Default Access Modifier

                     | Default   | Permitted declared accessibilities
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namespace            | public    | none (always implicitly public)

enum                 | public    | none (always implicitly public)

interface            | public    | none

class                | Internal   | All¹

struct               | private   | public, internal, private²

delegate             | private   | All¹

constructor          | private   | All¹

interface member     | public    | none (always implicitly public)

method               | private   | All¹

field                | private   | All¹

user-defined operator| none      | public (must be declared public)

Class Default   - Internal  use below link

https://www.c-sharpcorner.com/UploadFile/84c85b/default-scope-of-a-C-Sharp-class/