問題描述

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

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

運算符??"不能應用于System.DateTime?"類型的操作數和'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參數是第一個參數的不可為空版本時才有效.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:

無法確定條件表達式的類型，因為'System.DateTime'和'System.DBNull'之間沒有隱式轉換

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，運算符返回左操作數，否則返回右操作數.

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！