Cesium is a programming language targeting CLI, so it provides some extensions to call CLI code.
Here's EBNF of syntax extensions:
declaration_specifiers: cli_import_specifier declaration_specifiers
cli_import_specifier: '__cli_import' '(' StringLiteral ')'
type_specifier: __nint
type_specifier: __nuint
Any function declaration may be preceded with __cli_import("Fully.Qualified.Type::Method") which will mean that this function is to be associated with the corresponding CLI method from a referenced assembly.
__nint is a synonym for System.IntPtr in .NET or nint in C#.
__nint is a synonym for System.UIntPtr in .NET or nint in C#.
Cesium provides a built-in macro, __CESIUM__, defined to 1.