How to get number of checked rows in a gridview using c#

Question

Good afternoon everyone. I need a help.

 

I have a gridview and inside it is a checkbox. I want to get only the number of rows if the checkbox is checked. I tried the code below but it count the whole rows of the gridview.

 

if (checkbox.Checked == true)
                    {
                        lblCheckItems.Text = GridView1.Rows.Count.ToString();

                    }

 thanks for any assistance.