在这里，您可以为当前月份的第一天添加一个月，而不是从那天开始删除一天。

DateTime now = DateTime.Now;
var startDate = new DateTime(now.Year, now.Month, 1);
var endDate = startDate.AddMonths(1).AddDays(-1);

使用.Net API获取月份范围(只是另一种方式):

DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = new DateTime(date.Year, date.Month, DateTime.DaysInMonth(date.Year, date.Month));

DateTime结构只存储一个值，而不是一个值范围。MinValue和MaxValue是静态字段，用于保存DateTime结构实例的可能值范围。这些字段是静态的，与DateTime的特定实例无关。它们与DateTime类型本身相关。

推荐阅读:static (c#参考)

更新:获取月份范围:

DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);

更新:来自评论(@KarlGjertsen & @SergeyBerezovskiy)

DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddSeconds(-1);
//OR
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddTicks(-1);

如果你只关心日期的话

var firstDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind).AddMonths(1).AddDays(-1);

如果你想节省时间

var firstDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind).AddMonths(1).AddDays(-1);

很简单

Begin = new DateTime(DateTime.Now.Year, DateTime.Now.Month,1).ToShortDateString();
End = new DataFim.Text = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month)).ToShortDateString();

你可以做到的

DateTime dt = DateTime.Now; 
DateTime firstDayOfMonth = new DateTime(dt.Year, date.Month, 1);
DateTime lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);