On Scope of Variables in .NET(c#)

Reference Code

public void MyMethod()
{
    string x;
    List sampleList = populateList();

    foreach(MyType myType in sampleList)
    {
        string x;   // why is this not allowed?
        doSomethingwithX(x);
    }
}

I recently started learning C# and today ran into issue with code similar to the above. VS2010 flagged the commented code with this message a local variable named x cannot be declared in this scope because it would give a different meaning to variable x which is already used in parent or current scope to denote something else....

I dont get it...isnt that the whole essence of block statements and scoping...I know i can just change my variable names and go ahead.but i'd like to know WHY?

Jon Skeet
people
quotationmark

I dont get it...isnt that the whole essence of block statements and scoping...

No, not really. The intention of scoping isn't "to allow you to reuse names".

I know i can just change my variable names and go ahead.but i'd like to know WHY?

It reduces the possibilities for confusing code, basically. There are various situations where C# prevents you from writing code which is confusing. Not as many as we might like, of course, but where there's no clear benefit from allowing something, and a clear benefit from disallowing it, it makes sense to disallow it.

Would you ever want to work with code that had the same local variable name in scope twice? Wouldn't you always prefer the original developer to use a different name? I would - so I'm glad the compiler enforces that.

Note that this doesn't prevent the same local variable name being used twice in the same method - they just can't be in the same scope. So this is valid:

public void Foo()
{
    {
        int x = 10;
        ...
    }

    {
        int x = 10;
        ...
    }
}

But this isn't:

public void Foo()
{
    {
        int x = 10;
        ...
    }

    int x = 10;
    ...
}

If the second example is confusing, you need to bear in mind that the scope of a local variable is the block in which it was declared - not from the declaration point onwards.

people

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