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PODynamic recursive lambda expressions
 

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    <p>I want to create dynamic lambda expressions so that I can filter a list using a set of filtering parameters. This is what I have so far:</p> <p>The expression is built using the methods bellow, where T is the object type of the list</p> <pre><code> public static Expression&lt;Func&lt;T, bool&gt;&gt; GetExpression&lt;T&gt;(IList&lt;DynamicFilter&gt; filters) { if (filters.Count == 0) return null; ParameterExpression param = Expression.Parameter(typeof(T), "t"); Expression exp = null; if (filters.Count == 1) exp = GetExpression&lt;T&gt;(param, filters[0]); [...] return Expression.Lambda&lt;Func&lt;T, bool&gt;&gt;(exp, param); } private static Expression GetExpression&lt;T&gt;(ParameterExpression param, DynamicFilter filter) { MemberExpression member = Expression.Property(param, filter.PropertyName); ConstantExpression constant = Expression.Constant(filter.Value); [...] return Expression.Call(member, filterMethod, constant); } </code></pre> <p>I then call</p> <pre><code>List&lt;Example&gt; list = ...; var deleg = ExpressionBuilder.GetExpression&lt;Example&gt;(dynFiltersList).Compile(); list = list.Where(deleg).ToList(); </code></pre> <p>This works just as expected with an object that contains only simple types, but if there are complex types inside, the code doesn't work anymore. For example, let's say I have a member of custom type Field inside the Example class and Field has a string property Value. If <code>filter.PropertyName</code> would be 'Field0' (of type Field), the code would work just fine, but if I have 'Field0.Value' I would get an obvious error stating that there is no property named 'Field0.Value' inside class Example.</p> <p>I tried modifying the expression building method, like this:</p> <pre><code> MemberExpression member = null; if (filter.PropertyName.Contains('.')) { string[] props = filter.PropertyName.Split('.'); ParameterExpression param1 = Expression.Parameter(typeof(T).GetProperty(props[0]).PropertyType, "t1"); member = Expression.Property(param1, props[0]); } else { member = Expression.Property(param, filter.PropertyName); } </code></pre> <p>but then I got a <code>Lambda parameter not in scope</code> error when compiling the expression. I sort of understand why I get this error, but I don't know how to make this work.</p> <p>Bottom line is I need to make the expression building method work recursively when forming the MemberExpression. I ultimately need to obtain a <code>list = list.Where(deleg).ToList();</code> that translates to something like this <code>list = list.Where(obj =&gt; obj.Field0.Value == 'something').ToList();</code></p> <p>I've just started working with expressions so I don't really know too much in this area, but any help would be appreciated.</p> <p>Thanks</p>
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    1. USAndrei Rajala
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