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POCan I accept both a delegate type T and an Expression<T> in the same parameter?
 

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  1. POCan I accept both a delegate type T and an Expression<T> in the same parameter?
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    <p>I'm trying to write a helper class that represents fields on an object. The helper class needs to be able to both get the value of the field on a given instance AND return metadata about the underlying property that it can obtain by reflection.</p> <p>I'd like the helper class be created by a utility method that gets called something like the following:</p> <pre><code>public IEnumerable&lt;IFieldInfo&lt;SomeType&gt;&gt; Fields { get { yield return FieldHelper.GetFieldInfo&lt;SomeType&gt;(t =&gt; t.SomeField); yield return FieldHelper.GetFieldInfo&lt;SomeType&gt;(t =&gt; t.SomeOtherField); } } </code></pre> <p>Say that the <code>IFieldInfo</code> interface looks a bit like this:</p> <pre><code>interface IFieldInfo&lt;in T&gt; { public string PropertyName {get;} public object GetValue(T obj); } </code></pre> <p>In the context of my application, the <code>Fields</code> property will be accessed fairly often; the <code>GetValue()</code> method will be called fairly often, but the <code>PropertyName</code> (and the other metadata/reflectiony fields that I omitted for clarity) will be accessed less frequently. There are a lot of places where the Fields property needs to be implemented, and sometimes a lot of fields, so it's important for code clarity and maintenance that the code that <em>calls</em> <code>GetFieldInfo</code> be simple and clear; duplicating the list of fields or duplicating the parameter would be just <em>asking</em> for bugs where they get out of sync.</p> <p>So, how do I write <code>GetFieldInfo</code>? If I do:</p> <pre><code>public static IFieldInfo&lt;T&gt; GetFieldInfo&lt;T&gt;(Func&lt;T, object&gt; getter); </code></pre> <p>then I can implement <code>GetValue()</code> trivially by calling <code>getter(obj)</code> but I have no way to access the name.</p> <p>If, on the other hand, I do:</p> <pre><code>public static IFieldInfo&lt;T&gt; GetFieldInfo&lt;T&gt;(Expression&lt;Func&lt;T, object&gt;&gt; getter) </code></pre> <p>then I can extract the name of the property access that's represented by the expression, but the only way to implement <code>GetValue()</code> is to call <code>Compile()</code> on the expression, which I imagine to be a ridiculous amount of overhead for something that the compiler itself could compile for me, especially since <code>GetValue()</code> is used more often on these objects than <code>PropertyName</code> is.</p> <p>Is there any way to write my <code>GetFieldInfo</code> helper function to take a single parameter and interpret it as both an expression tree AND a delegate without the overhead of compiling the expression tree into a delegate at runtime?</p>
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    1. USStuart
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      1. POCan I accept both a delegate type T and an Expression<T> in the same parameter?
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      1. POCan I accept both a delegate type T and an Expression<T> in the same parameter?
      1. USDave Swersky
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    1. COCompile() is the solution. You can cache the results and the # of total invocations is statically bound. I doubt performance would be an issue if implemented that way.
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      1. POCan I accept both a delegate type T and an Expression<T> in the same parameter?
      1. USKirk Woll
    2. COThe number of total invocations isn't exactly statically bound; Fields can be called multiple times. Unless you're saying Expression itself would be a valid Dictionary key?
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      1. POCan I accept both a delegate type T and an Expression<T> in the same parameter?
      1. USStuart
 

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