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POHow to project Linq results into an Abstract constructor?
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  1. POHow to project Linq results into an Abstract constructor?
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    copied!<p>Yes, this is my fourth day in a row to ask a question about abstracts, sorry, I'll try and go answer some questions about SQLServer to return the favor to the community. Anyway...</p> <p>How can I project the results of a Linq query into a abstract base class collection? Here is my my method from my RecruiterBase abstract class (there is also a corresponding CandidateBase abstract class):</p> <pre><code>public IQueryable&lt;CandidateBase&gt; GetCandidates() { return from candidates in db.Candidates where candidates.RecruiterId == this.RecruiterId select candidates; } </code></pre> <p>The above method will throw a compile time error that an implicit conversion cannot be made between Candidate and CandidateBase.</p> <p>modifying db.Candidates to db.Candidates.Cast() lets everything compile but I get a runtime error that no coercion operator is defined between types Candidate and CandidateBase. </p> <p>I can't do: select New CandidateBase { ... } as CandidateBase since the abstract can't be implemented.</p> <p>Nor can I create an explicit conversion operator between Candidate and Candidate base because it would again require me to new up my abstract</p> <p>Nor can I project my results into an anonymous object and then cast to CandidateBase as I get the same runtime coercion exception between the anonymous type and CandidateBase type.</p> <p>This problem came about from yesterday's question, <a href="https://stackoverflow.com/questions/1337327/problem-with-covariant-return-types-from-an-abstract-method">Problem with Covariant return types from an abstract method</a></p> <p>The answer provided by Stan R was that I was making things to complicated. I went back, simplified everything (I left the implmentation in the base and removed it from the subs) and ended up with a working GetCanidates method implemented as such:</p> <pre><code>public IQueryable&lt;CandidateBase&gt; GetCandidates() { return (from candidates in db.Candidates where candidates.RecruiterId == this.RecruiterId select new CandidateA { CandidateId = candidates.CandidateId, LastName = candidates.LastName, RecruiterId = candidates.RecruiterId }).Cast&lt;CandidateBase&gt;(); } </code></pre> <p>The above method compiles and works, and I'm not trying to look a gift horse in the mouth, but now I have a reference to my subtype in my base type (when I project the results into CandidateA) and that just seems odd. Feel free to vote my question down if the reference to the subtype from within the base type is okay.</p> <p>Thanks.</p> <p>Full class defs:</p> <pre><code>public abstract class RecruiterBase { public int RecruiterId { get; set; } public string FirstName { get; set; } public string LastName { get; set; } public RecruiterBase() { } public RecruiterBase(int id) { DataClasses1DataContext db = new DataClasses1DataContext(); Recruiter rc = db.Recruiters.SingleOrDefault(r =&gt; r.RecruiterId == id); this.RecruiterId = rc.RecruiterId; this.FirstName = rc.FirstName; this.LastName = rc.LastName; } public IQueryable&lt;CandidateBase&gt; GetCandidates() { DataClasses1DataContext db = new DataClasses1DataContext(); return (from candidates in db.Candidates where candidates.RecruiterId == this.RecruiterId select new CandidateA { CandidateId = candidates.CandidateId, LastName = candidates.LastName, FirstName = candidates.FirstName, RecruiterId = candidates.RecruiterId } ).Cast&lt;CandidateBase&gt;(); } } public abstract class TempCandidateBase { public int CandidateId { get; set; } public string FirstName { get; set; } public string LastName { get; set; } public int? RecruiterId { get; set; } public CandidateBase() { } public CandidateBase(int id) { DataClasses1DataContext db = new DataClasses1DataContext(); Candidate candidate = db.Candidates.SingleOrDefault(c =&gt; c.CandidateId == id); this.CandidateId = candidate.CandidateId; this.FirstName = candidate.FirstName; this.LastName = candidate.LastName; this.RecruiterId = candidate.RecruiterId; } } public class RecruiterA : RecruiterBase { public RecruiterA() : base() { } public RecruiterA(int id) : base(id) { } } public class CandidateA : CandidateBase { public CandidateA() : base() { } public CandidateA(int id) : base(id) { } } </code></pre>
 

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