如何使用LinQ to Entities查询选择两个对象之一（取决于一个对象是否为空）。

Question

List<IStoreItem> StoreItems = new List<IStoreItem>();             

StoreItems = repository.ProductToCatMaps.Where(x => CatList.Contains(x.ExCatID))
             .Select(x => x.ProductShell ?? x.PCBuild);

The entity classes:

    public class ProductShell : IStoreItem
    {
    }

    public class PCBuild : IStoreItem
    {
    }

The ProductToCatMaps table contains the external category ID for each respective store item, I have two store items types: ProductShell and PCBuild. I wish to select either one depending on which is set to null, only one can contain a value (Foregn key to the respective types entry in the database ) which I need to select.

Answer 1

The translation of your intent to SQL is going to be nigh impossible (or perhaps just improbable*), because the SQL to get the ProductShell fully populated is going to be wholly different than the sql to get PCBuild. I would switch directions and do the last bit of your query in memory. Something like

StoreItems = repository.ProductToCatMaps
                       .Where(yourCondition)
                       .Select(x => new { x.ProductShell, x.PCBuild })
                       .AsEnumerable() // *** pulls above into memory
                       .Select(x => (IStoreItem)x.ProductShell ?? (IStoreItem)x.PCBuild);

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*Of course, I haven't tried your approach at home, so if it does in fact work (with some modification, I assume that by asking, what you have so far doesn't function), then kudos to the EF team.