Le Thanh Hung's blog

Problem

// User Class

public class User
{
  private int userID;
  private DateTime registeredDate;


  public User() {}

  public int UserID
  {
      get { return userID; }
      set { userID = value; }
  }

  public DateTime RegisteredDate
  {
      get { return registeredDate; }
      set { registeredDate = value; }
  }
}


// Create User from a DataReader
public virtual User CreateUserFromReader(IDataReader reader)
{
  User item = new User();

  item.UserID = (int)reader["UserID"];
    // throw exception if reader["RegisteredDate"] equals null 
    item.RegisteredDate = (DateTime)reader["RegisteredDate"]; 

  return item;
}

Because C# 1.1 does not support null value for DateTime so the example above will throw exception if reader["RegisteredDate"] equals null.
Solution
Old schoold soltution :
// Create User from a DataReader

public virtual User CreateUserFromReader(IDataReader reader)
{
  User item = new User();

  item.UserID = (int)reader["UserID"];
  if (!reader.IsDBNull(reader.GetOrdinal("RegisteredDate")))  
  {
      item.RegisteredDate = (DateTime)reader["RegisteredDate"];
  }
  else
  {      
      item.RegisteredDate = new DateTime(1900,1,1,0,0,0,0);
  }

  return item;
}

Now in C#2.0 we can resolve null problem by declaration RegisteredDate as nullable DateTime with ? symbol :

// User Class
public class User
{
  private int userID;
  private DateTime? registeredDate;


  public User() {}

  public int UserID
  {
      get { return userID; }
      set { userID = value; }
  }

  public DateTime? RegisteredDate
  {
      get { return registeredDate; }
      set { registeredDate = value; }
  }
}


// Create User from a DataReader
public virtual User CreateUserFromReader(IDataReader reader)
{
  User item = new User();

  item.UserID = (int)reader["UserID"];
  item.RegisteredDate =
   (reader.IsDBNull(reader.GetOrdinal("RegisteredDate"))) ?
      null : (DateTime)reader["RegisteredDate"];
  return item;
}

Nullable type
Nullable types are constructed using ? type modifier.
Example :
int? age;
string? name;
DateTime? birthday;

A nullable type has two public read-only properties: HasValue and Value. When HasValue is true, the Value property returns the contained value. When HasValue is false, an attempt to access the Value property throws an exception.

DateTime? registeredDay;

// throw exception if RegisteredDate is null
registeredDay = user.RegisteredDate; 
if (user.HasValue)
{
  registeredDay = user.RegisteredDate
}
else
{      
  registeredDay = null;
}

Null colalescence
C# 2.0 introduces a new operator called the null coalescing operator denoted by
double question marks ?? . If the instance is null, the value on the right is returned
otherwise the nullable instance value is returned.

int x = 10;
int? y = null;
int z = y ?? x;

response.write(z); // z = 10

y = 99;
int k = y ?? x;
response.write(k); // k = 99