DbNull.Value, is a valid reference to an instance of System.DbNull (System.DbNull is a singleton and System.DbNull.Value gives you a reference to the single instance of that class) that represents nonexistent* values in the database.

*We would normally say null, but I don't want to confound the issue.

So, there's a big conceptual difference between the two. The keyword null represents an invalid reference. The class System.DbNull represents a nonexistent value in a database field. In general, we should try avoid using the same thing (in this case null) to represent two very different concepts (in this case an invalid reference versus a nonexistent value in a database field).

Keep in mind, this is why a lot of people advocate using the null object pattern in general, which is exactly what System.DbNull is an example of.

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DataRow has a method that is called IsNull() that you can use to test the column if it has a null value - regarding to the null as it's seen by the database.

DataRow["col"]==null will allways be false.

use

DataRow r;
if (r.IsNull("col")) ...

instead.

小开

Null is similar to zero pointer in C++. So it is a reference which not pointing to any value.

DBNull.Value is completely different and is a constant which is returned when a field value contains NULL.

小开

From the documentation of the DBNull class:

Do not confuse the notion of null in an object-oriented programming language with a DBNull object. In an object-oriented programming language, null means the absence of a reference to an object. DBNull represents an uninitialized variant or nonexistent database column.

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DBNull.Value is annoying to have to deal with.

I use static methods that check if it's DBNull and then return the value.

SqlDataReader r = ...;
String firstName = getString(r[COL_Firstname]);


private static String getString(Object o) {
if (o == DBNull.Value) return null;
return (String) o;
}

Also, when inserting values into a DataRow, you can't use "null", you have to use DBNull.Value.

Have two representations of "null" is a bad design for no apparent benefit.


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