Question

Ive seen this code :

StringBuilder Foo<T> (T arg)
{
 if (arg is StringBuilder)
 return (StringBuilder) arg; // Will not compile
 ...
}

however :

StringBuilder Foo<T> (T arg)
{
 StringBuilder sb = arg as StringBuilder;
 if (sb != null) return sb;
 ...
}

will compile.

why is that ? what the compiler is afraid of ?

p.s. ive seen another solution which :

(StringBuilder) (object) arg

Answer 1

I think, cause there is no guranteed conversion between type T and StringBuilder.
Specifying it like arg as StringBuilder, if coversion fails, it will return a null and not exception, like in previouse case. Null is a valid case, in this function, like a returning type.