IndexOutOfRangeException in indexed getter

Question

In my indexed property I check whether the index is out of bounds or not. If it is, I throw an IndexOutOfBoundsException.

When I run the Code Analyst (in VS12) it complains with CA1065: Unexpected exception in unexpected location.

Referring to the description of CA1065, only

System.InvalidOperationException
System.NotSupportedException
System.ArgumentException
KeyNotFoundException

are allowed in an indexed getter.

Throwing IndexOutOfBoundsException seems natural to me, so what is the reasoning here? (And yes, I know I can turn the warning off, I just want to know the reasoning)

Please post the code for where you throw the exception and where you re catching the exception. — paparazzo, Sep 02 '12 at 16:09
It is strange. `System.String.this[int]` throws an `IndexOutOfRange`, so this guideline is not followed by a main class in the library. — H H, Sep 02 '12 at 16:26
@Blam -- the syntax is irrelevant here -- Code Analysis does not examine syntax. What he describes is clear and straight forward. No code sample is required. — BrainSlugs83, Oct 20 '13 at 07:14

score 8 · Accepted Answer · answered Sep 02 '12 at 17:40

8

A lot of classes use ArgumentOutOfRangeException for this, including List<T>. This is a subclass of ArgumentException so should satisfy the rule. I guess you could argue that for a vector etc accessed directly, there isn't actually a method call (it is a dedicated opcode - ldelem*), so the index in that case isn't actually an argument. Seems a weak argument, though.

answered Sep 02 '12 at 17:40

Marc Gravell

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I have to disagree on calling an indexer's parameter an index being a weak argument. As does MSDN. "Indexers allow instances of a class or struct to be indexed just like arrays." via http://msdn.microsoft.com/en-us/library/vstudio/6x16t2tx.aspx -- so why then, shouldn't an indexer behave like an array? – BrainSlugs83 Oct 20 '13 at 07:17

score 6 · Answer 2 · answered Sep 02 '12 at 17:45

6

See MSDN: IndexOutOfRangeException is system exception and reserved for accessing array elements. It is thrown by some MSIL instructions: ldelem., ldelema, stelem..

For developing classes use ArgumentOutOfRangeException.

answered Sep 02 '12 at 17:45

STO

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Marc beat you to it though :-) – Mario The Spoon Sep 02 '12 at 17:46
Also via MSDN (per my reply to Marc): "Indexers allow instances of a class or struct to be indexed just like arrays." -- so again, if the purpose of an indexer is to emulate an array, why would IndexOutOfRangeException be off limits? – BrainSlugs83 Oct 20 '13 at 07:18
That exception is intended to be used in runtime array bounds check (which could be turned off due optimization). But of course you can use it if you would like :) – STO Oct 21 '13 at 09:49

IndexOutOfRangeException in indexed getter

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