Java certainly does not guarantee "write once run everywhere", although Sun had a "Pure Java" branding effort at one time. What Microsoft did was extend the la

Java certainly does not guarantee "write once run everywhere", although Sun had a "Pure Java" branding effort at one time. What Microsoft did was extend the language to include an annotations feature (making it source-incompatible), and extending the JVM spec to include new opcodes (which crashed everyone else's JVM). Had they extended Java the conventional way there would not have been a problem.