c# - SQL 查询参数添加给出空引用异常

Question

我在这一行收到空引用异常:

cmd.Parameters.Add("@gross", DbType.Double).Value = gross_amount;

我已经尝试了一段时间了，但是 gross_amount 不是 NULL 仍然让我在 Commission_Load_Setup();

中出现空引用异常

检索值

if (purchaseBillTableDataGridView[8, rowindex].Selected == true)
{              
  quantity = purchaseBillTableDataGridView[8, rowindex].Value.ToString();
}

if (purchaseBillTableDataGridView[9, rowindex].Selected == true)
{
    rate = purchaseBillTableDataGridView[9, rowindex].Value.ToString();
}

if (purchaseBillTableDataGridView[10, rowindex].Selected == true)
{
    if (string.IsNullOrEmpty(quantity) && string.IsNullOrEmpty(rate))
    {
        return;
    }
    else
    {
        gross_amount = double.Parse(quantity) * double.Parse(rate);
        purchaseBillTableDataGridView[10, rowindex].Value = gross_amount;
        load_commission_setup();

    }
}

这是获取佣金的方法。

string paisa;
private void load_commission_setup()
{
    SqlCeConnection conn = null;
    SqlCeCommand cmd = null;
    SqlCeDataReader rdr = null;
    try
    {
        conn =new SqlCeConnection(@"Data Source=|DataDirectory|\Database.sdf;Persist Security Info=False");
        conn.Open();
        //   int rowindex = purchaseBillTableDataGridView.Rows.Count - 1;
        //  gross_amount = double.Parse(purchaseBillTableDataGridView[10, rowindex].Value.ToString());
        MessageBox.Show(Convert.ToString(gross_amount));
        //cmd.Parameters.Add("@gross", gross_amount);
        cmd.Parameters.Add("@gross", DbType.Double).Value = gross_amount;
        cmd = new SqlCeCommand(@"SELECT Paisa FROM CommissionSetupTable WHERE=@gross BETWEEN FromRate AND ToRate;", conn);
        rdr = cmd.ExecuteReader();
        if (rdr == null)
        {
        }
        else
        {
            while (rdr.Read())
            {
                paisa = rdr["Paisa"].ToString();
            }
            rdr.Close();
            cmd.Dispose();
        }
    }
    finally
    {
        conn.Close();
        int rowindex = purchaseBillTableDataGridView.Rows.Count - 1;
        purchaseBillTableDataGridView[11, rowindex].Value = paisa;
    }
}

谢谢 mohit，现在我只收到这个错误:

Answer 1

先创建命令

cmd = new SqlCeCommand(@"SELECT Paisa FROM CommissionSetupTable 
    WHERE=@gross BETWEEN FromRate AND ToRate;", conn);

然后添加参数

cmd.Parameters.Add("@gross", DbType.Double).Value = gross_amount;

否则你试图将参数添加到null命令对象