Recently, I wanted to add an optional parameter to an extension method. The original method looked like this:
public static class Extensions {
public static bool Foo(this IFoo target) {
target.DoIt(true);
}
}
This is obviously a simplified version but let's go from that.
What I did is:
public static class Extensions {
public static bool Foo(this IFoo target, bool flag = true) {
target.DoIt(flag);
}
}
All I did is introduce an optional parameter with a default value, right? What I was expecting was the compiler to generate the overloaded method, without the flag. This partially happened. Any code that I recompiled was able to compile and execute without any problems, even without the parameter like this:
...
IFoo foo = new FooBar();
foo.Foo();
...
However, any code that was built against the previous version of Foo() did not work, throwing the following exception:
Unhandled Exception: System.MissingMethodException: Method not found: 'Boolean Models.Class1.Foo()'.
at DefaultParamsTests.Program.Main(String[] args)
This is obviously a problem for us as we do have a public API that our customers do leverage and this would be a breaking change.
The solution is to explicitly create an overload:
public static class Extensions {
public static bool Foo(this IFoo target) {
target.DoIt(true);
}
public static bool Foo(this IFoo target, bool ) {
target.DoIt(true);
}
}
However, Resharper does suggest that I could introduce an optional parameter for method foo.
If I do follow the refactoring it basically does what I showed above. However, that won't work for existing code.
I have looked at the generated IL using both Reflector and dotPeek. Neither is showing the generation of the overload.
What am I missing?
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In the case of default parameters, the actual call site is re-written to use the default value. This means an overload isn't generated at all, your calling code is modified!
public void fun(int x = 3) { }
// in another file
fun(); // compiler re-writes to fun(3);
fun(7); // fun(7) as expected
// actual code generated by the c# compiler
fun(3);
fun(7);
If you use optional parameters in C# you need to re-compile all callers when the function
changes. For this reason it is strongly discouraged to put these kind of methods on a public interface (e.g. called by other's code). Using them inside your own linked projects is fine, since they should all compile at the same time
Optional parameters in methods in C# are resolved at compile-time at the call site. Your calling code that omits the parameter expands to include the parameter, calling the two-parameter version. This is the opposite of what you are presuming happens, with creating an overload.
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