如何将日期格式从yyyy/MM/dd转为dd/MM/yyyy？C#代码修改位置咨询

你需要在输出阶段对DateTime对象指定格式，DateTime本身是无格式的，格式只在转为字符串显示时生效，这就是你之前尝试无效的原因。

正确修改步骤： #

1. 保留你已经转换好的dateBorrowedDateTime和dateReturnedDateTime对象
2. 在Console.WriteLine中，把原来的detail[2]和detail[3]（原始字符串）替换成这两个DateTime对象的格式化输出

修改后的Console.WriteLine代码：

Console.WriteLine("{0,10} {1,15} {2,15} {3,15} {4, 10} {5, 10} {6, 10}",
detail[0], detail[1], 
dateBorrowedDateTime.ToString("dd/MM/yyyy"), 
dateReturnedDateTime.ToString("dd/MM/yyyy"), 
daysLoan, "", "");

额外优化建议： #

因为你明确知道CSV里的日期格式是yyyy/MM/dd，建议用DateTime.ParseExact来转换，避免系统区域设置差异导致转换失败（这可能是你之前转换后加代码出错的诱因）：

DateTime dateBorrowedDateTime = DateTime.ParseExact(dateBorrowed, "yyyy/MM/dd", System.Globalization.CultureInfo.InvariantCulture);
DateTime dateReturnedDateTime = DateTime.ParseExact(dateReturned, "yyyy/MM/dd", System.Globalization.CultureInfo.InvariantCulture);

记得要引用System.Globalization命名空间。

完整修改后的代码片段： #

using (StreamReader sr = new StreamReader("loaninfo.csv"))
{
    string? s = sr.ReadLine(); // read the heading
                               // display the heading
    if (s != null)
    {
        string[] heading = s.Split(',');
        Console.WriteLine("{0,10} {1,15} {2,15} {3,15} {4, 10} {5, 10} {6, 10}",
        heading[0], heading[1], heading[2], heading[3], "Days loan", "Days overdue", "Fine");
        // repeat until end of file
    }
    while ((s = sr.ReadLine()) != null)
    {
        string[] detail = s.Split(',');
        string dateBorrowed = detail[2];
        string dateReturned = detail[3];

        // 按指定格式解析日期，避免区域差异问题
        DateTime dateBorrowedDateTime = DateTime.ParseExact(dateBorrowed, "yyyy/MM/dd", System.Globalization.CultureInfo.InvariantCulture);
        DateTime dateReturnedDateTime = DateTime.ParseExact(dateReturned, "yyyy/MM/dd", System.Globalization.CultureInfo.InvariantCulture);

        int daysLoan = dateReturnedDateTime.Subtract(dateBorrowedDateTime).Days;
        
        // 输出时指定目标日期格式
        Console.WriteLine("{0,10} {1,15} {2,15} {3,15} {4, 10} {5, 10} {6, 10}",
        detail[0], detail[1], 
        dateBorrowedDateTime.ToString("dd/MM/yyyy"), 
        dateReturnedDateTime.ToString("dd/MM/yyyy"), 
        daysLoan, "", "");
    }
}

内容的提问来源于stack exchange，提问作者donut