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POC# property and ref parameter, why no sugar?
 

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  1. POC# property and ref parameter, why no sugar?
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    <p>I just ran across this error message while working in C#</p> <blockquote> <p>A property or indexer may not be passed as an out or ref parameter</p> </blockquote> <p>I known what caused this and did the quick solution of creating a local variable of the correct type, calling the function with it as the <code>out</code>/<code>ref</code> parameter and then assigning it back to the property:</p> <pre><code>RefFn(ref obj.prop); </code></pre> <p>turns into</p> <pre><code>{ var t = obj.prop; RefFn(ref t); obj.prop = t; } </code></pre> <p>Clearly this would fail if the property doesn't support get and set in the current context.</p> <p>Why doesn't C# just do that for me?</p> <hr> <p>The only cases where I can think of where this might cause problems are:</p> <ul> <li>threading</li> <li>exceptions</li> </ul> <p>For threading that transformation affects when the writes happen (after the function call vs. in the function call), but I rather suspect any code that counts on that would get little sympathy when it breaks.</p> <p>For exceptions, the concern would be; what happens if the function assigns to one of several <code>ref</code> parameters than throws? Any trivial solution would result in all or none of the parameters being assigned to when some should be and some should not be. Again I don't think this would be supported use of the language.</p> <hr> <p>Note: I understand the mechanics of why this error messages is generated. What I'm looking for is the rationale for why C# doesn't automatically implement the trivial workaround.</p>
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    1. USPeter Mortensen
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    1. COIt does not do the workaround because it is not always safe. The more fundamental problem is that a property of type `T` shouldn't just have a getter and setter method, but should also have an `ActUpon<U>([indexparams...,] ActionByRef<T,U> act, ref U param)` method. Such a method would allow a statement like `ListOfPoints[4].X += something` to be executed efficiently, using a static delegate and no closures (`ListOfPoints.ActUpon(4, (ref Point it, ref param) => {it.X += param;}, ref something);`).
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    2. COThe road to sugary compilers is paved with good intentions.
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    3. COA solid way to solve this with Expressions at the bottom of this answer: http://stackoverflow.com/a/3059448/176877
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      1. USChris Moschini
 

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