How to correctly deal with System.Nullable fiel

问题:

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?

回答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? Dunno. Depends on the your context.

回答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

回答3:

There are two approaches.

1. Use nullable types such as double?. Your entity class can have a property of double? and your client code will work with the nullable type.

   public class Person
   {
   
       public string FirstName { get; set; }
   
       public string LastName { get; set; }
       public double? Age { get; set; }
   
       public Person() 
       {
       }
       public Person( IDataRecord record )
       {
           FirstName = (string) record["first_name"];
           LastName = (string) record["last_name"];
           Age = (double?) record["age"];
       }
   }
   

2. Write a wrapper around the assignment that chooses a safe default value if NULL is present. I like to use an extension method off the data record class and put a SafeConvert method there.

回答4:

You can either cast it: double yourVariable = (double)ValueFromDb or you can just grab the Value of the double?: double yourVaraible = ValueFromDb.Value. Either is correct.