C# has a DateTime structure to hold date and time value in a single variable. The following script will gives you first and last date of given months in two lines of C# code. Assuming that the first and last dates we're looking for is for current month (now), the code would be like this:

DateTime FirstDate = new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);
DateTime LastDate = FirstDate.AddMonths(1).AddDays(-1);

What's next? You may bind each of those two variables to a time control (e.g. DatePicker) in WPF or execute the code directly in your C# code and get debugged after those two lines of code to see their values. This two lines of code is handy if you have 2 time controls (e.g. DatePicker) and you need to set their default values to first and last date of month respectively.

Notes:
Because of DateTime.Now is an expensive call, you may store it to a variable first before you use it in a function, like this:

DateTime Today = DateTime.Now;
DateTime FirstDate = new DateTime(Today.Year, Today.Month, 1);
DateTime LastDate = FirstDate.AddMonths(1).AddDays(-1);

Hope this helps someone!