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    <p>So from the point of view of expression the value is a constant. It can not be changed by the expression. </p> <p>What you have is a potentially open closure - i.e. the value can change between executions of the expression, but not during it. So it is a "constant". This is a paradigm difference between the world of functional programming and un-functional :) programming. </p> <p>Consider</p> <pre><code> int a =2; Expression&lt;Func&lt;int, int&gt;&gt; h = x=&gt; x+ a; Expression&lt;Func&lt;int, int&gt;&gt; j = x =&gt; x +2; a = 1; </code></pre> <p>the term a is a member access into an anonymous class that wraps up and access the a variable on the stack. The first node is a MemberAccess node then underneath that - the expression is a constant. </p> <p>For the code above:</p> <pre><code>((SimpleBinaryExpression)(h.Body)).Right {value(WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0).a} CanReduce: false DebugView: ".Constant&lt;WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0&gt;(WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0).a" Expression: {value(WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0)} Member: {Int32 a} NodeType: MemberAccess Type: {Name = "Int32" FullName = "System.Int32"} </code></pre> <p>And the constant underneath that:</p> <pre><code>((MemberExpression)((SimpleBinaryExpression)(h.Body)).Right).Expression {value(WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0)} CanReduce: false DebugView: ".Constant&lt;WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0&gt;(WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0)" NodeType: Constant Type: {Name = "&lt;&gt;c__DisplayClass0" FullName = "WindowsFormsApplication6.Form1+&lt;&gt;c__DisplayClass0"} Value: {WindowsFormsApplication6.Form1.} } } </code></pre> <p>The plain old 2 comes out to a:</p> <pre><code>((SimpleBinaryExpression)(j.Body)).Right {2} CanReduce: false DebugView: "2" NodeType: Constant Type: {Name = "Int32" FullName = "System.Int32"} Value: 2 </code></pre> <p>So I don't know if that helps you or not. You can kind of tell by looking at the parent node - or the type of object being accessed by the parent node.</p> <hr> <p>Adding as a result of your clarification - </p> <p>so when you say </p> <pre><code>user =&gt; user.Email == email </code></pre> <p>You mean look for all users with an email equal to a passed in parameter - however that link expression means something quite different. </p> <p>what you want to say is </p> <pre><code>Expression&lt;Func&lt;User, string, bool&gt;&gt; (user, email) =&gt; user.Email == email </code></pre> <p>This way the email will now be a parameter. If you don't like that there is one other thing you can do. </p> <p>The second example will work just fine - no extra params are needed consts will be consts. </p> <pre><code>t =&gt; t.Status != TaskStatus.Done &amp;&amp; t.Status != TaskStatus.Failed </code></pre> <hr> <p>Edit: adding another way:</p> <p>So one of the things that you had to do to get your code working was declare a string email outside the lambda - that is kind of clunky. </p> <p>You could better identify parameters by conventionally putting them in a specific place - like a static class. Then when going through the Lambda you don't have to look at some horrible cloture object - but a nice static class of your making. </p> <pre><code>public static class Parameter { public static T Input&lt;T&gt;(string name) { return default(T); } } </code></pre> <p>Then your code looks like this:</p> <pre><code>Expression&lt;Func&lt;User, bool&gt;&gt; exp = x =&gt; x.Email == Parameter.Input&lt;String&gt;("email"); </code></pre> <p>You can then traverse the tree - when you come to a call to to the Parameter static class you can look at the type and the name (in the arguments collection) and off you go.... </p>
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    1. POExpression Tree parsing, variables ends up as constants
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    1. COI'll take a look at the parent node as you suggest and see what that gets me! Thanks. I'll post up my findings here!
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    2. COWait a second - what might be going on is this ... do you have a local user defined, and it is trying to pull that instance in as a cloture. What you might want to do is Expression<Func<User, bool>> x=>x.email == email; I think you might have gotten confused between a local instance and a desired input?
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    3. COAdded some more to the question to clarify the problem. I'll still take a look at the parent node and see where that gets me.
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