Why would var be a bad thing?

Question

I\'ve been chatting with my colleagues the other day and heard that their coding standard explicitly forbids them to use the var keyword in C#. They had no idea

Answer 1

In most cases when uses sensibly (i.e. a simple type initializer where the type and value are the same), then it is fine.

There are some times when it is unclear that you've broken things by changing it - mainly, when the initialized type and the (original) variable type are not the same, because:

• the variable was originally the base-class
• the variable was originally an interface
• the variable was originally another type with an implicit conversion operator

In these cases, you can get into trouble with any type resolution - for example:

• methods that have different overloads for the two competing types
• extension methods that are defined differently for the two competing types
• members that have been re-declared (hidden) on one of the types
• generic type inference will work differently
• operator resolution will work differently

In such cases, you change the meaning of the code, and execute something different. This is then a bad thing.

Examples:

Implicit conversion:

static void Main() {
    long x = 17;
    Foo(x);
    var y = 17;
    Foo(y); // boom
}
static void Foo(long value)
{ Console.WriteLine(value); }
static void Foo(int value) {
throw new NotImplementedException(); }

Method hiding:

static void Main() {
    Foo x = new Bar();
    x.Go();
    var y = new Bar();
    y.Go(); // boom
}
class Foo {
    public void Go() { Console.WriteLine("Hi"); }
}
class Bar : Foo {
    public new void Go() { throw new NotImplementedException(); }
}

etc