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PO'The type arguments cannot be inferred from the usage'
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  1. PO'The type arguments cannot be inferred from the usage'
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    copied!<p>I am having some troubles with generics and inheritance. I have an abstract class called <code>CriteriaBase</code> whose job it is to determine if an entity <code>T</code> matches the criteria defined in any sub-classes. The sub-classes have to implement a method which returns a <code>Func</code> representing the criteria. The problem arises when I try to use generics for the <code>Func</code>. Hopefully some code will illustrate my problem.</p> <pre><code>public abstract class CriteriaBase&lt;T, U&gt; where T : ICrossoverable where U : IChromosome { protected abstract Func&lt;U, bool&gt; Criteria { get; } //some code removed for brevity private int GetNumberOfCriteriaMatches(T season) { //1. works //Func&lt;IChromosome, bool&gt; predicate = c =&gt; c.Genes == null; //return season.Chromosomes.Count(predicate); //2. doesn't work - The type arguments for method 'int //System.Linq.Enumerable.Count&lt;TSource&gt;(this IEnumerable&lt;TSource&gt;, //Func&lt;TSource, bool&gt;)' //cannot be inferred from the usage. Try specifying the type arguments //explicitly. return season.Chromosomes.Count(Criteria); } } </code></pre> <p>My intention is that the <code>CriteriaBase</code> class should be generic and completely reusable.</p> <p>An example sub-class:</p> <pre><code>public class TopTeamsPlayingEachOtherCriteria : CriteriaBase&lt;Season, MatchDay&gt; { //some code removed for brevity protected override Func&lt;MatchDay, bool&gt; Criteria { get { return matchDay =&gt; matchDay.Fixtures.Count( fixture =&gt; fixture.HomeTeam.TableGrouping.Ordering == 1 &amp;&amp; fixture.AwayTeam.TableGrouping.Ordering == 1) &gt; 1; } } } </code></pre> <p>The problem is in the <code>GetNumberOfCriteriaMatches()</code> method. Option 2 is how I originally wrote the code but I get the compile error as listed. If I use option 1 then the code compiles but it means that when I override <code>Criteria</code> in the sub-class, I have to use <code>IChromosome</code> instead of <code>MatchDay</code> which doesn't work (I need to access specific features of a <code>MatchDay</code>). In my simple mind, options 1 and 2 are equivalent. Option 2 simply replaces <code>IChromosome</code> with a generic type <code>U</code> which is restricted to a class that implements <code>IChromosome</code>.</p> <p>Is what I'm trying to achieve possible? If so, what am I missing/misunderstanding? If not, how should I approach this problem?</p> <p>For completeness (included at the end as I'm not sure how much it helps with the question), here are the two entities that I'm currently using for <code>T</code> (<code>Season</code>) and <code>U</code> (<code>MatchDay</code>).</p> <pre><code>public class Season : ICrossoverable { private readonly IEnumerable&lt;MatchDay&gt; _matchDays; public Season(IEnumerable&lt;MatchDay&gt; matchDays) { _matchDays = matchDays; } public IEnumerable&lt;MatchDay&gt; MatchDays { get { return _matchDays; } } //ICrossoverable implementation public IEnumerable&lt;IChromosome&gt; Chromosomes { get { return _matchDays; } } } public class MatchDay : IChromosome { private readonly int _week; private readonly List&lt;Fixture&gt; _fixtures; public MatchDay(int week, List&lt;Fixture&gt; fixtures) { _week = week; _fixtures = fixtures; } //some code removed for brevity public IEnumerable&lt;Fixture&gt; Fixtures { get { return _fixtures; } } //IChromosome implementation public IEnumerable&lt;IGene&gt; Genes { get { return Fixtures; } } } </code></pre>
 

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