In C# how does a declaration differ from a definition, i.e.:
In C++, this was rather obvious, but in C# from what I can tell from the ECMA standard and MSDN is that everything is a declaration and where the word definition is used, it is used to mean the same thing as declaration.
That distinction does not exist. You are referring to the scope of a declaration, the point of definition is irrelevant to the scope of the variable/field.
int x; // 'x' is declared
x = 10; // 'x' is assigned
int y = 20; // 'y' is both declared and assigned
That doesn't make a lot of sense. A method argument is declared in the method signature.
I think you are a bit confused regarding terminology in 1 and 3.
The C++ build model dates from an era where ferrite cores were a dollar a dozen. Multiple source code files compiled one at a time. With a single-pass compiler. A linker to glue everything together. That made separate declarations in header files and prototypes necessary.
The C# compiler takes ready advantage of modern machine resources. All source code files that constitute an assembly are compiled at the same time, the compiler makes at least two passes so it can parse declarations before method bodies. There's still a possible link model but it gets very rarely used.
The only notions of a declaration having to match a definition that I can think of is a delegate type having to match a target method and an interface member declaration having to match its concrete implementation.
where the word definition is used, it is used to mean the same thing as declaration
Correct.
The concept of 'declaration' as a soft/forward definition in C/C++ is needed because of the compilation model. C/C++ use (conceptually) single pass compilation, C# is multi-pass. Consider:
class Bar; // declaration: needed in C++, illegal in C#
class Foo
{
Foo f; // use of incomplete type, C++ can handle this
Bar b; // use of 'undefined' class Bar
}
class Bar // definition
{
}
C++ can't handle the Bar b field without a declaration first.