SqlDataReader 只能检查（Null 或非 null 值）之一

Question

I am writing a program (for personal use) that stores card names into a database. I want to implement a way to see if a name already exists and if it does it will go in a "Do_have_in_db_listbox" else it will go in a "do_not_have_list_box". This is my original method for doing so:

 while (retrieve.Read())
        {
            if(retrieve["Card"] != DBNull.Value)
            {
                listBox_haveInDB.Items.Add(word_for_query);
            }
            else if (retrieve["Card"] == DBNull.Value)
            {
                listBox_notINDb.Items.Add(word_for_query);
            }
        }

I've tried this without the While loop, I've tried variations of if, else, else if and conditions. But for whatever reason the else statement NEVER executes no mater what the first condition is. I've been looking and trying to trouble shoot but the only thing that worked for me was an exception handler:

retrieve.Read();
try
          {
              if(retrieve["Card"] != DBNull.Value)
              {
                  listBox_haveInDB.Items.Add(word_for_query);
              }
          }
          catch
          {
              listBox_notINDb.Items.Add(word_for_query);
          }

This is my way of getting around it. What am I doing wrong?

Answer 1

To evaluate database fields to determine whether their values are DBNull, you can pass the field value to the DBNull.Value.Equals method. However, this method is rarely used.

 DBNull.Value.Equals(retrieve["Card"] )

Try to use this syntax. That's the most reliable way:

if (!reader.IsDBNull( reader.GetOrdinal("Card")))
{
  listBox_haveInDB.Items.Add( ...your code
}

or you can use index as well

if (!reader.IsDBNull( column_index )

Answer 2

Is Card a field in the datatable? If there is any data in the datatable, the retrieve["Card"] will never be null.

I want to implement a way to see if a name already exists

If you want to check if word_for_query exists in datatable, you need to compare retrieve["Card"].ToString() with word_for_query , rather than DBNull.Value .

SqlDataReader retrieve = cmd.ExecuteReader();
try
{
    while (retrieve.Read())
    {
        if (retrieve["Card"].ToString() == word_for_query)
        {
            // if exists, show in listBox_haveInDB and return
            listBox_haveInDB.Items.Add(word_for_query);
            return;
        }
    }
    // not exists
    listBox_notINDb.Items.Add(word_for_query);
}
catch (Exception ex)
{
    Console.WriteLine("\nError:\n{0}", ex.Message);
}
finally
{
    retrieve.Close();
}