Cannot convert expression type SystemCollectionsGenericICollectionint to return type MicrosoftAspNetCoreMvcActionResultSystemCollectionsGenericICollectionint

This error occurs when there is a mismatch between the return type of a method and the type of the expression being returned. In this case, it seems that the method is expected to return an ActionResult of type ICollection, but the expression being returned is just an ICollection.

To fix this error, you can wrap the ICollection in an ActionResult by using the Ok method of the Controller class. For example:

public ActionResult<ICollection<int>> MyMethod()
{
    ICollection<int> myCollection = GetMyCollection();

    return Ok(myCollection);
}

This should convert the ICollection into an ActionResult<ICollection> and resolve the error