What is Type-safe?

Question

What does "type-safe" mean?

Answer 1

Type safety means that the compiler will validate types while compiling, and throw an error if you try to assign the wrong type to a variable.

Some simple examples:

// Fails, Trying to put an integer in a string
String one = 1;
// Also fails.
int foo = "bar";

This also applies to method arguments, since you are passing explicit types to them:

int AddTwoNumbers(int a, int b)
{
    return a + b;
}

If I tried to call that using:

int Sum = AddTwoNumbers(5, "5");

The compiler would throw an error, because I am passing a string ("5"), and it is expecting an integer.

In a loosely typed language, such as javascript, I can do the following:

function AddTwoNumbers(a, b)
{
    return a + b;
}

if I call it like this:

Sum = AddTwoNumbers(5, "5");

Javascript automaticly converts the 5 to a string, and returns "55". This is due to javascript using the + sign for string concatenation. To make it type-aware, you would need to do something like:

function AddTwoNumbers(a, b)
{
    return Number(a) + Number(b);
}

Or, possibly:

function AddOnlyTwoNumbers(a, b)
{
    if (isNaN(a) || isNaN(b))
        return false;
    return Number(a) + Number(b);
}

if I call it like this:

Sum = AddTwoNumbers(5, " dogs");

Javascript automatically converts the 5 to a string, and appends them, to return "5 dogs".

Not all dynamic languages are as forgiving as javascript (In fact a dynamic language does not implicity imply a loose typed language (see Python)), some of them will actually give you a runtime error on invalid type casting.

While its convenient, it opens you up to a lot of errors that can be easily missed, and only identified by testing the running program. Personally, I prefer to have my compiler tell me if I made that mistake.

Now, back to C#...

C# supports a language feature called covariance, this basically means that you can substitute a base type for a child type and not cause an error, for example:

 public class Foo : Bar
 {
 }

Here, I created a new class (Foo) that subclasses Bar. I can now create a method:

 void DoSomething(Bar myBar)

And call it using either a Foo, or a Bar as an argument, both will work without causing an error. This works because C# knows that any child class of Bar will implement the interface of Bar.

However, you cannot do the inverse:

void DoSomething(Foo myFoo)

In this situation, I cannot pass Bar to this method, because the compiler does not know that Bar implements Foo's interface. This is because a child class can (and usually will) be much different than the parent class.

Of course, now I've gone way off the deep end and beyond the scope of the original question, but its all good stuff to know :)