C# - Nullable type or ? on variable

Nullable<T> type or  ? on variable

Represents a value type that can be assigned null.

It's a nullable value. Structs, by default, cannot be nullable, they must have a value, so in C# 2.0, the Nullable<T> type was introduced to the .NET Framework.

C# implements the Nullable<T> type with a piece of syntactic sugar, which places a question mark after the type name, thus making the previously non-nullable type, nullable.

Explanation 

   //Cannot Be Null

   DateTime

   DateTime dt = null;   // Error: Cannot convert null to 'System.DateTime'

                         //because it is a  non-nullable value type

   //Can Be Null

   DateTime?             //Nullable < DateTime >               

   DateTime? dt = null;  // no problems

Example 1

using System;

using System.Collections.Generic;

using System.Data;

using System.Drawing;

using System.IO;

using System.Linq;

using System.Linq.Expressions;

using System.Text;

using System.Web;

using System;

class Sample

{

    // Define the "titleAuthor" table of the Microsoft "pubs" database.

    public struct titleAuthor

    {

        // Author ID; format ###-##-####

        public string au_id;

        // Title ID; format AA####

        public string title_id;

        // Author ORD is nullable.

        public short? au_ord;

        // Royalty Percent is nullable.

        public int? royaltyper;

    }

    public static void Main()

    {

        // Declare and initialize the titleAuthor array.

        titleAuthor[] ta = new titleAuthor[3];

        ta[0].au_id = "712-32-1176";

        ta[0].title_id = "PS3333";

        ta[0].au_ord = 1;

        ta[0].royaltyper = 100;

        ta[1].au_id = "213-46-8915";

        ta[1].title_id = "BU1032";

        ta[1].au_ord = null;

        ta[1].royaltyper = null;

        ta[2].au_id = "672-71-3249";

        ta[2].title_id = "TC7777";

        ta[2].au_ord = null;

        ta[2].royaltyper = 40;

        // Display the values of the titleAuthor array elements, and

        // display a legend.

        Display("Title Authors Table", ta);

        Console.WriteLine("Legend:");

        Console.WriteLine("An Author ORD of -1 means no value is defined.");

        Console.WriteLine("A Royalty % of 0 means no value is defined.");

    }

    // Display the values of the titleAuthor array elements.

    public static void Display(string dspTitle,

                               titleAuthor[] dspAllTitleAuthors)

    {

        Console.WriteLine("*** {0} ***", dspTitle);

        foreach (titleAuthor dspTA in dspAllTitleAuthors)

        {

            Console.WriteLine("Author ID ... {0}", dspTA.au_id);

            Console.WriteLine("Title ID .... {0}", dspTA.title_id);

            Console.WriteLine("Author ORD .. {0}", dspTA.au_ord ?? -1);

            Console.WriteLine("Royalty % ... {0}", dspTA.royaltyper ?? 0);

            Console.WriteLine();

        }

    }

}

// The example displays the following output:

//     *** Title Authors Table ***

//     Author ID ... 712-32-1176

//     Title ID .... PS3333

//     Author ORD .. 1

//     Royalty % ... 100

//    

//     Author ID ... 213-46-8915

//     Title ID .... BU1032

//     Author ORD .. -1

//     Royalty % ... 0

//    

//     Author ID ... 672-71-3249

//     Title ID .... TC7777

//     Author ORD .. -1

//     Royalty % ... 40

//    

//     Legend:

//     An Author ORD of -1 means no value is defined.

//     A Royalty % of 0 means no value is defined.