Why does using an implicit collection initializer for dictionaries cause a runtime error in C#?

Question

I have an async method with the following signiature:

public async Task UpdateMerchantAttributes(UpdateMerchantRequestModel model).

The model only has one property:

public Dictionary<string, string> Attributes { get; set; }

For a while I was calling it in a test in the following way:

await client.UpdateMerchantAttributes(new UpdateMerchantRequestModel { Attributes = 
    {
        {"businessType", "0"}
    }
});

This compiled just fine, but caused a NullReferenceException at runtime on that line. I was confused about this because client was not null, and nothing else is referenced in that line (or so it seems at a glance). Then I tried adding an explicit Dictionary declaration, like so:

await client.UpdateMerchantAttributes(new UpdateMerchantRequestModel { Attributes =
    new Dictionary<string, string>
    {
        {"businessType", "0"}
    }
});

And now it passes fine. It was my mistake, but this mistake would've cost me much less time if it'd been a compile error rather than a runtime null ref exception. So I'm curious, why does this happen? Does the compiler think that I'm trying to define a dynamic and that somehow resolves to null?

Indeed, I'm not sure why you *wouldn't* expect it to be a runtime error. It is no different than any other. — Sudsy1002, Mar 23 '18 at 12:32
tip: `public Dictionary Attributes { get; } = new Dictionary();` - concise and convenient (and fixes the problem, which is for exactly the reasons that @mjwills notes) — Marc Gravell, Mar 23 '18 at 12:34

mjwills · Accepted Answer · 2018-03-23T12:39:36.623

7

The first form:

Attributes = 
    {
        {"businessType", "0"}
    }
}

is syntactic sugar for .Add(key, value). That is all. It isn't concerned with creating the dictionary (so, in effect, you are adding to a null dictionary).

Your second form is one way to solve it. Another, slightly more bulletproof way (since it protects you against the first form), is to use @MarcGravell's suggestion:

public Dictionary<string, string> Attributes
    { get; } = new Dictionary<string,string>(); // set; after the get; is also OK

or to populate Attributes in the constructor.

edited Mar 23 '18 at 12:39

answered Mar 23 '18 at 12:33

mjwills

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Ahh, I see. So without `new` it's an implied add. Makes sense. Thanks so much – Jansky Mar 23 '18 at 13:20
With, or without, the `new` it is an implied add @Jansky . – mjwills Mar 23 '18 at 22:14

Why does using an implicit collection initializer for dictionaries cause a runtime error in C#?

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