Call button click event

In the checkchanged event of the checkbox  you can just call the
btnSave_Click event.

Ex:

private void CheckBox1_CheckedChanged(object sender, System.EventArgs e)
{
     btnSave_Click(sender, e);
}
or
btnSave_Click(null,null);
or
 ((IPostBackEventHandler)btnSave).RaisePostBackEvent(null);


If you want to call dropdown selected index changed method then do the following steps.


Ex:     Object sender=new Object();

      EventArgs myargs=new EventArgs();

      // Call the Drop down list Selected Index changed method

      ddlMyDropDown1_SelectedIndexChanged(sender, myargs);