C＃SQL从表中选择一个数据

Question

I have a database(username(key),Fullname,Pass), I want to search for the username and write out the Fullname for this username in a console. I have these:

static DataTable dt = new DataTable();
static SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=""path\login.mdf"";Integrated Security=True");

con.Open();
string username="mate";
    SqlDataAdapter sda = new SqlDataAdapter("SELECT Nev FROM Login WHERE username='" + username+ "'", con);
        sda.Fill(dt);
        string fullname;
        if (dt.Rows[0][0].ToString() == "1")then?

"Nev" means Full name

I want it to write the Fullname into the "fullname" string, then just:

Console.Writeline("Welcome {0}",fullname);

Can you help me with this code?
I just don't know how to continue my code. Users can login and register to the app, and when they login i want to greet them like "Hello FullName" when they register their username, fullname and password is stored, and they login with username & pass. I just don't know how to find the Fullname if i know the username. (I'm a beginner in SQL)

Answer 1

I think this is what you're after:

string fullname = null;
using(var con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=""path\login.mdf"";Integrated Security=True")) {
    string username = "mate";
    SqlCommand cmd = new SqlCommand("SELECT Nev FROM Login WHERE username=@Username", con);
    cmd.Parameters.AddWithValue("@Username", username);

    con.open();
    fullname = cmd.ExecuteScalar()?.ToString();
}

if (fullname == null) {
    Console.WriteLine("The username could not be found!");
}

The using ensures that the connection is cleaned up once the block is exited. Also, you should use Parameters to be more resilient to SQL injection.

Answer 2

For your specific problem, try this code:

if(dt.Rows.Count > 0)
      fullname = dt.Rows[0]["Nev"].ToString();

But here's a better and complete solution to your problem:

string userName = "mate";
string fullName = string.Empty;

string connectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=path\login.mdf;Initial Catalog=Login;Integrated Security=True";
string query = "SELECT Nev FROM Login WHERE username=@UserName;";

using (SqlConnection connection = new SqlConnection(connectionString))
{
     SqlCommand command = new SqlCommand(query, connection);
     command.Parameters.Add("@UserName", SqlDbType.NVarChar);
     command.Parameters["@UserName"].Value = userName;

     try
     {
         connection.Open();
         fullName = command.ExecuteScalar().ToString();
     }
     catch (Exception ex)
     {
         Console.WriteLine(ex.Message);
     }
}

Console.WriteLine("Hello {0}!", fullName);