如何显示可为空的DateTime类型的字符串表示？

Question

用户提供"从"和"到"月份以生成报告(例如,从1个月回到13个月;如果他们在2016年2月15日选择此项,则返回值为2015年1月1日, 2016年1月1日).

我想允许用户从"from"或"to"组合框中选择最远的后面或最近的月份.我只想用最远的时间作为"从"和最接近的时间作为"to"以避免他们的混淆(他们可以做任何看起来很自然的事情).

所以我从这段代码开始:

int fromMonths = Convert.ToInt32(comboBoxProduceUsageFrom.Text);
DateTime RptParamsNominalFromDate = ReportSchedulerConstsAndUtils.SubtractMonthsFrom(fromMonths, nextGenerateAndSendDate);
int toMonths = Convert.ToInt32(comboBoxProduceUsageTo.Text);
DateTime RptParamsNominalToDate = ReportSchedulerConstsAndUtils.SubtractMonthsFrom(toMonths, nextGenerateAndSendDate);

..然后我想将"从"日期设置为最晚的时间,并将"到"设置为更接近时间.我第一次尝试这个:

DateTime RptParamsFromDate;
DateTime RptParamsToDate;
if (RptParamsNominalFromDate > RptParamsNominalToDate)
{
    RptParamsFromDate = RptParamsNominalToDate;
    RptParamsToDate = RptParamsNominalFromDate;
}
else if (RptParamsNominalToDate > RptParamsNominalFromDate)
{
    RptParamsFromDate = RptParamsNominalFromDate;
    RptParamsToDate = RptParamsNominalToDate;
}

...但是失败了," 使用未分配的局部变量'RptParamsFromDate'"(和"RptParamsToDate "的错误相同).

所以我尝试给DateTimes一个值/非值,如下所示:

DateTime RptParamsFromDate = null;
DateTime RptParamsToDate = null;

...但是这给了我," 无法将null转换为'System.DateTime',因为它是一个不可为空的值类型 "

所以我再次设置动作并尝试对DateTimes进行nullablizing:

DateTime? RptParamsFromDate = null;
DateTime? RptParamsToDate = null;

...但后来我得到," 'System.Nullable'不包含'ToLongDateString'的定义,并且没有扩展方法'ToLongDateString'接受类型'System.Nullable'的第一个参数'(你是否错过了使用指令或汇编参考？) "

这是由于这段代码:

RptParamsFromDate.ToLongDateString()

在这个块中:

MessageBox.Show(string.Format(
    "Using the current configuration, the Produce Usage report would next be sent on {0} and emailed to {1}; the report would cover data from {2} to {3}",
    nextGenerateAndSendDate.ToLongDateString(), 
    emailRecipients,
    RptParamsFromDate.ToLongDateString(),
    RptParamsToDate.ToLongDateString()));

那么我该怎么做才能显示DateTime值并仍然安抚那个脾气暴躁的野兽呢？

UPDATE

结合来自SLaks和crashmstr的信息,我最终得到了以下工作方法:

private void buttonTestProdUsageSettings_Click(object sender, EventArgs e)
{
    // Show example of when the report will run, and using which parameters,
    // using the current configuration
    DateTime nextGenerateAndSendDate = GetNextProduceUsageGenerateAndSendDate();
    string emailRecipients = string.Join(",", emailAddresses.ToArray());
    int fromMonths = Convert.ToInt32(comboBoxProduceUsageFrom.Text);
    DateTime RptParamsNominalFromDate = ReportSchedulerConstsAndUtils.SubtractMonthsFrom(fromMonths, nextGenerateAndSendDate);
    int toMonths = Convert.ToInt32(comboBoxProduceUsageTo.Text);
    DateTime RptParamsNominalToDate = ReportSchedulerConstsAndUtils.SubtractMonthsFrom(toMonths, nextGenerateAndSendDate);
    if (RptParamsNominalFromDate.Equals(RptParamsNominalToDate))
    {
        MessageBox.Show("The \"from\" and \"to\" values must differ; please try again.");
        return;
    }

    // Allow user to enter either the nearest or furthest value in either the "from" or the "to":
    DateTime? RptParamsFromDate = null;
    DateTime? RptParamsToDate = null;
    if (RptParamsNominalFromDate > RptParamsNominalToDate)
    {
        RptParamsFromDate = RptParamsNominalToDate;
        RptParamsToDate = RptParamsNominalFromDate;
    }
    else if (RptParamsNominalToDate > RptParamsNominalFromDate)
    {
        RptParamsFromDate = RptParamsNominalFromDate;
        RptParamsToDate = RptParamsNominalToDate;
    }

    MessageBox.Show(string.Format(
        "Using the current configuration, the Produce Usage report would next be sent on {0} and emailed to {1}; the report would cover data from {2} to {3}",
        nextGenerateAndSendDate.ToLongDateString(), 
        emailRecipients,
        RptParamsFromDate.HasValue ? RptParamsFromDate.Value.ToLongDateString() : "No \"from\" Date",
        RptParamsToDate.HasValue ? RptParamsToDate.Value.ToLongDateString() : "No \"to\" Date"));
}

Answer 1

您正在尝试使用可空类型的值.

为此,您需要访问其.Value属性,该属性返回常规DateTime值.

请注意,如果它实际上为null,则会抛出异常.