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POCreate a reusable "meta-predicate" function accepting lambda operand parameters
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  1. POCreate a reusable "meta-predicate" function accepting lambda operand parameters
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    copied!<p>Is it possible to create a "meta-predicate" by writing a a function that takes 2 (or 4 if necessary) lambdas which represent the left and right side properties (operands) and have it generate an predicate. Something like the following code sample:</p> <pre><code>public Expression&lt;Func&lt;Something,bool&gt;&gt; StringEquals&lt;Something&gt;(Expression&lt;Something&gt; leftOperand, Expression&lt;Something&gt; leftOperandSelector){ return (rightOperandSelector, leftOperandSelector) =&gt; ( (rightOperandSelector == leftOperandSelector) || ( string.IsNullOrEmpty(rightOperandSelector) &amp;&amp; string.IsNullOrEmpty(leftOperandSelector) ) ); } </code></pre> <p>OR:</p> <pre><code>public Expression&lt;Func&lt;Something,bool&gt;&gt; DatesActive&lt;Something&gt;(Expression&lt;Something&gt; startDateOperandSelector, Expression&lt;Something&gt; endDateOperandSelector){ return (startDateOperandSelector, endDateOperandSelector) =&gt; ( (startDatePropertySelector &gt;= DateTime.Now) &amp;&amp; (endDatePropertySelector &lt;= DateTime.Now) ); } </code></pre> <p>Where <em>SomeStringProperty</em> for each side or <em>startDatePropertySelector</em> or <em>endDatePropertySelector</em> is defined by a lambda? I have not figured out how to dynamically pass the Operand of the predicate expression.</p> <p>Ideally I would like to be able to inline it like this:</p> <pre><code>return new Expression&lt;Func&lt;Request,bool&gt;&gt;[]{ r =&gt; (r.Id != request.Id) &amp;&amp; (!r.Reviewed), StringEquals(r =&gt; r.VendorName, request=&gt;request.VendorName), NotExpired(r =&gt; r.ContractStart, request=&gt;request.ContractEnd), ... }; </code></pre> <p>*<em>Does someone have an idea on the best way to approach this? My interest is in creating "meta"-expressions for easy use where I am using the same expression repeatedly over multiple properties. Concrete example is given just for reference/explanation. Very open to knowing if this is silly and if there is a better approach, happy to learn. *</em></p> <p>More background below if desired.</p> <p><strong>Background:</strong> After a simpler form of this Expression was in place I was forced to refactor to handle the fact that EntityFramework doesn't treat equality how you would expect in that instead of interpreting C# like the following: <code>(rightSide.SomeStringProperty == leftSide.SomeStringProperty)</code> as a two part SQL expression like this</p> <pre><code>( (rightSide.SomeStringProperty IS NULL AND leftSide.SomeStringProperty IS NULL) OR (rightSide.SomeStringProperty = leftSide.SomeStringProperty) ) </code></pre> <p>it more literally tranlates it as:</p> <pre><code>(rightSide.SomeStringProperty = leftSide.SomeStringProperty) </code></pre> <p>which of course will not return values for where both sides are null. Apparently this has been corrected in EF6 (<strong>Correction</strong>: @Slauma points out this is available in EF5 via the UseCSharpNullComparisonBehavior. I am using EF4 and cannot upgrade for this release.)</p> <p>I want to avoid more repetitive code like the following:</p> <pre><code> var where = new Expression&lt;Func&lt;Request,bool&gt;&gt;[]{ r =&gt; (r.Id != request.Id) &amp;&amp; (!r.Reviewed) &amp;&amp; ( (r.Address == request.Address) || (string.IsNullOrEmpty(r.Address) &amp;&amp; string.IsNullOrEmpty(request.Address)) ) &amp;&amp; ( (r.City == request.City) || (string.IsNullOrEmpty(r.City) &amp;&amp; string.IsNullOrEmpty(request.City)) ) &amp;&amp; ( (r.Province == request.Province) || (string.IsNullOrEmpty(r.Province) &amp;&amp; string.IsNullOrEmpty(request.Province)) ) &amp;&amp; ( (r.PostalCode == request.PostalCode) || (string.IsNullOrEmpty(r.PostalCode) &amp;&amp; string.IsNullOrEmpty(request.PostalCode)) ) &amp;&amp; ( (r.Website == request.Website) || (string.IsNullOrEmpty(r.Website) &amp;&amp; string.IsNullOrEmpty(request.Website)) ) }; </code></pre>
 

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