Why is '??' an operatore in C#?

/prog/, I want to know what is more more accepted method of two different if statements sharing an identical routine. I'm not good at explaining, so I think you'll work out what I mean by the following examples:

So, method one:
void func_main(){
    int i = 0;

    if (...bool1...){
        i = 1;
    }
    else if (...bool2...){
        i = 2;
    }
    else {
        return;
    }
   
    ...functions...
}

Method two:

void func_sub(){
    ...functions...
}

void func_main(){
    int i = 0;
   
    if (...bool1...){
        i = 1;
        func_sub();
    }
    else if (...bool2...){
        i = 2;
        func_sub();
    }
}
Method three:

void func_main(){
    int i = 0;
   
    if (...bool1... || ...bool2...){
        if (...bool1...){
            i = 1;
        }
        else if (...bool2...){
            i = 2;
        }
        ...functions...
    }
}