我开发了一个 Windows 应用程序（C#.net）。我得到了例外，
你调用的对象是空的。
堆栈跟踪是
在 C:\FinalOnam\CableOperatorSoftware\OnamVideoCable\OnamVideoCable\Bill Generate.cs:line 179 中的 OnamVideoCable.Bill_Generate.btnBill_Click(Object sender, EventArgs e)
在 System.Windows.Forms.Control.OnClick(EventArgs e)
在 System.Windows.Forms.Button.OnMouseUp（MouseEventArgs 事件）
在 System.Windows.Forms.Control.WmMouseUp（消息和 m，MouseButtons 按钮，Int32 点击）
在 System.Windows.Forms.Control.WndProc（消息和 m）
在 System.Windows.Forms.ButtonBase.WndProc（消息和 m）
在 System.Windows.Forms.Button.WndProc（消息和 m）
在 System.Windows.Forms.Control.ControlNativeWindow.WndProc（消息和 m）
在 System.Windows.Forms.NativeWindow.DebuggableCallback（IntPtr hWnd，Int32 msg，IntPtr wparam，IntPtr lparam）
在 System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW（味精和味精）
在 System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop（Int32 dwComponentID，Int32 原因，Int32 pvLoopData）
在 System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner（Int32 原因，ApplicationContext 上下文）
在 System.Windows.Forms.Application.ThreadContext.RunMessageLoop（Int32 原因，ApplicationContext 上下文）
在 C:\FinalOnam\CableOperatorSoftware\OnamVideoCable\OnamVideoCable\Program.cs:line 19 中的 OnamVideoCable.Program.Main()
在 System.AppDomain._nExecuteAssembly（程序集程序集，字符串 [] 参数）
在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
在 System.Threading.ExecutionContext.Run（ExecutionContext executionContext，ContextCallback 回调，对象状态）
在 System.Threading.ThreadHelper.ThreadStart()
代码：
private void btnBill_Click(object sender, EventArgs e)
{
int CustNo=0;
int PayMonth;
objB.OB = Convert.ToDouble(txtOB.Text);
if (ddCustSelect.Text == "All Customers")
{
try
{
DataTable dtCustNo = objCust.GetCustInfoForBill();
foreach (DataRow drCustNo in dtCustNo.Rows)
{
CustNo = Convert.ToInt32(drCustNo["CustNo"]);
DataTable dtR = new DataTable();
dtR=objR.GetLastReceiptInfo(CustNo);
if (dtR != null)
{
if (dtR.Rows.Count > 0)
{
DataRow drR = dtR.Rows[0];
//groupBox1.Visible = true;
//lblReceipt.Text = Convert.ToString(drR["RNo"]);
//lblPaidAmt.Text = Convert.ToString(drR["Amount"]);
// lbloutstand.Text = Convert.ToString(drR["Balance"]);
//lblMonthRate.Text = Convert.ToString(drR["MonthlyRate"]);
DateTime paydate = Convert.ToDateTime(drR["date"]);
//lblPayMonth.Text = Convert.ToString(drR["paydate"]);
PayMonth = paydate.Month;
objB.LastOut = Convert.ToDouble(drR["Balance"]);
total = calculate_bill(PayMonth, ddMonth.SelectedIndex + 1, Convert.ToDouble(drR["Balance"]), Convert.ToDouble(drR["MonthlyRate"]));
}
else
{
PayMonth = ddMonth.SelectedIndex + 1;
DataTable dtRate = objRate.GetExistCustRate(CustNo);
DataRow drRate = dtRate.Rows[0];
//total = calculate_bill(PayMonth, ddMonth.SelectedIndex + 1, 0, Convert.ToDouble(drRate["Total"]));
//objB.LastOut = 0;
total = Convert.ToDouble(drRate["Total"]);
objB.LastOut = total;
}
}
else
{
PayMonth = ddMonth.SelectedIndex + 1;
DataTable dtRate = objRate.GetExistCustRate(CustNo);
DataRow drRate = dtRate.Rows[0];
//total = calculate_bill(PayMonth, ddMonth.SelectedIndex + 1, 0, Convert.ToDouble(drRate["Total"]));
//objB.LastOut = 0;
total = Convert.ToDouble(drRate["Total"]);
objB.LastOut = total;
//MessageBox.Show("Table is null");
}
objB.CustNo = Convert.ToInt32(CustNo);
objB.Month = ddMonth.Text;
objB.Year = ddYear.Text;
objB.Total = total;
DataTable dtBill = new DataTable();
dtBill = objB.checkBill();
if (dtBill != null)
{
if (dtBill.Rows.Count > 0)
{
// MessageBox.Show("Bill is Already Generated");
continue;
}
else
{
objB.GenerateBill();
}
}
else
{
//MessageBox.Show("2nd table is null");
objB.GenerateBill();
}
}
MessageBox.Show("Bill is generated");
}
catch (Exception ex)
{
// MessageBox.Show(ex.Message + CustNo);
throw ex;
}
}
if (ddCustSelect.Text == "Single Customer")
{
try
{
//panelSingle.Visible = true;
// groupBox1.Visible = true;
objB.CustNo = Convert.ToInt32(ddCustNo.Text);
objB.Month = ddMonth.Text;
objB.Year = ddYear.Text;
objB.Total = Convert.ToDouble(lblTotalBill.Text);
if (lbloutstand.Visible)
{
objB.LastOut = Convert.ToDouble(lbloutstand.Text);
}
else
{
objB.LastOut = objB.Total;
}
DataTable dtBill = objB.checkBill();
if (dtBill != null)
{
if (dtBill.Rows.Count > 0)
{
MessageBox.Show("Bill is Already Generated");
}
else
{
if (objB.GenerateBill())
{
MessageBox.Show("Bill is generated..");
}
else
{
MessageBox.Show("Error while generating bill");
}
}
}
else
{
MessageBox.Show("2nd table null");
}
}
catch(Exception)
{
}
}
}//----this is line no 179