I understand partial methods can be used to split the definition of a method across multiple files. I'm curious though if it's permissible to have each definition of a method across multiple files contain code?
For example, say I have a method private partial void Foo(). Let's say I've got it defined in file A and file B. Can both instances have code contained in the method, or just one or the other? I guess I'd be surprised if this were permitted.
No, you can't. If you could, when you call Foo(), which code would execute first? If both versions were dealing with (and modifying) global state, it would be very important to know the order of execution.
Anyway, it makes no sense. So no, you can't.
As a simple example of the potential nastiness of the erratic behavior emerging from such a possibility, suppose you could, and suppose you had the following code:
public partial class MyClass {
private int count = 0;
public partial void NastyMethod() {
count++;
}
}
public partial class MyClass {
public partial void NastyMethod() {
Console.WriteLine(count);
}
}
When you call NastyMethod(), what value would it print? No sense!
Now another strange problem. What to do with parameters? And return values?
public partial class MyClass2 {
public partial bool HasRealSolution(double a, double b, double c) {
var delta = b*b - 4*a*c;
return delta >= 0;
}
}
public partial class MyClass2 {
public partial void HasRealSolution(double a, double b, double c) {
return false;
}
}
And now, how could one possibly give a sense to this code? Which return should we consider after calling HasRealSolution(1, 2, 1)? How is it ever conceivable to have 2 different, simultaneous, return values* for a single method? We are not dealing with nondeterministic finite automata!
To those who would impose that in this hypothetical world my inexistent partial methods should be void, replace the returns with setting a value on some private field to that class. The effect is almost the same.
* Note that what I'm talking here is not a single return value composed of two values, such as a Tuple. I'm talking here about TWO return values. (???)
This is really a comment to @Bruno's answer and may not be completely relevant to the question:
The decision to not allow partial methods to have multiple implementations is an arbitrary decision made in the design of the language. Your argument is a good reason why you might decide against allowing such a thing in your language but there isn't really a technical limitation. You can very easily decide to allow multiple implementation and let the compiler decide the order of execution of implementations. Actually, C# specification already has cases with undefined order for partial classes:
// A.cs:
partial class Program {
static int x = y + 42;
static void Main() {
System.Console.WriteLine("x: {0}; y: {1}", x, y);
}
}
// B.cs:
partial class Program {
static int y = x + 42;
}
This is valid C# according to the C# Specification v3.0 but the output can be:
x: 42; y: 84
or
x: 84; y: 42
and the compiler is allowed to generate either of them. It doesn't even generate a warning.
The C# language requires a partial method to to have a void return type. The partial method signature can be defined at most once. Likewise, the implementation should be defined at most once.
No. Only a single version of a partial method may contain code. Defining more than one implementation of a partial method with an implementation will result in a compiler error.
No, this is not possible. Also, you have a slight misunderstanding. The purpose of partial methods is not to "split the definition of a method across multiple files".
Rather, a partial method is designed to split the definition of class across multiple files. This is helpful, especially with automatically generated code, since it lets you define "optional" methods that you are free to implement, but also free to ignore.
That is not possible. You define the signature in one place, and the implementation in another place.
For details, see MSDN.