Question

This errors:

double z;
int? x = 0;
int? y = 0;

z = (double)x -= (double)y;

..this does not error:

double z;
int? x = 0;
int? y = 0;

double x_con = (double)x;
double y_con = (double)y;

z = x_con -= y_con;

Simple question. Why?

Answer 1

If fails on this alone, without the z:

(double)x -= (double)y;

Simply because the left hand side of an assignment operator must be a variable, property or indexer, and not another expression, like your cast is.

Your second example however has the assignment written like this:

x_con -= y_con;

And that is perfectly fine of course since x_con is a variable.