设置 DataSource 属性后无法修改项集合

comboBox.DataSource = ds.Tables[0];
comboBox.DisplayMember = "Com_CompanyName";
comboBox.ValueMember = "Com_CompanyNo";
comboBox.Items.Insert(0, "--请选着—");

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DataTable dt = ds.Tables[0];
DataRow dr = dt.NewRow();
dr["Com_CompanyName"] = "--选择所有--";
dt.Rows.InsertAt(dr, 0);

         comboBox.DataSource = ds.Tables[0];
         comboBox.DisplayMember = "Com_CompanyName";
         comboBox.ValueMember = "Com_CompanyNo";