C# is a case-sensitive language. That is, the use of uppercase vs. lowercase in C# is deliberate and mandatory. For example:
Variable names that are spelled the same way but which differ in their use of case, e.g., x (lowercase) vs. X (uppercase), represent different variables.
All keywords are expressed in all lowercase: public, class, int, bool, and so forth. Don't get "creative" about capitalizing these, as the compiler will violently object!
Capitalization of the name of the Main method is mandatory.