Set Attributes of TextBox

See below my observations:

1. Use the var keyword to simplify the code and improve readability:

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e) {
  if (e.Row.RowType == DataControlRowType.DataRow) {
    var txtColumn = (TextBox)e.Row.Cells[0].Controls[0];
    txtColumn.CssClass = "form-control daterange-single";
    txtColumn.Font.Size = new FontUnit("12px");
  }
}

 2. Use a more descriptive variable name instead of `txtColumn`, such as `textBox`.

3. Consider moving the CSS class assignment outside the RowDataBound event handler if it is a static value for all TextBox controls in the GridView. This way, it will not be set repeatedly for each row during the event.

// Set the CSS class for TextBox controls in the GridView (outside the event handler)
protected void GridView1_RowCreated(object sender, GridViewRowEventArgs e) {
  if (e.Row.RowType == DataControlRowType.DataRow) {
    var txtColumn = (TextBox)e.Row.Cells[0].Controls[0];
    txtColumn.CssClass = "form-control daterange-single";
  }
}

// Set the font size for TextBox controls in the GridView (inside the event handler)
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e) {
  if (e.Row.RowType == DataControlRowType.DataRow) {
    var textBox = (TextBox)e.Row.Cells[0].Controls[0];
    textBox.Font.Size = new FontUnit("12px");
  }
}