What happens to C# 4 optional parameters when compiling against 3.5?
- by Bertrand Le Roy
Here’s a method declaration that uses optional parameters: public Path Copy( Path destination, bool overwrite = false, bool recursive = false)
Something you may not know is that Visual Studio 2010 will let you compile this against .NET 3.5, with no error or warning.
You may be wondering (as I was) how it does that. Well, it takes the easy and rather obvious way of not trying to be too smart and just ignores the optional parameters. So if you’re compiling against 3.5 from Visual Studio 2010, the above code is equivalent to:
public Path Copy( Path destination, bool overwrite, bool recursive)
The parameters are not optional (no such thing in C# 3), and no overload gets magically created for you.
If you’re building a library that is going to have both 3.5 and 4.0 versions, and you want 3.5 users to have reasonable overloads of your methods, you’ll have to provide those yourself, which means that providing a version with optional parameters for the benefit of 4.0 users is not going to provide that much value, except for the ability to provide named parameters out of order. I guess that’s not so bad…
Providing all of the following overloads will compile against both 3.5 and 4.0:
public Path Copy(Path destination)public Path Copy(Path destination, bool overwrite)public Path Copy( Path destination, bool overwrite = false, bool recursive = false)
