The C# team posts answers to common questions and describes new language features
Named and optional (default) parameters are available starting from C# 4.0. For more information, see Named and Optional Arguments (C# Programming Guide).
In languages such as C++, a default value can be included as part of the method declaration:
void Process(Employee employee, bool bonus = false)
This method can be called either with:
in the second case, the parameter bonus is set to false.
C# doesn't have this feature.
One reason we don't have this feature is related to a specific implementation of the feature. In the C++ world, when the user writes:
the compiler generates
In other words, the compiler takes the default value that is specified in the method prototype and puts it into the method call - it's just as if the user wrote 'false' as the second parameter. There's no way to change that default value without forcing the user of the class to recompile, which is unfortunate.
The overloading model works better in this respect. The framework author just defines two separate methods, and the single-parameter one calls the two-parameter method. This keeps the default value in the framework, where it can be modified if necessary.
It would be possible for a compiler to take something like the C++ definition and produce the overloads, but there are a few issues with that approach.
The first one is that the correlation between the code that the user writes and the code the compiler generates is less obvious. We generally try to limit magic when possible, as it makes it harder for programmers. The second issue has to do with things like XML doc comments and intellisense. The compiler would have to have special rules for how it generates doc comments for the overloaded methods, and intellisense would need to have smarts to collapse the overloaded methods into a single method.
Writing overloads yourself is a bit less convenient, but we think it's an acceptable solution.
[Author: Eric Gunnerson]
這是我長久的疑問，也是長久以來C#被罵到臭頭之處，因為C 、VB6、VB.NET，就連T-SQL都有default value，為什麼C#沒有呢?我翻譯了Eric Gunnerson的博客，Eric是C# team的重要成員，我想某種程度可以解釋為什麼C#到了2.0還是不願意提供default value的原因。