Скачать или смотреть How to Allow Duplicate Items in a C# List with NHibernate

Struggling to add duplicate elements to a list in C# using NHibernate? Learn how to effectively use collections and avoid the overwrite issue with helpful solutions.
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How to Allow Duplicate Items in a C# List with NHibernate

When working with lists in C# , you might encounter situations where you want to add duplicate elements, especially when dealing with collections of objects linked to a database context, like in NHibernate. This guide addresses the common issue of adding duplicates in a list of objects and provides a solution that leverages NHibernate's configurations.

The Problem

You have a class Allegato, which inherits from a base class, and you want to add instances of Allegato to a list within another class called Corso. However, when trying to add multiple Allegato instances, only the first one successfully gets added because the Equals method in Allegato checks the Id field. Since the Id is not assigned at this point (it will be generated by the database), the subsequent additions are treated as duplicates and are not allowed.

Here’s a snippet of the relevant code:

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In the Corso class, you are trying to add multiple Allegato objects to the Allegati collection:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

As mentioned by a user in a forum, the issue arises from NHibernate treating your collection as a Set, which inherently does not allow duplicates. To resolve this, you can switch the collection type in your NHibernate mapping from Set to Bag. A Bag allows for duplicates, hence accommodating your requirement to add multiple Allegato instances.

Steps to Implement the Solution

Update NHibernate Mapping: Change the collection mapping from Set to Bag in your NHibernate configuration file or Fluent NHibernate mapping. This allows duplicates in the collection.

Check Your Model: Ensure your model reflects the change, and it’s clearly defined as a Bag. Here’s an example:

[[See Video to Reveal this Text or Code Snippet]]

Add Items to the Collection: Now that you are using a Bag, you can freely add multiple Allegato instances to the Allegati collection without Equals conflict hindering the addition.

Sample Implementation

Make sure your collection in the Corso class can accommodate multiple instances as shown below:

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Conclusion

Allowing duplicates in a list in C# when using NHibernate can be a roadblock if not addressed correctly. By changing the collection type from Set to Bag, you can add multiple instances of the same object to your collection without encountering duplicate restriction errors. This change is simple yet effective for overcoming challenges associated with object equality checks based on IDs.

Feel free to reach out with any further questions or for assistance with specific implementations!