How to correctly deal with System.Nullable<T> fields of LinqToSql classes?

Question

I've got a table with some nullable double fields. Working with LinqToSQL trying to use the field directly I get

Argument type System.Nullable is not assignable to parameter type double

How do I deal with this correctly?

Answer 1

The problem has nothing to do with Linq. It's got to do with conversion between double and double?/Nullable<double>.

Implicit conversion from double? to double is not allowed:

double? foo ;
double  bar = foo ;

• You can reference the double value directly:

    double? foo ;
    double  bar = foo.Value ;
  

  This will throw a NullReferenceException if the Nullable<T> is null (that is, .HasValue is false).

• You can cast it:

    double? foo ;
    double  bar = (double) foo ;
  

  Again, you'll get an exception if the Nullabl<T> is null.

• You can use the null coalescing operator to assign a default value if the Nullable<T> is null:

    double? foo ;
    double  bar = foo ?? -1.0 ;
  

  This, natch, avoids the NullReferenceException` problem.

• You can use the ternary operator in the same vein as the null coalescing operator:

    double? foo ;
    double  bar = ( foo.HasValue ? foo.Value : -2 ) ;
  

• Finally, you can use regular conditional logic to follow an alternative path:

    double? foo ;
    double  bar ;
  
    if ( foo.HasValue )
    {
      doSomething(foo.Value) ;
    }
    else
    {
      doSomething() ;
    }
  

Those are about the options. Which is correct? I don't know. It depends on your context.

Answer 2

You can use

double value = theObject.TheProperty.GetValueOrDefault();

When you have a Nullable<T> and you need a value of type T. This will normalize any null to the default value of T, which will be 0.0 for a double. If you want something other than the default, you can work around it another way.

double value = theObject.TheProperty ?? 1d; // where 1 replaces any null