how to make changes to selected row comboBox if its checkbox is ticked

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C#

I have a gridview and in gridView i have checkbox and combobox.
i need to select particular item from combobox only if the check box is not ticked, if it is ticked it must not select it.

i used following code:

for (int i = 0; i <= GridView1.Rows.Count - 1;  i++)
            {
                object obj = GridView1.Rows[i].Cells[0];
                if (((obj != null)))
                {
                    DataGridViewCheckBoxCell checkBox1 = (DataGridViewCheckBoxCell)obj;
                    object objValue = checkBox1.Value;
                    if (((objValue != null)))
                    {
                        bool @checked = (bool)objValue;
                        if ((@checked))
                        {
                            SplitMethod.ReadOnly = true;
                        }
                        else
                        {
                            SplitMethod.ReadOnly = false;
                        }
                    }
                }
            }

know the problem if i select first row checkbox so it enable me to select rest of combobox as well.

simply:
each row in grid view has check box and combo box, so when i select each row check box so it must stop me to make changes to only that combobox not rest of the rows.
i must me able to make changes to other rows combobox if it check box is not selected.