Using the local variable before it is declared #9677
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Hello, Could somebody please explain why the line below is valid? int value = (value = 1) > 0 ? 2 : -2;If I replace var value = (value = 1) > 0 ? 2 : -2;
// error CS0841: Cannot use local variable 'value' before it is declared (SharpLab) I'm confused because in both cases it seems I'm using a local variable "before it is declared". |
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Replies: 1 comment 3 replies
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In hte latter case, we don't know the type of 'value', and to determine its type, we have to look at the RHS. But the RHS uses 'value', whose type we are trying to figure out. So we can't figure out the type of the RHS. So we can't figure out the type of 'value'. In the former case, there is no issue. we knwo the type. It is explicit. It is 'int'. As such, we don't have to go figure out the type. We can just bind and compile the RHS. The RHS is perfectly legal. The variable is never read before it is written, for example. |
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Note: The error message here could be improved. Instead of saying "Cannot use local variable 'value' before it is declared " It could say something more like "Could not determine the type of 'value' because it is referenced in its own initializer." |
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Thanks for the explanation! Yep, in the case I have seen the message like you propose, I would fully understand the context. And just to be sure: is it consistent with the language spec to use the variable in a similar context (assigning a value) on the right side of the declaration of that same variable? |
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Yes. This is 100% legal, and has been since c# 1.0. THere is no problem using a variable within its own initializer. The only rule that matters in such a situation is that it not be read before first written. |
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In hte latter case, we don't know the type of 'value', and to determine its type, we have to look at the RHS. But the RHS uses 'value', whose type we are trying to figure out. So we can't figure out the type of the RHS. So we can't figure out the type of 'value'.
In the former case, there is no issue. we knwo the type. It is explicit. It is 'int'.
As such, we don't have to go figure out the type. We can just bind and compile the RHS. The RHS is perfectly legal. The variable is never read before it is written, for example.