Decimal parsing - why does it work the way it works.

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This is a follow on from another question, similar but not the same

why-I-need-to-parse-if-object-value-is-decimal

Taking a vb.net app, implicit conversion set to error in the compiler.

So - e.g.
Dim MyDecimal as Decimal=decimal.parse(0.001.tostring)

MyDecimal=MyDecimal*0.45355

Error: Implicit Conversion from Double to Decimal.

Ok. So then I have to do something strange like:

MyDecimal=MyDecimal*decimal.parse(0.45355.tostring)

Why does it have to be done this way - or is there a better way to manage this. Seems like a waste of a cpu cycle to convert it to a string, then to parse it as a decimal. Why cant the system just understand (i) that 0.45355 is decimal, because it is and (ii) why cant just decimal.parse(0.45355) work anyway if it doesnt understand that 0.45355 is a decimal.
Even if the number has less decimal points i.e. 0.4 it still moans that its a double.

Im just trying to understand the logic behind why the compiler thinks this? Thanks!

What I have tried:

Guess I have to fill this in just for the query to be accepted, but trying different decimal point seems to have no effect. if double "is the standard" for decimal, then why have decimal at all?

Posted 18-Feb-20 21:13pm

Member 12561559

Updated 18-Feb-20 21:28pm

v3

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[no name] 19-Feb-20 3:24am

How is about this:

Dim dec  As Decimal
dec = 2.375D

Member 12561559 19-Feb-20 10:00am

Thanks!

2 solutions

Solution 2

The Decimal constructor has an overload for Double. See Decimal Constructor (System) | Microsoft Docs[^].

Posted 18-Feb-20 21:29pm

CPallini

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Member 12561559 19-Feb-20 3:43am

Thanks! sorry I could not give you the accept solution, just griff got in there first - otherwise, 5 stars from me :)

CPallini 19-Feb-20 4:04am

You are welcome.

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OriginalGriff · Accepted Answer · 2020-02-18T21:22:00

Solution 1

Because you don't specify it, the constant literal 0.45355 is an implicit Double value, not a decimal - and since they take precedence over Decimal in calculating (so as not to lose precision) the result of Decimal * Double is Double.

Append a "D" to the end of your literal value and the system will know it's a Decimal value and act accordingly:

VB

MyDecimal = MyDecimal * 0.45355D

Posted 18-Feb-20 21:22pm

OriginalGriff

Comments

Member 12561559 19-Feb-20 3:42am

Ok, that makes sense to be honest. So again, as Im trying to implement the parse for most things as convert.to is outdated from a previous question I supplied that someone mentioned - I have a date:

Dim CleanDaysToGo as int32=0
CleanDaysToGo = NextCleanDate.Value.Subtract(Now).TotalDays
doesn't work because of datetime issues, so utilising the parse.datetime and parse.int32 to turn the result into an int32 on the outer, like so:

Dim CleanDaysToGo as int32=0
CleanDaysToGo = Int32.Parse(DateTime.Parse(NextCleanDate.Value.ToString).Subtract(Now).TotalDays.ToString)

NextCleanDate is a of nullable datetime equal to say, 25 february 2020 - so 6 days from now - and CleanDaysToGo should hold that value - but it flops with an error, only on the runtime with "input string is not in correct format".

Sorry to seem so dumb with this and following up a question, with a question, but it might be useful for others who run into the same issues. :) Cheers

Richard MacCutchan 19-Feb-20 3:55am

You need to read the documentation on types and classes. You appear to be under the misapprehension that the compiler can guess your intentions.