odd variable scope in switch statement

Question

This question reminded me of an old unanswered question in my mind about switch:

    int personType = 1;
    switch (personType)
    {
        case 1:
            Employee emp = new Employee();
            emp.ExperienceInfo();
            break;
        case 2:
            Employee emp = new Employee(); 
            //Error: A local variable named 'emp' is already defined in this scope
            emp.ManagementInfo();
            break;
        case 3:
            Student st = new Student();
            st.EducationInfo();
            break;
        default:
            MessageBox.Show("Not valid ...");
    }

why is emp recognized in 'case 2'? in C++ (if I am not wrong) we could use multiple cases together, but in C# that is impossible and we should close case 1 with break so the following code seems right in C++ and wrong in C#:

case 1:
case 2:
   SomeMethodUsedByBothStates();

When we can not have such behaviour so why should we be able to declare emp in case 1 and see it in case 2? If never two cases happen together so why should the object be seen in both?

Answer 1

Cases do not create scope in c++ or in c#. All of those variables declared inside a case are in the same scope, that of the switch statement. You need to use braces if you want those variables to be local to some specific case:

switch (personType)
{
    case 1: {
        Employee emp = new Employee();
        emp.ExperienceInfo();
        break;
    }
    case 2: {
        Employee emp = new Employee(); 
        // No longer an error; now 'emp' is local to this case.
        emp.ManagementInfo();
        break;
    }
    case 3: {
        Student st = new Student();
        st.EducationInfo();
        break;
    }
    ...
}

Answer 2

The second code you showed is perfectly fine in C#, assuming case 2 had a break or return:

case 1:
    // no code here...
case 2:
    SomeMethodUsedByBothStates();
    break;

Empty cases are allowed to fall through.
What is not allowed is to have code in a case-branch that falls through. So, the following would be invalid:

case 1:
    SomeMethodUsedOnlyByCase1();
    // no break here...
case 2:
    SomeMethodUsedByBothStates();
    break;

---

The question about the scope is a different one. Basically, the scope is the switch statement itself, not a case-branch.

To make your example compile, simply give the case-branches scopes of their own by adding curly braces:

int personType = 1;
switch (personType)
{
    case 1:
    {
        Employee emp = new Employee();
        emp.ExperienceInfo();
        break;
    }
    case 2:
    {
        Employee emp = new Employee(); 
        emp.ManagementInfo();
        break;
    }
    case 3:
    {
        Student st = new Student();
        st.EducationInfo();
        break;
    }
    default:
        MessageBox.Show("Not valid ...");
}