title: Problem inserting string or NULL into SQL Server database
body: <p>I'm having trouble coming up with the proper syntax for allowing either a string or a NULL to be passed to the database. Here's my code:</p>
  
  <pre><code>string insertString = String.Format(
      @"INSERT INTO upload_history (field1, field2, field3) 
      VALUES ('{0}', '{1}', '{2}')",
      varField1, varField2, varField3);
  </code></pre>
  
  <p>I used single quotes around the variable placeholders so that the database would properly accept a string value. However, if NULL is passed, it ends up going into the database as the string "NULL".</p>
  
  <p>Is there a way I can leave the single quotes out of the InsertCommand string and conditionally add single quotes to my variables?</p>