Nullable Convert ? : operand problem

[mihkeltt]

Just one question, why wouldn't the following lines compile:

decimal? decVar = 5;
int? intVar = decVar == null ? null :
Convert.ToInt32(decVar.GetValueOrDefault());

The compiler shows the following message: "There is no implicit conversion
between 'null' and 'int'."

A similar if-else construction works just fine.

Mihkel

 

[Marc Gravell]

As far as the ternary is concerned, it's two values are incompatible,
since "null" is interpreted as a null object, not the int? missing
value. You can re-educate it with a cast:

int? intVar = decVar == null ? (int?) null :
Convert.ToInt32(decVar.GetValueOrDefault());

Marc

 

[Martin Hey]

you've to convert the null to a nullable int

decimal? decVar = 5;
int? intVar = decVar == null ? (int?)null :
Convert.ToInt32(decVar.GetValueOrDefault());

 

[Jon Skeet [C# MVP]]

Just one question, why wouldn't the following lines compile:

decimal? decVar = 5;
int? intVar = decVar == null ? null :
Convert.ToInt32(decVar.GetValueOrDefault());

The compiler shows the following message: "There is no implicit conversion
between 'null' and 'int'."

A similar if-else construction works just fine.

An int isn't convertible to null, and null isn't convertible to int -
you need to explain to the compiler which type to use that both values
*can* be converted to:

int? intVar = decVar==null ? (int?) null : Convert.ToInt32(...);
or
int? intVar = decVar==null ? null : (int?)Convert.ToInt32(...);

Note that the code can be simplified somewhat:

However, for this code it's easier to just use:

int? intVar = (int?) decVar;

Jon

 

[mihkeltt]

then how is this statement different from the first one i've presented:

int? intVar = null;

this would work just fine.

 

[Jon Skeet [C# MVP]]

then how is this statement different from the first one i've presented:

int? intVar = null;

this would work just fine.

That's converting from null to int?, which is fine.

The conditional operator has to decide what the type of the overall
expression is. It does this by taking the two result types, and seeing
if it can convert from either one to the other. In this case, it
can't.

Jon

 

[mihkeltt]

thanks for the informative and quick reply, Jon.

Mihkel

 

[Ben Voigt [C++ MVP]]

As far as the ternary is concerned, it's two values are incompatible,
since "null" is interpreted as a null object, not the int? missing
value. You can re-educate it with a cast:

int? intVar = decVar == null ? (int?) null :
Convert.ToInt32(decVar.GetValueOrDefault());

Furthermore (and to eliminate precedence confusion), don't compare Nullable
values to null.

Instead, use the HasValue property:

int? intVar = decVar.HasValue? Convert.ToInt32(decVar.GetValueOrDefault()):
(int?) null;