Does a C# indexer return a variable or value?

Question

Mutable structs are error-prone; dictionary[0].Inflate(1,1) doesn't behave the same as array[0].Inflate(1,1) would when T is a Rectangle (since array[0] is a variable, whereas dictionary[0] is a value).

If I make a custom indexer for SomeClass:

public T this[int x, int y] { get { return arr[y*100+x]; } }

Is someclass[x,y] a variable or value or neither? Presuming T is, of course, a struct.

Do you mean Value Type and Reference Type, not value and variable? — psubsee2003, Jan 04 '14 at 10:49
Looks like you are mixing up *variable* and *reference type* — Sergey Berezovskiy, Jan 04 '14 at 10:49
@psubsee2003 I believe *value* and *variable* are the correct terminology for when T is a struct. — Mr. Smith, Jan 04 '14 at 10:51
@Mr.Smith I don't think so. A struct is always a Value Type. It can never be anything different. A variable is not a type at all. — psubsee2003, Jan 04 '14 at 10:53
@Mr.Smith From MSDN: Variables represent storage locations. Every variable has a type that determines what values can be stored in the variable. — Sergey Berezovskiy, Jan 04 '14 at 10:53
@Mr.Smith ok, makes sense, it was a choice of terminology that I was not familiar with. — psubsee2003, Jan 04 '14 at 10:58

score 4 · Accepted Answer · answered Jan 04 '14 at 10:56

An indexer access expression is initially classified as an "indexer access". From section 7.6.6.2 of the C# 4 spec:

The result of processing the indexer access is an expression classified as an indexer access

And then from section 7.1:

A property access or indexer access is always reclassified as a value by performing an invocation of the get-accessor or set-accessor.

So basically you can think of it as being classified as a value.

However, an array access is classified as a variable expression. From section 7.6.6.1 of the C# 4 spec:

The result of evaluating an array access is a variable of the element type of the array [...]

That's why this is fine:

string[] x = { "", "", "" };
SomeMethodWithRef(ref x[0]);

But this isn't:

List<string> x = new List<string> { "", "", "" };
SomeMethodWitHRef(ref x[0]);

Just to be absolutely certain I'm reading this right, does that imply my indexer is returning a *copy* of the struct `T`? — Mr. Smith, Jan 04 '14 at 10:58
@Mr.Smith: Yes, in exactly the same way as a simple assignment would copy the value. — Jon Skeet, Jan 04 '14 at 11:01

Does a C# indexer return a variable or value?

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