Try this. The code is tested.
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protected void rbtnSelect_CheckedChanged(object sender, EventArgs e)
{
RadioButton selectButton = (RadioButton)sender;
GridViewRow row = (GridViewRow)selectButton.Parent.Parent;
int a = row.RowIndex;
foreach (GridViewRow rw in gvwMixes.Rows)
{
if (selectButton.Checked)
{
if (rw.RowIndex != a)
{
RadioButton rd = rw.FindControl("rbtnSelect") as RadioButton;
rd.Checked = false;
}
}
}
}