我收到错误“将数据类型 nvarchar 转换为实数时出错."

问题描述

我的代码如下，请告诉我问题出在哪里，以便我们可以按仓库 ID 降价 7%?

The code which I have is below, can you please inform me where the issue might be so we can drop the price by warehouse ID with 7%?

        private void button4_Click(object sender, EventArgs e)
    {
        try
        {
            myConnection = new SqlConnection(frm.cs);
            myCommand = new SqlCommand("update Inventory set Price=@Price where WarehouseCode=6", myConnection);
            myConnection.Open();
            myCommand.Parameters.AddWithValue("@Price", "@Price * 0.7");
            myCommand.ExecuteNonQuery();
            myConnection.Close();
            MessageBox.Show("Update successfully!");
            DisplayData();
            if (myConnection.State == ConnectionState.Open)
            {
                myConnection.Dispose();
            }
        }

推荐答案

当你的 仓库 ID 和 折扣率 由最终用户指定，我建议您的代码如下:

As you are not assigning any value to @Price in the C# code while your Warehouse Id and the Discount rate are subject to be assigned by the end-user, I recommend your code be like this:

   private void button4_Click(object sender, EventArgs e)
   {
        try
        {
            var discountRate = 0.07; //could be Convert.ToDouble(textBox1.Text) or something else
            var warehouseId = 6;    //again, could be Convert.ToInt32(textBox2.Text) or something else
            myConnection = new SqlConnection(frm.cs);
            myCommand = new SqlCommand("update Inventory set Price=Price*(1-@DiscountRate) " +
                                        "where WarehouseCode=@WarehouseId", myConnection);
            myConnection.Open();
            myCommand.Parameters.AddWithValue("@DiscountRate", discountRate);
            myCommand.Parameters.AddWithValue("@WarehouseId", warehouseId);
            myCommand.ExecuteNonQuery();
            myConnection.Close();
            MessageBox.Show("Update successfully!");
            DisplayData();
            if (myConnection.State == ConnectionState.Open)
            {
               myConnection.Dispose();
            }
        }
        catch
        {

        }
    }

我还建议您重新考虑您的查询，因为它会使用相同的值更新所有产品价格，您可以考虑传递参数 @ProductId 并且您的查询是

I would also recommend you think again about your query as it will update all products price with the same value, you may consider passing parameter @ProductId and your query be

update Inventory set Price=Price*(1-@DiscountRate) 
where WarehouseCode=@WarehouseId and ProductCode=@ProductId

当然是这样.

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