现有一个AspxGridView，外面有一个保存按钮，当点击按钮的时候，把AspxGridView里的CheckBox的Checked值取出来，怎么取呢，在按钮的Click事件里写：

for (int i = 0; i < G1.VisibleRowCount; i++)
            {
                CheckBox chkselect = G1.FindRowCellTemplateControl(i, (GridViewDataColumn)G1.Columns[2], "chkSelect") as CheckBox;
                CheckBox chkupdate = G1.FindRowCellTemplateControl(i, (GridViewDataColumn)G1.Columns[3], "chkUpdate") as CheckBox;
                CheckBox chkDelete = G1.FindRowCellTemplateControl(i, (GridViewDataColumn)G1.Columns[4], "chkDelete") as CheckBox;

chkselect.checked=true;}

报错：未将对象引用到对象的实例