Best syntax for a = (x == null) ? null : x.func()

Well, you could use an extension method like this:

public static TResult NullOr<TSource, TResult>(this TSource source,
    Func<TSource, TResult> func) where TSource : class where TResult : class
{
    return source == null ? null : func(source);
}

Then:

var a = some_long_expression.NullOr(x => x.Method());

Or (depending on your version of C#)

var a = some_long_expression.NullOr(Foo.Method);

where Foo is the type of some_long_expression.

I don’t think I would do this though. I’d just use the two line version. It’s simpler and less clever – and while “clever” is fun for Stack Overflow, it’s not usually a good idea for real code.