Make dictionary read only in C#

Question

I have a Dictionary> and would like to expose the member as read only. I see that I can return it as a IReadOnlyDictionar

Answer 1

First, you'll have to create a new dictionary with the desired content types:

var dicWithReadOnlyList = dic.ToDictionary(
    kv => kv.Key,
    kv => kv.Value.AsReadOnly());

Then you can just return the new dictionary, since IReadOnlyDictionary is a supertype of Dictionary.

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Why do you need to do that? Because Dictionary is not a supertype of Dictionary, even if A is a supertype of B. Why? Consider the following example:

var dic = new Dictionary();
Dictionary dic2 = dic;      // Imagine this were possible...

dic2.Add(someT, someA);           // ...then we'd have a type violation here, since
                                  // dic2 = dic requires some B as the value.

In other words, TValue in Dictionary is not covariant. From an object-orientied point of view, covariance should be possible in the read-only version of the dictionary, but there are legacy issues in the .NET framework which prevent this (see the part starting with "UPDATE" in this question for details).