How do parameterized queries help against SQL injection?

Question

In both queries 1 and 2, the text from the textbox is inserted into the database. What's the significance of the parameterized query here?

1. Passing txtTagNumber as a query parameter

   SqlCommand cmd = new SqlCommand("INSERT INTO dbo.Cars " +"VALUES(@TagNbr);" , conn);
   cmd.Parameters.Add("@TagNbr", SqlDbType.Int);
   cmd.Parameters["@TagNbr"].Value = txtTagNumber.Text;
   

2. Converting txtTagNumber to an integer before constructing the query

   int tagnumber = txtTagNumber.Text.ToInt16(); /* EDITED */
   INSERT into Cars values(tagnumber.Text); /* then is it the same? */
   

Also, here I would use Regular Expression validation to stop insertion of illegal characters.

Answer 1

Parameterized queries do proper substitution of arguments prior to running the SQL query. It completely removes the possibility of "dirty" input changing the meaning of your query. That is, if the input contains SQL, it can't become part of what is executed becase the SQL is never injected into the resulting statement.