String为Nullable返回false

Question

Why does:

 string s = "";
 bool sCanBeNull = (s is Nullable);
 s = null;

sCanBeNull equate to false?

I'm writing a code generator and need to ensure every type passed to it is nullable, if it isn't already.

       //Get the underlying type:
       var type = field.FieldValueType;

        //Now make sure type is nullable:
        if (type.IsValueType) 
        {
            var nullableType = typeof (Nullable<>).MakeGenericType(type);
            return nullableType.FullName;
        }
        else
        {
            return type.FullName;
        }

Do I need to have to explicitly check for a string or am I missing something?

Answer 1

is tells you whether a value is of a particular type, or one derived from that particular type.

Nullable is a generic struct that allows for nullable versions of non-nullable values.

string is not a Nullable

To tell if a type can have null values use the fact that for all such types the default value is null , while for all other types it is not:

default(string) == null; // true

Answer 2

string is a reference type so it is not Nullable as that is reserved for value types.

In fact:

var nullable = new Nullable<string>();

Gives a compile time error.

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

Answer 3

The rational behind System.Nullable is to be able to represent undefined Value Types using the null keyword. It does not mean that you can check whether some variable can be set to null using someVar is Nullable .

If at compile-time you cannot know beforehand whether some variable would be a value-type or a reference-type, you can use:

!someVar.GetType().IsValueType

But usually a generic argument would be a better approach.

Answer 4

Nullable is just a normal class like MyClass or System.String ( Nullable<T> is struct though).

So if you type:

class _Nullable {}

struct _Nullable<T> {}

class Program
{
    static void Main()
    {
       string a = "";
       Console.Write(a is _Nullable);
    }
}

You wouldn't be surprised if it returns false right ?

If you want to check if something is nullable or not you can use if(!(a is ValueType))

       string a = "";
       Console.Write("a is nullable = {0}", !(a is ValueType));

Output : a is nullable = true