問題描述

我有一個(gè) DateTime?，我正嘗試使用 DbParameter 將其插入到字段中.我正在像這樣創(chuàng)建參數(shù):

I've got a DateTime? that I'm trying to insert into a field using a DbParameter. I'm creating the parameter like so:

DbParameter datePrm = updateStmt.CreateParameter();
datePrm.ParameterName = "@change_date";

然后我想將 DateTime? 的值放入 dataPrm.Value 中，同時(shí)考慮 nulls.

And then I want to put the value of the DateTime? into the dataPrm.Value while accounting for nulls.

我最初以為我會很聰明:

I thought initially I'd be clever:

datePrm.Value = nullableDate ?? DBNull.Value;

但由于錯誤而失敗

運(yùn)算符??"不能應(yīng)用于System.DateTime?"類型的操作數(shù)和'System.DBNull'

Operator '??' cannot be applied to operands of type 'System.DateTime?' and 'System.DBNull'

所以我想這只有在第二個(gè)參數(shù)是第一個(gè)參數(shù)的不可為空版本時(shí)才有效.然后我去了:

So I guess that only works if the second argument is a non-nullable version of the first argument. So then I went for:

datePrm.Value = nullableDate.HasValue ? nullableDate.Value : DBNull.Value;

但這也不起作用:

無法確定條件表達(dá)式的類型，因?yàn)?System.DateTime'和'System.DBNull'之間沒有隱式轉(zhuǎn)換

Type of conditional expression cannot be determined because there is no implicit conversion between 'System.DateTime' and 'System.DBNull'

但我不想在這些類型之間轉(zhuǎn)換！

But I don't want to convert between those types!

到目前為止，我唯一能開始工作的是:

So far the only thing I can get to work is:

if (nullableDate.HasValue)
  datePrm.Value = nullableDate.Value;
else
  datePrm.Value = DBNull.Value;

這真的是我寫這篇文章的唯一方式嗎?有沒有辦法讓單行使用三元運(yùn)算符工作?

Is that really the only way I can write this? Is there a way to get a one-liner using the ternary operator to work?

更新:我真的不明白為什么 ??版本不起作用.MSDN 說:

Update: I don't really get why the ?? version doesn't work. MSDN says:

??如果不為空，運(yùn)算符返回左操作數(shù)，否則返回右操作數(shù).

The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.

這正是我想要的！

更新 2: 嗯，最后很明顯:

datePrm.Value = nullableDate ?? (object)DBNull.Value;

推薦答案

啊哈！我找到了比@Trebz 更有效的解決方案！

Ah ha! I found an even more efficient solution than @Trebz's!

datePrm.Value = nullableDate ?? (object)DBNull.Value;

這篇關(guān)于C# ADO.NET:空值和 DbNull —— 有沒有更高效的語法?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！