Function CheckExistingUsername(ByVal username As String) As Boolean

        Dim UserExists As Boolean = False
        Dim strConn As String = System.Configuration.ConfigurationManager.ConnectionStrings("Fullconnection").ConnectionString
        Dim MySQL As String = "Select username from tUser where Username=@Username"
        Dim MyConn As New SqlConnection(strConn)
        Dim objDR As SqlDataReader
        Dim Cmd As New SqlCommand(MySQL, MyConn)
        Cmd.Parameters.Add(New SqlParameter("@Username", username))

        Try
            MyConn.Open()
            objDR = Cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection)

            While objDR.Read()
                UserExists = True
            End While

        Catch ex As Exception
            LogError("GlobalClass.vb :: CheckExistingUsername", ex.ToString)
        Finally
            MyConn.Close()
            CheckExistingUsername = UserExists
        End Try
    End Function

Function CheckExistingUsername(ByVal username As String) As Boolean

        Dim UserExists As Boolean = False
        Dim strConn As String = System.Configuration.ConfigurationManager.ConnectionStrings("Fullconnection").ConnectionString
        Dim MySQL As String = "Select username from tUser where Username=@Username"
        Dim MyConn As New SqlConnection(strConn)
        Dim objDR As SqlDataReader
        Dim Cmd As New SqlCommand(MySQL, MyConn)
        Cmd.Parameters.Add(New SqlParameter("@Username", username))

        Try
            MyConn.Open()
            objDR = Cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection)

            While objDR.Read()
                UserExists = True
            End While

        Catch ex As Exception
            LBL_Info.Visible = True
            LBL_Info.Text = "Error: " & ex.Message
        Finally
            CheckExistingUsername = UserExists
        End Try

    End Function

Check if username is in use

validation

VB.NET