I want to mark a generic class to support nullable value. The following is my class
publicclass DataField<T> where T:System.Nullable<T> { public T Value; } |
But it can't. Why?
Please help me....
public class NullableClass<T> { // empty } public class UseNullableClass { private NullableClass<int?> nullVariable; } |
is that your mean ?
you can do like this
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?
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.
Thank you for your help.
I can solve my problem
public class DataField<T> where T:INullableValue
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....
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.
