Angular4 Exception: Can't bind to 'ngClass' since it isn't a known property of 'input'

Since RegisterComponent was declared in RegisterRouter(what's the name for module?) module then you have to import CommonModule there:

@NgModule({
  imports: [
    RouterModule.forChild(routes),
    FormsModule,
    ReactiveFormsModule,
    CommonModule      <================== this line
  ],
  declarations: [
    RegisterComponent // this component wants to have access to built-in directives
  ],
  exports: [RouterModule]
})
export class RegisterRouter { }

For anyone who's still having these issue even after importing CommonModule

I had a library that I just could not get to compile. I had several modules.. which were importing other smaller/components:

One of the components in one of my smaller modules had [ngClass] in it.. it would cause lib to not compile and throw Can't bind to 'ngClass' since it isn't a known property of 'div'

Apparently... i was exporting all my components for that modules... but not the module itself in the public-api.ts file (the module that did the import of CommonModule):

export * from './lib/af-layout/af-layout.module'; // <==== THAT WAS MISSING
export * from './lib/af-layout/components/af-layout-header/af-layout-header.component';

It took me a long time to find that.. as the error was directing me towards CommonModule not being imported.. not that an export was missing.

Lazy Loading Share module issue common solution:

Both module must import CommonModule

In shared module:

@NgModule({
  imports: [CommonModule],
  ...
})

Module where you want to use component:

@NgModule({
  imports: [CommonModule, ...],
  ...
})