How to create a class that uses a generic type that allows nullable and non-nullable types.

Question

I'm trying to create a class to use as a field in other objects that can hold a temporary unsaved value until a SaveChanges method is called on the object, so that I can pass the value to a stored proc, which will update any non-null fields with the new value, and leave fields with null as their original values.

I want to be able to do this:

private Field<int> MyInt;
private Field<int?> MyNullableInt;
private Field<string> MyString;

and then be able to use the Field objects like this:

MyInt = new Field<int>(1);    // initialise with a value
MyInt.Value = 2;              // assign a new value internally marking it as having a new value
int x = MyInt.Value;          // read the current value
int? newInt = MyInt.NewValue; // read the newly set value, or null if there is no new value

I want to be able to create fields that are:

• value types, such as int, bool, etc that have a NOT NULL constraint in the db.
• value types, such as int?, bool?, etc that allow NULLs in the db.
• reference types, such as string, that are always nullable.

I'm still learning about Generics, but I'm starting to understand them, however, I'm a bit stuck on this because I can create a Field object as needed above that works with a non-nullable type, or a nullable type, but not both.

This is what I've come up with so far:

protected class Field<T> where T : struct {
    private T _Field;
    private bool _IsNew;

    public Field(T InitialValue) {
        _Field = InitialValue;
        _IsNew = false;
    }

    public T Value {
        get { return _Field; }
        set { 
            if (!_Field.Equals(value)) { 
                _Field = value; 
                _IsNew = true; 
            } 
        }
    }

    public Nullable<T> NewValue {
        get {
            if (_IsNew) {
                _IsNew = false;
                return _Field;
            }
            return null; 
        }
    }

    public bool IsNew { get { return _IsNew; } }

}

This works well. I can also change the type constraint to where T : class, and change Nullable<T> to just T on the NewValue method, which makes it work with nullable types, but how do I get it working with all data types? Do I really need to create two different classes to handle the two different scenarios?

Thanks, Ben

Answer 1

what prevents from doing what you want? My guess it is the where constraint. If you drop it I do not see what the problem is.

EDIT

You cannot have it both ways. Either you generic class knows something about nulls being assigned to the value, and then you cannot have a value type (like int) as an allowed type, or you do not.

If former is the case your generic class needs the constraint, and you cannot use it for value types like int

In latter case there is no need for the constraint and you still can assign null to the value IF the generic paremeter is of reference type