C# property and ref parameter, why no sugar?

Question

I just ran across this error message while working in C#

A property or indexer may not be passed as an out or ref parameter

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 out/ref parameter and then assigning it back to the property:

RefFn(ref obj.prop);

turns into

{
    var t = obj.prop;
    RefFn(ref t);
    obj.prop = t;
}

Clearly this would fail if the property doesn't support get and set in the current context.

Why doesn't C# just do that for me?

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The only cases where I can think of where this might cause problems are:

• threading
• exceptions

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.

For exceptions, the concern would be; what happens if the function assigns to one of several ref 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.

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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.

Answer 1

Because you're passing the result of the indexer, which is really the result of a method call. There's no guarantee that the indexer property also has a setter, and passing it by ref would lead to a false security on the developer's part when he thinks that his property is going to be set without the setter being called.

On a more technical level, ref and out pass the memory address of the object passed into them, and to set a property, you have to call the setter, so there's no guarantee that the property would actually be changed especially when the property type is immutable. ref and out don't just set the value upon return of the method, they pass the actual memory reference to the object itself.