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POHow to create a System.Linq.Expressions.Expression for Like?
 

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  1. POHow to create a System.Linq.Expressions.Expression for Like?
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    <p>I created a filterable BindingList <a href="http://www.nablasoft.com/alkampfer/index.php/2008/11/22/extend-bindinglist-with-filter-functionality/" rel="nofollow noreferrer">from this source</a>. It works great:</p> <pre><code>list.Filter("Customer == 'Name'"); </code></pre> <p>does what it should. The internals work like a parser, that converts the expression <code>==</code> or <code>!=</code> into <code>System.Linq.Expressions.Expression</code>. In this case, <code>==</code> becomes <code>System.Linq.Expressions.Expression.Equal</code>.</p> <p>Unfortunately <code>System.Linq.Expressions.Expression</code> does not contain a like operator and I don't know how to solve this.</p> <p>The initial code looks like this:</p> <pre><code>private static Dictionary&lt;String, Func&lt;Expression, Expression, Expression&gt;&gt; binaryOpFactory = new Dictionary&lt;String, Func&lt;Expression, Expression, Expression&gt;&gt;(); static Init() { binaryOpFactory.Add("==", Expression.Equal); binaryOpFactory.Add("&gt;", Expression.GreaterThan); binaryOpFactory.Add("&lt;", Expression.LessThan); binaryOpFactory.Add("&gt;=", Expression.GreaterThanOrEqual); binaryOpFactory.Add("&lt;=", Expression.LessThanOrEqual); binaryOpFactory.Add("!=", Expression.NotEqual); binaryOpFactory.Add("&amp;&amp;", Expression.And); binaryOpFactory.Add("||", Expression.Or); } </code></pre> <p>Then I created an expression that will do what I want:</p> <pre><code>private static System.Linq.Expressions.Expression&lt;Func&lt;String, String, bool&gt;&gt; Like_Lambda = (item, search) =&gt; item.ToLower().Contains(search.ToLower()); private static Func&lt;String, String, bool&gt; Like = Like_Lambda.Compile(); </code></pre> <p>e.g.</p> <pre><code>Console.WriteLine(like("McDonalds", "donAld")); // true Console.WriteLine(like("McDonalds", "King")); // false </code></pre> <p>But <code>binaryOpFactory</code> requires this:</p> <pre><code>Func&lt;Expression, Expression, Expression&gt; </code></pre> <p>The predefined expressions seem to be exactly that:</p> <pre><code>System.Linq.Expressions.Expression.Or; </code></pre> <p>Can anyone tell me how to convert my expression?</p>
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    1. USJürgen Steinblock
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      1. USJhonny D. Cano -Leftware-
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    1. COAnd how does your LIKE operate? I can help you build an Expression, but I need to understand how you want it to work first... regex? contains? etc?
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      1. USMarc Gravell
    2. COThat does not matter. The final implementation will propably be with regexp. Basically I have a Func<String, String, bool> to that I pass 2 Strings and get true or false as returnvalue. My Problem is that I do not understand the Implementation of Objects in System.Linq.Expressions.Expression Namespace, which seem to be Func<Expression, Expression, Expression> (look at the generic type argements of binaryOpFactory) so I cannot create my own comparison.
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      1. USJürgen Steinblock
    3. CORe comment: understanding the Expression API can take some doing... I try to cover a few basics on my blog; Jon's book (C# in Depth) also gives a high level overview.
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      1. USMarc Gravell
 

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