Does C# allow double semicolon ; ; if so, are there any special ways?

Question

I am writing a statement and it compiles, but the compiler [VS] never tells me that I put the semicolon two times.

This means in ASP.NET MVC 3

return         


        

Answer 1

That a double ;; is allowed, is for historical reasons. It's is a hangover from C style languages (which C# is based on).

C & C++ have the concept of pre-processor macros which are replaced in the code before the code is compiled e.g. trivial example AddAndSquare is a macro, not a function

#define AddAndSquare(X,Y) (X+Y)*(X+Y)
int Foo() {
   int a = 1, b = 2;
   return AddAndSquare(a, b);
}

goes to the compiler as

int Foo() {
   int a = 1, b = 2;
   return (A+B)*(A+B);
}

You can redefine macros to be different to their initial definition, also you can redefine them so they don't exist at all.

Given an assertion macro #define ASSERT(c) if(!c) throw new AssertionFailedException() you can have your coded littered with ASSERT statements.

void Foo(int x) {
    int y = x + 2;
    ASSERT(y != 0);
   int z = x / y;
    . . . .
}

Now consider that you only want the asserts in debug builds, but not in release builds, for release you redefine the macro to be empty (literally #define ASSERT). Now when Foo goes to the compiler for a release build, it looks like this

void Foo(int x) {
    int y = x + 2;
    ;
   int z = x / y;
    . . . .
}

There's now an empty statement where the ASSERT was, because there may or may not be a statement there (depending on build configuration), the compiler needs to be able to handle an empty statement.

Why this convention was kept in C# where there are nothing like C macros, I have no idea, but possibly because it causes little or no harm.

I would guess that multiple ; are elided by the compiler before it starts parsing code, therefore your unreachable ; is ignored by the compiler.