invalid attempt to read when no data is present, c#

Question: How can I correct the folowing error at line 18?

Error: invalid attempt to read when no data is present

After I changed lines 10 and 17, respectively, to read:

command.CommandText = "SELECT * From RodCtiteria";

if (reader.GetString(4) != null)

I got the error shown on the attached image with more detailed info.

public string ReadCriteria(string dataPoint)
        string criteria = "";
        using (SqlConnection connection = new SqlConnection("Data Source=USER-PC;Initial Catalog=ROD_July18;Integrated Security=True"))
            SqlCommand command = new SqlCommand();

            //string YYYY = rc.YYYY;// Session["YYYY"].ToString();
            command.Connection = connection;
            command.CommandText = "SELECT Top 1 [" + dataPoint + "] From RodCtiteria";
            command.CommandType = CommandType.Text;

            SqlDataReader reader = command.ExecuteReader();
            if (reader.HasRows)
                if (reader.GetString(0) != "")
                    criteria = reader.GetString(0).ToString();     //<-- error here****************************
        return criteria;

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LVL 34
Mike EghtebasDatabase and Application DeveloperAsked:
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Robert SchuttSoftware EngineerCommented:
You forgot to actually Read() the data, also there is a function for testing null:
                if (reader.HasRows) {
                    if (!reader.IsDBNull(0)) // instead of: reader.GetString(0) != ""
                        criteria = reader.GetString(0);

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käµfm³d 👽Commented:
And really there is no reason to even check HasRows:

SqlDataReader reader = command.ExecuteReader();

if (reader.Read())
    (reader.GetString(0) != "")

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Éric MoreauSenior .Net ConsultantCommented:
since you read a single column from a single row, you should use ExecuteScalar instead of Reader. It is optimized for that kind of read:
Mike EghtebasDatabase and Application DeveloperAuthor Commented:
Thank you.
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