如何正确处理System.Nullable< T>LinqToSql类的字段? [英] How to correctly deal with System.Nullable<T> fields of LinqToSql classes?

问题描述

我有一个带有一些可为null的double字段的表.与LinqToSQL一起尝试直接使用该字段，我得到了

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

参数类型System.Nullable不能分配给参数类型double

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

我该如何正确处理?

推荐答案

问题与Linq无关.这与 double 和 double?/ Nullable< double> 之间的转换有关.

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

不允许从 double?到 double 的隐式转换:

Implicit conversion from double? to double is not allowed:

double? foo ;
double  bar = foo ;

• 您可以直接引用double值:

  • You can reference the double value directly:

      double? foo ;
      double  bar = foo.Value ;
    

    如果 Nullable< T> 为null(即 .HasValue 为false)，则会抛出 NullReferenceException .

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

    您可以投射它:

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

    同样，如果 Nullabl< T> 为null，则会出现异常.

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

    如果 Nullable< T> 为null，则可以使用null合并运算符分配默认值:

    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 ;
    

    natch可以避免NullReferenceException`问题.

    This, natch, avoids the NullReferenceException` problem.

    您可以将三元运算符与空合并运算符一起使用:

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

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

  • 最后，您可以使用常规条件逻辑来遵循替代路径:

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

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

  这些与选项有关.哪个是对的?我不知道.这取决于您的情况.

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

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