Add new row to GridView on each button-Click

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ADO.NET

//======================================================
public partial class AddMaterial : System.Web.UI.Page
{
        DataTable dt = new DataTable();
        DataColumn dc = new DataColumn();
//=======================================================
protected void Button1_Click(object sender, EventArgs e)
   {
       if (dt.Columns.Count == 0)
        {
            dt.Columns.Add("Category", typeof(string));
            dt.Columns.Add("Mtl", typeof(string));
        }

        DataRow NewRow = dt.NewRow();
        NewRow[0] = TextBox1.Text;
        NewRow [1]=TextBox2.Text;
        dt.Rows.Add(NewRow); 
        GridView1.DataSource = dt;
        GridViewl.DataBind();
       
   }

//=========================================================
}

On each button-click each row to be added to the grid-view.
(ie. if user clicked 3times there should be 3rows).
The above code always showing 1 row even after multiple click.
Please provide corrected solution..

[Improved readability. Corrected textspeak.]