SELECTing data from database in vb and outputting data into label - sql

Question

I have the following code which SELECTs data from a database and outputs a value to a label on the form:
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim strConn As String = System.Configuration.ConfigurationManager.ConnectionStrings("yourConnectionString").ToString()
Dim sql As String = "SELECT aid FROM tbl_RAPA WHERE username=#username"
Dim conn As New Data.SqlClient.SqlConnection(strConn)
Dim objDR As Data.SqlClient.SqlDataReader
Dim Cmd As New Data.SqlClient.SqlCommand(sql, conn)
Cmd.Parameters.AddWithValue("#username", User.Identity.Name)
conn.Open()
objDR = Cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection)
While objDR.Read()
Label1.Text = objDR("aid")
End While
End Sub
However, if the value in the database is empty, the program runs into an error. Is there a way for me to do this so the program just returns an empty value rather than crashing?
The error message i am given is System.InvalidCastException: 'Unable to cast object of type 'System.DBNull' to type 'System.Windows.Forms.Label'.' on the line Label1.Text = objDR("aid")

Answer 1

Database objects generally need to be closed and disposed. Using...End Using blocks will do this for you even if there is an error.
Since you are only expecting one piece of data you can use .ExecuteScalar which provides the first column of the first row of the result set. This method returns an object.
Try to always use the the .Add method with Parameters. See http://www.dbdelta.com/addwithvalue-is-evil/
and
https://blogs.msmvps.com/jcoehoorn/blog/2014/05/12/can-we-stop-using-addwithvalue-already/
and another one:
https://dba.stackexchange.com/questions/195937/addwithvalue-performance-and-plan-cache-implications
Here is another
https://andrevdm.blogspot.com/2010/12/parameterised-queriesdont-use.html
I had to guess at the database type so, check your database for the real value.
Don't update the User Interface until after the connection is closed and diposed. (End Using). I declared aid before the Using block so, it could be used after the block. Check if the object, aid, is not Nothing before adding it to the label's Text.
Imports MySql.Data.MySqlClient
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim aid As Object
Using conn As New MySqlConnection(ConfigurationManager.ConnectionStrings("yourConnectionString").ToString),
cmd As New MySqlCommand("SELECT aid FROM tbl_RAPA WHERE username=#username", conn)
cmd.Parameters.Add("#username", MySqlDbType.VarChar).Value = User.Identity.Name
aid = cmd.ExecuteScalar
conn.Open()
End Using
If Not IsNothing(aid) Then
Label1.Text = aid.ToString
End If
End Sub

Answer 2

I would add this before While objDR.Read() as a precaution in case your query returns no rows:
if objDR.HasRows
...
Then, to handle the null values (this is probably what you mean by empty):
If Not String.IsNullOrEmpty(objDR.Item("aid")) Then
Label1.Text = objDR("aid")
Else
Label1.Text = "Null !"
End if
You could also use ExecuteScalar() if you are only expecting one record. But you would need to handle the situation where no matching record is found.