private void dataGridView1_RowsRemoved(object sender, DataGridViewRowsRemovedEventArgs e)
{
    if (this.dataGridView1.RowCount == 0)
    {
        // 已经全部删了
    }
}

------解决方案--------------------
if (dataGridView1.Rows.Count == 0)

------解决方案--------------------
条件dataGridView1.Rows.Count == 0就这么简单

------解决方案--------------------
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if (dataGridView1.Rows.Count == 0)