This is fine as the expression is evaluated left to right in C# and the if
will be exited when the first condition is found that evaluates to false.
So if your ptr is null, the ptr.Length code will never execute.

You can do it this way if you like, but it isn't necessary.

Hi Jamie,

They do the same thing, but I prefer the first one because it fits nicely on
one line.  BTW, it's more common in C# to write it as follows:

if (ptr != null && ptr.Length > 0)
{
   ...
}

The compiler prevents accidental assignment to ptr because it requires the
expression to evaluate to a boolean, not byte[].

Yes, C# will short-circuit that if the first condition is false.  That is
part of the C# spec and every compiler should honor that.  I believe in the
C/C++ world, that sort of thing is up to the compiler vendor.  In C# you can
count on it being there (unless you have a nonstandard compiler vendor).

A side note: you should write ptr != null instead of the other way around.  
It reads better to someone looking at your code.  All the reasons you did it
the other way around in C are no longer valid in C#.