c# difference between null and dbnull

In my experience DBNull can be interpreted as database null. If the database returns a null, you need to check it against DBNull.value to catch it.

Cases where you would check for null

In the code

int? i=null;

if (System.DBNull.Value.Equals(i))

{

    Console.WriteLine("DBNull");

}

else if (i == null)

{

    Console.WriteLine("null");//code enters here

}

null is what gets printed out.

In the code below, we try to access a non existent field

DataSet ds = new DataSet();

if (System.DBNull.Value.Equals(ds.Tables[""]))

{

    Console.WriteLine("DBNull");

}

else if (ds.Tables[""]== null)

{

    Console.WriteLine("null");//code enters here

}

          

 again null is printed out.

Cases where you would check for DBNull.value 

Here dr is an instance of the SqldataReader class. Assume that the database returns null as the value for the column "xxx"

while (dr.Read())

{

    if (dr["xxx"]==null)

    {

        Console.WriteLine("NULL");

    }

    else if (System.DBNull.Value.Equals(dr["xxx"]))

    {

        Console.WriteLine("DBNull");//code enters here

    }

}

DBNull is what gets printed out

However note that neither checking null of nor DBNull.value will work if the index is out of range. If we check for an index or a column that doesn't exist, it will throw an index out of range exception.