string strsql = "select * from userinfo where 1=1";
if(!string.IsNullOrEmpty(txtUserName.Text.Trim()))
 strsql+= " and username='"+txtUserName.Text.Trim()+"'";
if(!string.IsNullOrEmpty(cmbAge.Text.Trim()))
 strsql+= " and age='"+cmbAge.Text.Trim()+"'";
if(!string.IsNullOrEmpty(datePickerBirthday.Text))
 strsql+=" and Birthday ='"+datePickerBirthday.Text+"'";