SqlDataAdapter#Fill: `SelectCommand.connection` property has not been initilized - mysql

Question

I am making a Student Management System for our thesis. When I click the login button after I put the username and password, this error shows up in da.Fill(dt):
InvalidOperationException was unhandled
Fill: SelectCommand.connection property has not been initilized.
Here is my code in login button
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
cs = "Data Source=.\SQLEXPRESS;Initial Catalog=demo;Integrated Security=True"
con = New SqlConnection(cs)
Dim username As String = TextBox1.Text
Dim password As String = TextBox2.Text
cmd = New SqlCommand("select username,password from login where
username='" + TextBox1.Text + "'and password'" + TextBox2.Text + "' ")
da = New SqlDataAdapter(cmd)
dt = New DataTable()
da.Fill(dt)
If (dt.Rows.Count > 0) Then
name = TextBox1.Text
MessageBox.Show("Login Successful", "success!",
MessageBoxButtons.OK, MessageBoxIcon.Information)
content.Show()
Else
MsgBox("Invalid Login Information!", MessageBoxButtons.OK,
MessageBoxIcon.Error)
End If
End Sub
End Class
When I click the login button I should get to the home page.
This is the login:
and this is the home:

Answer 1

You have to specify which connection to use in your command.
cmd = New SqlCommand("select username,password from login where
username='" + TextBox1.Text + "'and password'" + TextBox2.Text + "' ", con)
Please note, you are concatenating string to build your SQL Query. This is VERY unsecure. This lead to SQL Injection! Please at least double quote in string variable, and check int variable that are variable. But I strongly suggest you to use parametrized variable (See sp_executeSql).
cmd = New SqlCommand("select username,password from login where
username='" + TextBox1.Text.replace("'", "''") + "'and password'" + TextBox2.Text.replace("'", "''") + "' ", con)

Answer 2

Comments and explanations in-line.
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim cs = "Data Source=.\SQLEXPRESS;Initial Catalog=demo;Integrated Security=True"
Dim Exists As Boolean
'The Using block ensures that your database objects are closed and disposed
'even if there is an error.
Using con = New SqlConnection(cs)
'All you need to know is if the record exists. You do not need to return
'the values you just entered.
'Pass the connection to the constructor of the command
Using cmd = New SqlCommand("If Exists (Select username, password From login Where
username=#User and password = #Password;", con)
'Use parameters. It not only helps protect your database against SQL Injection but
'simplifies your SQL statement
cmd.Parameters.Add("#User", SqlDbType.VarChar).Value = TextBox1.Text
cmd.Parameters.Add("#Password", SqlDbType.VarChar).Value = TextBox2.Text
'You do not need a data adapter or data table for this
'Use execute scalar when you are returning a single value
con.Open()
Exists = CBool(cmd.ExecuteScalar)
End Using
End Using
If Exists Then
Name = TextBox1.Text
MessageBox.Show("Login Successful", "success!", MessageBoxButtons.OK, MessageBoxIcon.Information)
content.Show()
Else
MessageBox.Show("Invalid Login Information!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
End If
End Sub
EDIT
Never store passwords as plain text.