vb.net - If - Strange Behavior in Detecting Type -

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i discovered strange behavior in vb.net today in trying work nullable datetime data. pulling datetime value out of xml file inserting database, , want allow empty value. thought should use if prevent casting errors:

dim lastrun datetime? = _     if(rowdata("lastrun") = "", nothing, ctype(rowdata("lastrun"), datetime))

it seems should return value of nothing in case if false, or value of date time lastrun if value not blank. instead, when if condition returns false, value of datetime.minvalue, causes exception on insert database due sql datetime underflow.

i able fix using datetime? cast in last parameter, behavior seems odd me. expected type datetime? because that's variable type. also, narrowest type can allow both possible result values datetime?, since either datetime or nothing. , yet somehow decides result value should datetime , guess typecasts nothing datetime.minvalue? going on here?

part of problem i'm used c#, , equivalent expression rowdata["lastrun"] == "" ? null : (datetime)rowdata["lastrun"]) doesn't compile (as expected), because there's "no implicit conversion between datetime , null."

nothing not same null in c#, mixture between null , default(t). when use nothing on value type(like structure datetime) it's default value datetime.minvalue.


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