Decimal nullable problem

  • Thread starter Thread starter Luigi
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Luigi

Hi all,
I have an expression like this:

decimal? ValoreComplessivoNettoDelfondoValue = (totaleAttivitaValue.HasValue
|| totalePassivitaValue.HasValue) ? ((totaleAttivitaValue ?? 0) -
(totalePassivitaValue ?? 0)) : null;

where every variable is of type Decimal?.

But VS 2005 returns me the error "Impossibile implicit conversion between
decimal and null".

Where's the problem?

Thanks a lot.
 
It happens that Luigi formulated :
Hi all,
I have an expression like this:

decimal? ValoreComplessivoNettoDelfondoValue = (totaleAttivitaValue.HasValue
(totalePassivitaValue ?? 0)) : null;

where every variable is of type Decimal?.

But VS 2005 returns me the error "Impossibile implicit conversion between
decimal and null".

Where's the problem?

Thanks a lot.
Click to expand...

use "(decimal?)null" instead of "null", then it will compile.

The problem is that "(totalePassivitaValue ?? 0)" has a resulttype of
"decimal", not "decimal?". So when the compiler tries to decide on a
returntype of the conditional expression (..?..:..) it sees "decimal"
in the "true expression" and a null in the "false expression". And it
can't match the two.
If you cast the null to a nullable decimal, then the compiler sees
"decimal" and "decimal?" and knows there is an implicit conversion, so
the result-type will be "decimal?".

Hans Kesting
 
I have an expression like this:

decimal? ValoreComplessivoNettoDelfondoValue = (totaleAttivitaValue.HasValue
|| totalePassivitaValue.HasValue) ? ((totaleAttivitaValue ?? 0) -
(totalePassivitaValue ?? 0)) : null;

where every variable is of type Decimal?.

But VS 2005 returns me the error "Impossibile implicit conversion between
decimal and null".

Where's the problem?
Click to expand...

Well, that many conditionals and null-coalescing operators makes the
code very difficult to read for a start. I would break it up into
separate statements - I suspect you'll find the problem goes away then
anyway. I believe the issue is demonstrated by this rather simpler
code:

decimal? foo = true ? 5m : null;

The types of the rightmost two operands are "decimal" and "<null>" and
there's no implicit conversion from either one to the other. If you
cast either of the operands to "decimal?" it will work fine:

decimal? foo = true ? (decimal?) 5m : null;
or
decimal? foo = true ? 5m : (decimal?) null;

You could do the same with your original code - casting the null is
probably simpler than casting the calculation.

Jon
 
Hans Kesting said:
use "(decimal?)null" instead of "null", then it will compile.

The problem is that "(totalePassivitaValue ?? 0)" has a resulttype of
"decimal", not "decimal?". So when the compiler tries to decide on a
returntype of the conditional expression (..?..:..) it sees "decimal"
in the "true expression" and a null in the "false expression". And it
can't match the two.
If you cast the null to a nullable decimal, then the compiler sees
"decimal" and "decimal?" and knows there is an implicit conversion, so
the result-type will be "decimal?".
Click to expand...

Thank you Hans, now it's working (with a cast).

Luigi
 
Jon Skeet said:
Well, that many conditionals and null-coalescing operators makes the
code very difficult to read for a start. I would break it up into
separate statements - I suspect you'll find the problem goes away then
anyway. I believe the issue is demonstrated by this rather simpler
code:

decimal? foo = true ? 5m : null;

The types of the rightmost two operands are "decimal" and "<null>" and
there's no implicit conversion from either one to the other. If you
cast either of the operands to "decimal?" it will work fine:

decimal? foo = true ? (decimal?) 5m : null;
or
decimal? foo = true ? 5m : (decimal?) null;

You could do the same with your original code - casting the null is
probably simpler than casting the calculation.
Click to expand...

Well, thank you Jon, another good point of view to keep in mind.

Luigi
 
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