Remember that string takes fixing memory until the program ends.
Null and Empty
In C#, a variable's value is null if it is not set, or if it has been explicitly set to null. A null value typically means the value is not known, or has not yet been set, and is frequently a source of errors in programs, because methods cannot be invoked on null instances of types. An empty string is simply a string with a length of zero. It's a valid instance of a string, but one that currently has no characters in it.
It shows how null can cause an exception. String.IsNullOrEmpty(emptyString) can help.
Replacing Parts of Strings
Date and Time：
Refer to：Working with Dates and Times
It is totally same as it is in C++.