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POBuilding a MicroRuleEngine using LinqExpressions
 

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  1. POBuilding a MicroRuleEngine using LinqExpressions
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    <p>So I am building a MicroRuleEngine (Would love to see this take off as an OpenSource project) and I am running into a null reference Error When executing the compiled ExpressionTree and I am not exactly sure why. Rules against the simple properties work but going against Child Properties aka Customer.Client.Address.StreetName etc. do not work.</p> <p>Below is the MicroRuleEngine</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Linq.Expressions; namespace Trial { public class MicroRuleEngine { public bool PassesRules&lt;T&gt;(List&lt;Rule&gt; rules, T toInspect) { bool pass = true; foreach (var rule in rules) { var cr = this.CompileRule&lt;T&gt;(rule); pass = pass &amp;&amp; cr.Invoke(toInspect); if (!pass) return pass; } return pass; } public Func&lt;T, bool&gt; CompileRule&lt;T&gt;(Rule r) { var paramUser = Expression.Parameter(typeof(T)); Expression expr = BuildExpr&lt;T&gt;(r, paramUser); // build a lambda function User-&gt;bool and compile it return Expression.Lambda&lt;Func&lt;T, bool&gt;&gt;(expr, paramUser).Compile(); } Expression BuildExpr&lt;T&gt;(Rule r, ParameterExpression param) { Expression propExpression; Type propType;// typeof(T).GetProperty(r.MemberName).PropertyType; ExpressionType tBinary; if (r.MemberName.Contains('.')) { // support to be sorted on child fields. String[] childProperties = r.MemberName.Split('.'); var property = typeof(T).GetProperty(childProperties[0]); var paramExp = Expression.Parameter(typeof(T), "SomeObject"); propExpression = Expression.MakeMemberAccess(paramExp, property); for (int i = 1; i &lt; childProperties.Length; i++) { property = property.PropertyType.GetProperty(childProperties[i]); propExpression = Expression.MakeMemberAccess(propExpression, property); } propType = propExpression.Type; propExpression = Expression.Block(new[] { paramExp }, new[]{ propExpression }); } else { propExpression = MemberExpression.Property(param, r.MemberName); propType = propExpression.Type; } // is the operator a known .NET operator? if (ExpressionType.TryParse(r.Operator, out tBinary)) { var right = Expression.Constant(Convert.ChangeType(r.TargetValue, propType)); // use a binary operation, e.g. 'Equal' -&gt; 'u.Age == 15' return Expression.MakeBinary(tBinary, propExpression, right); } else { var method = propType.GetMethod(r.Operator); var tParam = method.GetParameters()[0].ParameterType; var right = Expression.Constant(Convert.ChangeType(r.TargetValue, tParam)); // use a method call, e.g. 'Contains' -&gt; 'u.Tags.Contains(some_tag)' return Expression.Call(propExpression, method, right); } } } public class Rule { public string MemberName { get; set; } public string Operator { get; set; } public string TargetValue { get; set; } } } </code></pre> <p>And This is the Test that is Failing</p> <pre><code>[TestMethod] public void ChildPropertyRuleTest() { Container container = new Container() { Repository = "TestRepo", Shipment = new Shipment() { OrderNumber = "555" } }; MicroRuleEngine mr = new MicroRuleEngine(); var rules = new List&lt;Rule&gt;() { new Rule() { MemberName = "Shipment.OrderNumber", Operator = "Contains", TargetValue = "55" } }; var pases = mr.PassesRules&lt;Container&gt;(rules, container); Assert.IsTrue(!pases); } </code></pre>
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    1. USErik Funkenbusch
    1. USrunxc1 Bret Ferrier
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    1. COWhat is a micro rule engine?
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      1. USRitch Melton
    2. COA micro rule engine is just a small rule engine. I ended up Open Sourcing the library here https://github.com/runxc1/MicroRuleEngine
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      1. USrunxc1 Bret Ferrier
 

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