C# 为什么当我插入 DateTime null 时，SQL Server 中有“0001-01-01”？

Why when I insert a DateTime null I have "0001-01-01" in SQL Server?

提问by Alex

I try to insert the value null (DateTime) in my database for a field typed 'date' but I always get a '0001-01-01'. I don't understand, this field "allow nulls" and I don't know why I have this default value.

我尝试在我的数据库中为键入 'date' 的字段插入值 null (DateTime) 但我总是得到一个'0001-01-01'。我不明白，这个字段“允许空值”，我不知道为什么我有这个默认值。

I'm using C# asp .net with MVC (Entity Framework), this is my code :

我将 C# asp .net 与 MVC（实体框架）一起使用，这是我的代码：

Budget_Synthesis newBS = new Budget_Synthesis
{
    Budget_Code = newBudgetCode,
    Last_Modified_Date = null
};
db.Budget_Synthesis.AddObject(newBS);

Last_Modified_Date is typed System.DateTime? so I don't know why they change this 'null'.

Last_Modified_Date 的类型是 System.DateTime？所以我不知道他们为什么要改变这个“空”。

If I try to display the value on my application I get 01/01/0001 00:00:00

如果我尝试在我的应用程序上显示该值，我会得到 01/01/0001 00:00:00

And 0001-01-01with SSMS

和0001-01-01SSMS

Someone can explain me why I can't get a real 'NULL' ?

有人可以解释我为什么我不能得到一个真正的 'NULL' 吗？

Best regards

此致

采纳答案by Hugo Pedrosa

I think this is the value corresponding to the null

我认为这是与空值对应的值

回答by Nikola Dimitroff

When saying you are trying to put a null DateTime, are you using a Nullable<DateTime>(a.k.a DateTime?) or simply DateTime? The latter is a value type and its default value is precisely 01/01/0001 00:00:00

当你说你试图放置一个 null 时DateTime，你是使用Nullable<DateTime>（又名DateTime?）还是简单的DateTime？后者是一个值类型，它的默认值正好是01/01/0001 00:00:00

回答by Spaceman

If Last_Modified_Dateis of type DateTime, you can't have "real null" because DateTime structure- as others already said- is not nullable. So your sample code will not even compile.

如果Last_Modified_Date是 type DateTime，你不能有“真正的空”，因为DateTime 结构- 正如其他人已经说过的 - 不可为空。所以你的示例代码甚至不会编译。

If Last_Modified_Dateis of type DateTime?(Nullable<DateTime>) your code is correct, but -as @Nikola Dimitroff said in his answer- you can't have "real null" in your database because the default value for DateTime?is 01/01/0001 00:00:00.

如果Last_Modified_Date是DateTime?( Nullable<DateTime>)类型，您的代码是正确的，但是 - 正如@Nikola Dimitroff 在他的回答中所说 - 您的数据库中不能有“真正的空”，因为默认值为DateTime?01/01/0001 00:00:00 .

The "real null" you are looking for is DBNull.Value, but you can use it only for System.DBNulltype; if you assign Last_Modified_Date = DBNull.Value, whatever the type of Last_Modified_Dateis, your code will not compile.

您正在寻找的“真正的空”是DBNull.Value，但您只能将它用于System.DBNull类型；如果你赋值Last_Modified_Date = DBNull.Value，无论Last_Modified_Date是什么类型，你的代码都不会编译。