行过滤器-'无法对System.Double和System.String执行'='操作。当值为空时

Question

因此，每当我运行以下代码时，当comboBox中包含值时，它就会起作用，但是如果值是null，则会显示：

  

“无法对System.Double和System.String执行'='操作。”

        DataView dv = new DataView(dt);
        dv.RowFilter =
                       "Price = '" + comboBox3.Text +
                       "' OR Model = '" + comboBox1.Text +
                       "' OR Supplier = '" + comboBox2.Text +
                       "' OR TotalPower = '" + comboBox6.Text +
                       "' OR Battery = '" + comboBox5.Text +
                       "' OR TireSize = '" + comboBox4.Text +
                       "' OR TireType = '" + comboBox9.Text +
                       "' OR Brakes = '" + comboBox8.Text +
                       "' OR WeightCapacity = '" + comboBox7.Text +
                       "' OR MinSpeed = '" + comboBox11.Text +
                       "' OR MaxSpeed = '" + comboBox10.Text + "'";

        dataGridView.DataSource = dv;