Casting a generic dictionary containing a generic dictionary

Question

I have:

var someConcreteInstance = new Dictionary<string, Dictionary<string, bool>>();

and I wish to cast it to an interface version, i.e.:

someInterfaceInstance = (IDictionary<string, IDictionary<string, bool>>)someConcreteInstance;

'someInterfaceInstance' is a public property:

IDictionary<string, IDictionary<string, bool>> someInterfaceInstance { get; set; }

This compiles correctly, but throws a runtime casting error.

Unable to cast object of type 'System.Collections.Generic.Dictionary`2[System.String,System.Collections.Generic.Dictionary`2[System.String,System.Boolean]]' to type 'System.Collections.Generic.IDictionary`2[System.String,System.Collections.Generic.IDictionary`2[System.String,System.Boolean]]'.

What am I missing? (Problems with the nested generic type/Property?)

Can't you change your original inner-dictionary type to `IDictionary<>` ? This would simplify things, and usually this doesn't require big changes in the code that fills the dictionary. — digEmAll, Apr 12 '11 at 14:20
@digEmAll: as per Eric's answer, that simply moves the problem, it doesn't solve it. — nicodemus13, Apr 12 '11 at 15:00
No, I was asking why don't you do: `var someConcreteInstance = new Dictionary>();` instead of your original line ? This would fix the problem (you won't even need a cast), but of course you could have some reason for leaving your original code... — digEmAll, Apr 12 '11 at 15:07
possible duplicate of [IDictionary in .NET 4 not covariant](http://stackoverflow.com/questions/2149589/idictionarytkey-tvalue-in-net-4-not-covariant) — Ryan Gates, Dec 13 '13 at 17:21

Eric Lippert · Answer 1 · 2011-04-12T15:14:52.517

12

The other answers are right, but just to be crystal clear as to why this is illegal, consider the following:

interface IAnimal {}
class Tiger : IAnimal {}
class Giraffe : IAnimal {}
...
Dictionary<string, Giraffe> d1 = whatever;
IDictionary<string, IAnimal> d2 = d1; // suppose this were legal
d2["blake"] = new Tiger(); // What stops this?

No mortal hand can stop you putting a tiger into a dictionary of IAnimals. But that dictionary is actually constrained to contain only giraffes.

For the same reason you can't go the other way either:

Dictionary<string, IAnimal> d3 = whatever;
d3["blake"] = new Tiger(); 
IDictionary<string, Giraffe> d4 = d3; // suppose this were legal
Giraffe g = d4["blake"]; // What stops this?

Now you're putting a tiger in a variable of type giraffe.

Generic interface covariance is only legal in C# 4 if the compiler can prove that situations like this cannot arise.

edited Apr 12 '11 at 15:14

answered Apr 12 '11 at 14:22

Eric Lippert

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are you sure the last line is clear as you've written it? My original code compiles fine, but throws a runtime error, so the compiler seems to permit such situations. – nicodemus13 Apr 12 '11 at 14:59
2

@nicodemus13: Your code has a cast operator in it; mine does not. The cast operator means "take this conversion which would have been illegal at compile time and make it legal; if it turns out to be illegal at runtime, throw an exception". And hey, guess what? That's what's happening. You're telling the compiler "assume that this illegal conversion is going to work out just fine" and then it doesn't, and you get an exception. **The purpose of a reference type cast operator is to move the burden of type checking from the compiler to the runtime.** The burden doesn't vanish, it just moves. – Eric Lippert Apr 12 '11 at 15:13
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"Giraffe, giraffe, burning bright/In the forests of the night". Good thing the C# type system prevents such tragedies as this! – Jeffrey L Whitledge Apr 12 '11 at 15:23

score 4 · Accepted Answer · edited May 23 '17 at 10:30

4

IDictionary does not support covariance.

Look here IDictionary<TKey, TValue> in .NET 4 not covariant

edited May 23 '17 at 10:30

Community

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answered Apr 12 '11 at 14:03

Aliostad

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Oh, is that the problem! (even in .net 4?) Do you have any suggestions on how to get the result I want? – nicodemus13 Apr 12 '11 at 14:04
Depends what you are trying to do. Post another question on what you are trying to achieve and ask for the best design. – Aliostad Apr 12 '11 at 14:09

score 1 · Answer 3 · answered Apr 12 '11 at 14:04

The most you will be able to do is

IDictionary<string, Dictionary<string, bool>> viaInterface = someConcreteInstance

The reason your inner dictionary cannot be referenced differently here (or via a cast) is that while Dictionary<string, bool> is an IDictionary<string, bool>, not all IDictionary objects will be Dictionary objects. As such, obtaining a pure interface cast would then seemingly allow you to add other <string, IDictionary<string, bool>> pairs to the original collection, when clearly there could be type violations for the original object. Therefore, this is not supported.

Casting a generic dictionary containing a generic dictionary

3 Answers3

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