C# - CS1503 - 参数 1：无法从“字符串”转换为“整数”

Question

Before you reply, I've checked my code over & over again and have also searched for about an hour now for a similar answer. The compiler keeps throwing the error CS1503, I'm not really sure how to fix this. It's on lines 36 and 37 and I've commented the lines that have the error, 36 and 37. It's for a database that is supposed to search for the license plate and output the other data in the table.

public partial class Login : Form
{
    string connectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=J:\Plate Reader\DB\InfoDB.mdf;Integrated Security=True;Connect Timeout=30";
    SqlDataReader mdr;

    public Login()
    {
        InitializeComponent();
    }

    private void btnSearch_Click(object sender, EventArgs e)
    {
        using (SqlConnection sqlCon = new SqlConnection(connectionString))
        {
            sqlCon.Open();
            string selectQuery = "SELECT * From Plate WHERE Plate='" + txtPlate.Text;
            SqlCommand command = new SqlCommand(selectQuery, sqlCon);

            mdr = command.ExecuteReader();

            if (mdr.Read())
            {
                labelName.Text = mdr.GetString("Name");  //Right here
                labelWanted.Text = mdr.GetInt32("Bounty").ToString(); //and here
            }
            else
            {
                MessageBox.Show("No Data For This Plate");
            }
        }
    }
}

Answer 1

As explained in comments, GetString as well GetInt32 requires a parameter of type integer. That integer is the position of the field in the Select field list. If you do not want to use a position you could write

 labelName.Text = mdr.GetString(mdr.GetOrdinal("Name")); 

and this simple line could be easily transformed in an extension method adding a method to a static class whose code is

public static class ReaderExtensions
{
     public static string GetString(this SqlDataReader source, string fieldName)
     {
         return source.GetString(source.GetOrdinal(fieldName));
     }
}

and this finally allows you to write

string labelText = mdr.GetString("Name");

of course the same could be written also for a GetInt32 that accepts a field name. By the way, if I am not mistaken the MySql version has these overloads directly in the assembly

Answer 2

GetInt32 is expecting an 'int' parameter.

You are passing in a string.

You need to send in the ordinal position of your columns.

use this instead:

mdr.GetInt32(mdr.GetOrdinal("Name"));

You might want to consider using a micro ORM like Dapper to make things easier.

Answer 3

The reason is that GetString(i) and GetInt32(i) methods requires zero-based column ordinal as an parameter. If you want to retrieve data from the column by name you should use indexer :

labelName.Text = mdr["Name"].ToString();  
labelWanted.Text = mdr["Bounty"].ToString();