Nullable Value and Generic

Nullable Value and Generic

• Afia

  Thank you for your help.
  
  I can solve my problem
  
  public class DataField<T> where T:INullableValue

  
  
  

• MartinUSS

  1. string  is not actually valid. Nullable<T> has a struct constraint so only value types can be used with Nullable<T>.
  
  As far as I'm aware there is no way to use constraints to say: Only allow reference types and nullable types. The only way I can see to do this is to remove the constraints and run some sort of type check within the static constructor of the generic type.
  
  2. Generics is a bit tricky here. Is your situation DataField<int > is not actually assignable to DataField<INullableValue>, even though int implements INullableValue. The same works for base and derived classes. For example, If you have the following classes:
  
  

  public class MyClass<T> { } public class A { } public class B : A { }

  
  
  This will not compile:
  
  

  MyClass<A> myclass = new MyClass<B>();

  
  
  The only common base class that MyClass<A> and MyClass<B> have is System.Object. Object is also the only common base class that DataField<int > and DataField<INullable> have in common.

  
  
   

  
  
  

• Chuck S.

  public class NullableClass<T> {     // empty } public class UseNullableClass {     private NullableClass<int > nullVariable; }

  
  
  is that your mean
  you can do like this
  
  
  

• RichardWu

  Sorry,
  
  I misread your post, I thought you were inheriting Nullable<T>.
  
  For some reason, this restriction was placed for generics.
  
  Replace Nullable<T> with INullableValue, which Nullable<T> inherits.
  
  

• mms02

  That's because Nullable<T> is a Value Type (struct), and you can't inherit Value Types.
  
  You can implement System.INullableValue.  However, by default all reference types have a concept of a null value. For example:
  
  DataField<String> field = null;
  
  Field now contains a null reference. What are you trying to achieve
  
  

• ScottWK

  I have several issues for this approach
  
  this is my class
   public class DataField<T> where T:INullableValue
   {
    public T Value;
    public DataField() { }
   }
  
  1. I cannot use string in this approach
  DataField<int > field=new DataField<int >; work
  DataField<long > field=new DataField<long >; work
  DataField<string > field=new DataField<string >; not work
  DataField<string> field=new DataField<string>; not work
  
  How to solve this problem.  I want to enforce the data type is nullable value when create DataField object
  
  2. I cannot add the DataField into collection
     Dictionary<int, DataField<INullableValue>> dataFields= new Dictionary<int, DataField<INullableValue>>();
     
     DataField<int > x = new DataField<int >();
     dataFields.Add(1,x);
     DataField<long > y = new DataField<long >();
     dataFields.Add(2,y);
  
  It does not work.....
  Please help me....
  
  
• Nullable Value and Generic