I wanted to filter the Data in asp.net but my Data is not filtering i have some logical error So please help me for this case i will be very thanks full to those who will help me please consider my code and replay me with code if  you can so please i am waiting for your replay thanks again

    my asp.cs file is 
     protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            { DisplayStudentInformation(); }


        }
        private void DisplayStudentInformation()
        {
            string filter = "%" + filterTextBox.Text + "%";
             if (filter == String.Empty)
                filter = "%";
                try
                {
                    using (SqlDataReader reader = DAC.GetCompanyInformation(filter))
                    {//reader.Read();
                        StudentGridView.DataSource = reader;

                        StudentGridView.DataBind();

                    }
                }
                catch (SqlException ex)
                {
                    StatusLabel.Text = ex.Message;
                }

        }

    my .aspx file is 
    asp:Table ID="Tabel" runat ="server">
       asp:TableRow>
       asp:TableCell>
       asp:Label ID="filterLabel" runat ="server" Text ="Company Name Filter:" AssociatedControlID="filterTextBox">
       /asp:TableCell>
       asp:TableCell>
       asp:TextBox ID="filterTextBox" runat="server" MaxLength ="50">
       /asp:TableCell>
       asp:TableCell>
       asp:Button ID="refreshButton" runat ="server" Text ="Filter" CausesValidation="false"  /> 
       /asp:TableCell>
       /asp:TableRow>
       /asp:Table>

    My DAC file is 
     public static SqlDataReader GetCompanyInformation(string filter)
            {

                SqlDataReader reader;
                string sql = "SELECT * FROM Student WHERE LastName LIKE @prmLastName ";
                using(SqlCommand command = new SqlCommand (sql,ConnectionManager.GetConnection()))
                {//In ExecuteReader we pass the CommandBehavior as singleResult because we need the Single result and also passing the close connection when Datais retriev
                  // command.Parameters.Add("@prmLastName", SqlDbType.VarChar, 25).Value = filter;
                    command.Parameters.AddWithValue("@prmLastName", filter);
                    reader = command.ExecuteReader(CommandBehavior.SingleResult | CommandBehavior.CloseConnection); 
                }
                return reader;

            }  

Note:- When i don't use the If(!Ispostback)
Condition and simply pass the DisplayStudentInformation(); method in my page load then Data can be filter but with If(!IspostBack ) condition which is also important for updating the data and for other purpose . Data can be filter . Se aim of the expert is that Filter the Data in a gridview using condition of IF(!IspostBack ) means without the removing is post back condition Filter the Data . I have been ask other about this question but no body solve this so please help me i will be very thanks full to you all ok