When working with custom (user-created) .bdh files, some or all built-in BDL functions are underlined and marked as Function or event handler not declared, even though the main .bdf file compiles successfully.
The BDL parser in the code editor cannot determine which .bdh files have been included before the current .bdh file in the main .bdf file. Make your custom .bdh files self-contained. This means: for all underlined API functions add a use statement for the corresponding .bdh file, where the function is defined.