Verificar si existe una fila en una tabla

SELECT count(1) from table where key = value;

declare @_var INT = 0
 set @_var = (select count(1) from SQLTabla where folio = 11801 and RUT_proveedor = '123456-0')
 print @_var

select @EmpID = ID from dbo.Employee

Or

set @EmpID =(select id from dbo.Employee)

fuente1
fuente2

Conectar sql en una clase separada

Imports System.Data.SqlClient
Imports System.IO

Public Class SqlCon

    Public sql As SqlConnection
    Public str As String = ""

    Public Sub sqlOpen()
        str = StringConex()
        sql = New SqlConnection(str)
        sql.Open()
    End Sub

    Public Sub sqlClose()
        sql.Close()
        sql.Dispose()
    End Sub

    Public Function StringConex() As String
        Dim rd As New StreamReader("conex.txt")
        Dim linea As String = rd.ReadLine()
        rd.Close()
        rd.Dispose()
        Return linea
    End Function
End Class

Public Class Form1
Private Sub btn_ok_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_ok.Click
        Dim con As New SqlCon
        Try
            con.sqlOpen()
            MessageBox.Show("Conexión OK!")
            con.sqlClose()
        Catch ex As Exception
            MessageBox.Show(ex.ToString)
        End Try
End Sub
End Class

Bloquear ComboBox no editable

Private Sub ComboBox1_KeyPress(ByVal sender As System.Object, _
                                   ByVal e As System.Windows.Forms.KeyPressEventArgs) _
                               Handles ComboBox1.KeyPress
        e.Handled = True
    End Sub

Fecha(s) DateTimePicker

Dim fecha_emis As String = DateTimePicker1.Value.ToString("yyyy-MM-dd")
Dim fecha_venc As String = DateTimePicker2.Value.ToString("yyyy-MM-dd")

Error en DataTable

error datagridview

Private Sub DataGridView1_DataError(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DataGridViewDataErrorEventArgs) Handles DataGridView1.DataError

        If e.Context = DataGridViewDataErrorContexts.Formatting OrElse DataGridViewDataErrorContexts.PreferredSize Then
            e.ThrowException = False
        End If

    End Sub

Encoding

Dim enc As Encoding = Encoding.GetEncoding("ISO-8859-1")
Encoding.GetEncoding("ISO-8859-1")

System.Text.Encoding.GetEncoding(&H6FAF)

System.IO.File.WriteAllLines("archivo.txt", txt.ToArray, System.Text.Encoding.GetEncoding(1252))

Validar letras y números con expresión regular

Public Function letras_y_numeros(ByVal s As String) As Boolean
        Dim match As Boolean = False
        Dim rgx As New Regex("^[a-zA-Z0-9]+$")
        match = rgx.IsMatch(s)
        Return match
    End Function

fuente: http://stackoverflow.com/questions/1181419/verifying-that-a-string-contains-only-letters-in-c-sharp