How can I add user-supplied input to an SQL statement?

Question

I am trying to create an SQL statement using user-supplied data. I use code similar to this in C#:

var sql = \"INSERT INTO myTable (myField1, myField2) \" +
         


        

Answer 1

Use parameterized SQL.

Examples

(These examples are in C#, see below for the VB.NET version.)

Replace your string concatenations with @... placeholders and, afterwards, add the values to your SqlCommand. You can choose the name of the placeholders freely, just make sure that they start with the @ sign. Your example would look like this:

var sql = "INSERT INTO myTable (myField1, myField2) " +
          "VALUES (@someValue, @someOtherValue);";

using (var cmd = new SqlCommand(sql, myDbConnection))
{
    cmd.Parameters.AddWithValue("@someValue", someVariable);
    cmd.Parameters.AddWithValue("@someOtherValue", someTextBox.Text);
    cmd.ExecuteNonQuery();
}

The same pattern is used for other kinds of SQL statements:

var sql = "UPDATE myTable SET myField1 = @newValue WHERE myField2 = @someValue;";

// see above, same as INSERT

or

var sql = "SELECT myField1, myField2 FROM myTable WHERE myField3 = @someValue;";

using (var cmd = new SqlCommand(sql, myDbConnection))
{
    cmd.Parameters.AddWithValue("@someValue", someVariable);
    using (var reader = cmd.ExecuteReader())
    {
        ...
    }
    // Alternatively: object result = cmd.ExecuteScalar();
    // if you are only interested in one value of one row.
}

A word of caution: AddWithValue is a good starting point and works fine in most cases. However, that the value you pass in needs to exactly match the data type of the corresponding database field. Otherwise, you might end up in a situation where the conversion prevents your query from using an index. Note that some SQL Server data types, such as char/varchar (without preceding "n") or date do not have a corresponding .NET data type. In those cases, Add with the correct data type should be used instead.

Why should I do that?

• It's more secure: It stops SQL injection. _{(Bobby Tables won't delete your student records.)}

• It's easier: No need to fiddle around with single and double quotes or to look up the correct string representation of date literals.

• It's more stable: O'Brien won't crash your application just because he insists on keeping his strange name.

Other database access libraries

• If you use an OleDbCommand instead of an SqlCommand (e.g., if you are using an MS Access database), use ? instead of @... as the placeholder in the SQL. In that case, the first parameter of AddWithValue is irrelevant; instead, you need to add the parameters in the correct order. The same is true for OdbcCommand.

• Entity Framework also supports parameterized queries.