error show on There is already an open DataReader associated with this Command which must be closed first - vb.net

Question

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim n As Integer = 0
str = "select Vote from vote where party='Green'"
cmd = New SqlCommand(str, con)
con.Open()
dr = cmd.ExecuteReader()
if dr.HasRows Then
dr.Read()
n = dr("Vote").ToString()
n = n + 1
Label8.Text = n.ToString()
End If
str = "update Vote set vote='" + n.ToString() + "' where party='Green'"
cmd = New SqlCommand(str, con)
cmd.ExecuteNonQuery()
dr.Read()
cmd.Dispose()
con.Close()
End Sub

Answer 1

The message seems to be pretty clear. You have to close the DataReader before trying to use the connection for another operation
MSDN says
While the SqlDataReader is being used, the associated SqlConnection is
busy serving the SqlDataReader, and no other operations can be
performed on the SqlConnection other than closing it. This is the case
until the Close method of the SqlDataReader is called. For example,
you cannot retrieve output parameters until after you call Close.
....
dr.Close()
str = "update Vote set vote='" + n.ToString() + "' where party='Green'"
cmd = New SqlCommand(str, con)
cmd.ExecuteNonQuery()
' ??? dr.Read()
cmd.Dispose()
con.Close()
By the way, your whole code could be simplified using an ExecuteScalar to retrieve the last vote value and dumping the DataReader
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim n As Integer = 0
str = "select Vote from vote where party='Green'"
Using cmd = New SqlCommand(str, con)
con.Open()
Dim n as Integer
Dim result = cmd.ExecuteScalar()
if result IsNot Nothing Then
n = Convert.ToInt32(result) + 1
else
n = 1
End If
Label8.Text = n.ToString()
str = "update Vote set vote='" + n.ToString() + "' where party='Green'"
cmd.CommandText = str
cmd.ExecuteNonQuery()
con.Close()
End Using
End Sub
In this scenario you use a global connection object and this should be avoided. Remember that it is always better to create, open, use and destroy the connection when you need to use it. SqlClient classes could use the Connection Pooling infrastructure that could help a lot in managing precious resources like a connection

Answer 2

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
try
Dim n As Integer = 0
str = "select Vote from vote where party='Green'"
cmd = New SqlCommand(str, con)
con.Open()
dr.Close()
dr = cmd.ExecuteReader()
if dr.HasRows Then
dr.Read()
n = dr("Vote").ToString()
n = n + 1
Label8.Text = n.ToString()
End If
str = "update Vote set vote='" + n.ToString() + "' where party='Green'"
cmd = New SqlCommand(str, con)
cmd.ExecuteNonQuery()
dr.Read()
Finally
cmd.Dispose()
con.Close()
End Try
End Sub

Answer 3

Add multiple active results set to your connection string as shown below
Example:
"Server=Server;Database=MyDatabase;Trusted_Connection=True;MultipleActiveResultSets=true;"