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Collision of providers

Imagine you have Module3 where you imported Module2 and Module1. You did this import because you need Service2 and Service1 from these modules, respectively. You are viewing how these services work, but for some reason Service1 does not work as expected. You start debugging and it turns out that Service1 is exported from both modules: Module2 and Module1. You expected that Service1 would only be exported from Module1, but the version exported from Module2 actually worked.

To prevent this from happening, if you import two or more modules that export non-identical providers with the same token, Ditsmod will throw the following error:

Error: Importing providers to Module3 failed: exports from Module2 and Module1 causes collision with Service1. If Module3 declared in your application (it is not imported from node_modules), you should add Service1 to resolvedCollisionsPer* in this module. For example: resolvedCollisionsPerReq: [ [Service1, Module1] ].

Specifically in this case:

  1. Module1 substitute and then exports the provider with the token Service1;
  2. and Module2 substitute and then exports the provider with the token Service1;
  3. providers with token Service1 are not identical in Module1 and Module2, i.e. from Module2 can be exported, for example, object { token: Service1, useValue: {} }, and from Module1 Service1 can be exported as a class.

And since both of these modules are imported into Module3, this causes a "provider collisions", because the developer may not know which of these substitutions will work in Module3.

Collision resolution​

If Module3 is declared in your application (it is not imported from node_modules), the collision is resolved by adding to resolvedCollisionsPer* an array of two elements, with the provider's token in the first place and the module from which the provider needs to be taken in the second place:

import { Module1, Service1 } from './module1.js';
import { Module2 } from './module2.js';

@featureModule({
  imports: [Module2, Module1],
  resolvedCollisionsPerReq: [ [Service1, Module1] ]
})
export class Module3 {}

If you have installed Module3 using packages manager (npm, yarn, etc.), there is no point in modifying this module locally to resolve the collision. This situation can only occur if Module2 and Module1 are exported from the root module, so you need to remove one of these modules from there. And, of course, after that you will have to explicitly import the deleted module into those modules where it is needed.