Nullable class? | 易学教程

问题

I have this class

public class FilterQuery
    {
        public FilterQuery() { }

        public string OrderBy { set; get; }
        public string OrderType { set; get; }        

        public int? Page { set; get; }
        public int ResultNumber { set; get; }

    }

I would like to use it like this

public IQueryable<Listing> FindAll(FilterQuery? filterQuery)

Is this possible?

回答1:

This immediately begs the question of why. Classes are by definition reference types and thus are already nullable. It makes no sense to wrap them in a nullable type.

In your case, you can simply define:

public IQueryable<Listing> FindAll(FilterQuery filterQuery)

and call FindAll(null) to pass no filter query.

If you're using C#/.NET 4.0, a nice option is:

public IQueryable<Listing> FindAll(FilterQuery filterQuery = null)

which means you don't even have to specify the null. An overload would do the same trick in previous versions.

Edit: Per request, the overload would simply look like:

public IQueryable<Listing> FindAll()
{
    return FindAll(null);
}

回答2:

There's no need to do that; all class types are reference types, which means that it is nullable by definition. Indeed, there is no way to enforce that it's non-null at compile time.

The Nullable<T> struct is designed to allow value types (which are, by definition, not null) to represent a null value.

It's also worth noting that the ability to compare Nullable<T> with null (or Nothing in VB.NET) is syntactic sugar; because Nullable<T> is a struct, it cannot actually be null. In the case where it represents a null value, HasValue is false and Value is default(T).

来源：https://stackoverflow.com/questions/3132667/nullable-class

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