All programs can be decompiled to a degree. However, the vast bulk of the useful information in your source code is removed during compilation. The source code that a decompiler produces is a pale imitation of the original.
The variable names, function names, class names etc. will not be available after decompilation. So the best that a decompiler can do is to turn your functions that look like this:
double CalculateWidgetStrength(int WidgetType, int WidgetFrobishness);
into rather meaningless code like this:
double Function85(int p1, int p2);
And even succeeding in doing that much accurately can be very hard for a decompiler.
