Why are the properties of anonymous types in C# read-only?

Question

In C#, the properties of anonymous types are read-only:

var person = new { Surname = "Smith", OtherNames = "John" };
person.Surname = "Johnson";  // ERROR: .Surname is read-only

Of course I can declare a real class if I want writable fields or properties, but regardless, what is the reasoning behind this design decision to make the properties read-only?

Safety, maybe? If you didn't specify they were writable, don't let them be written? Besides, those are properties, not fields. — John Saunders, Jul 06 '09 at 22:03
Good point, I'll change the text. I did also wonder why they couldn't be fields declared with "readonly"... — Roman Starkov, Jul 06 '09 at 22:05
http://stackoverflow.com/questions/622664/what-is-immutability-and-why-should-i-worry-about-it — Mehrdad Afshari, Jul 06 '09 at 22:22

score 56 · Accepted Answer · edited Jan 07 '13 at 23:05

56

Interesting article on that here. From there ...

... [B]y ensuring that the members do not change, we ensure that the hash is constant for the lifetime of the object.This allows anonymous types to be used with collections like hashtables, without actually losing them when the members are modified. There are a lot of benefits of immutabilty in that, it drastically simplifies the code that uses the object since they can only be assigned values when created and then just used (think threading)

edited Jan 07 '13 at 23:05

phloopy

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answered Jul 06 '09 at 22:09

JP Alioto

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Yes there are a lot of benefits to immutability, but there are a lot of benefits to mutability as well - you could use the above argument to argue that Lists should be immutable as well. This decision makes absolutely no sense. – BlueRaja - Danny Pflughoeft Mar 30 '13 at 00:05
2

It is interesting because anonymous types in vb.net are mutable... – IEnjoyEatingVegetables Apr 16 '20 at 16:22

score 0 · Answer 2 · answered May 05 '22 at 00:23

Unfortunately features like "top level statements" seem to be more important than the ability to pass complex data around and manipulate it as needed without having to create a class for that data. Its a HUGE gap in C# who's omission makes little sense to me. You basically have to use Dictionary<string,object> as this is the only way to do this. But let me ask any C# designers who might encounter this page, which is cleaner and less verbose between these 2 code snippets?

        var data = new 
        {
            Username = "My Username",
            FullName = "John Smith",
            Age = 22,
            Salary = 11.5,               
            IsEmployee = true,
            DOB = DateTimeOffset.UtcNow,              
        };
        data.FullName = "Hello worlder";

        var data2 = new Dictionary<string, object>
        {
            ["Username"] = "My Username",
            ["FullName"] = "John Smith",
            ["Age"] = 22,
            ["Salary"] = 11.5,
            ["IsEmployee"] = true,
            ["DOB"] = DateTimeOffset.UtcNow,
        };
        data["FullName"] = "john smith";

Why are the properties of anonymous types in C# read-only?

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