选择语句不适用于C＃

Question

I am trying to get data from local database using this code:

SqlConnection conn = new SqlConnection(@"Data Source = .\SQLEXPRESS; Initial Catalog = serin; Integrated Security = True");
conn.Open();

string query = "SELECT UserPassword FROM UserList WHERE UserName = @UserName";

SqlCommand cmd = new SqlCommand(query, conn);
//SqlParameter param = new SqlParameter("@UserName", SqlDbType.VarChar, txt_UserName.TextLength);
//param.Value = txt_UserName.Text;

//cmd.Parameters.Add(param);
cmd.Parameters.AddWithValue("@UserName", "aliserin");

When I try in this way, it works. But when I try to take input from a textbox, it returns NULL. I have tried as commented lines, but they also didn't work. I thought maybe it is about type but it doesn't look so.

Any help will be appreciated.

I'm using Visual Studio 2017 and SQL Server 2017.

Answer 1

Change this line

cmd.Parameters.AddWithValue("@UserName", "aliserin");

to this

cmd.Parameters.AddWithValue("@UserName", txt_UserName.Text);

NOTE: Can we stop using AddWithValue() already? thanks to @marc_s

Answer 2

don't use a parameter name by the same name as the column... I have historically seen this as a confusion as it sees both a column as the same and uses the column, not the parameter. Try

string query = "SELECT UserPassword FROM UserList WHERE UserName = @parmUserName";
SqlCommand cmd = new SqlCommand(query, conn);
cmd.Parameters.AddWithValue("@parmUserName", txt_UserName.Text );