C# ADO.NET:null 和 DbNull——有更高效的语法吗? [英] C# ADO.NET: nulls and DbNull -- is there more efficient syntax?

问题描述

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";

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

I thought initially I'd be clever:

datePrm.Value = nullableDate ?? DBNull.Value;

but that fails with the error

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

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;

but that doesn't work either:

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

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

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

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

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?

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

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

That's exactly what I want!

Update2: Well it was kind of obvious in the end:

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

解决方案

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

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

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