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

Question

I have a table with multiple fields with a null value. Working with LinqToSQL, trying to use the field directly, I get

The argument type System.Nullable is not assigned to the parameter type double

How can I handle this?

Answer 1

The problem has nothing to do with Linq. This is due to the conversion between doubleand double?/ Nullable<double>.

Implicit conversion from double?to is doublenot allowed:

double? foo ;
double  bar = foo ;

• You can directly refer to a double value:

  double? foo ;
  double  bar = foo.Value ;
  

  This will cause NullReferenceExceptionif Nullable<T>null (i.e. .HasValue, false).

• :

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

  , , Nullabl<T> null.

• null coalescing , Nullable<T> null:

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

  , natch, NullReferenceException`.

• , :

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

• , , :

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

. ? . .

Answer 2

double value = theObject.TheProperty.GetValueOrDefault();

Nullable<T>, T. null T, 0.0 double. - , .

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

Answer 3

.

• , double?. double? NULL.

  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"];
      }
  }
  

• , , NULL. SafeConvert.

Answer 4

: double yourVariable = (double)ValueFromDb double?: double yourVaraible = ValueFromDb.Value. .