c# - is order of field important in anonymous types automatic initialization?

Question

I got a scenario to create the anonymous list from the anonymous types, and i achieved that using

    public static List<T> MakeList<T>(T itemOftype)
    {
        List<T> newList = new List<T>(); 
        return newList; 
    } 

    static void Main(string[] args)
    {
       //anonymos type
       var xx = new
       {                             
          offsetname = x.offName,
          RO = y.RO1
       };

       //anonymos list
       var customlist = MakeList(xx);

       //It throws an error because i have given the wrong order
       customlist.Add(new { RO = y.RO2, offsetname = x.offName });
       customlist.Add(new { RO = y.RO3, offsetname = x.offName });

       //but this works
       customlist.Add(new { offsetname = x.offName, RO = y.RO2 });
       customlist.Add(new { offsetname = x.offName, RO = y.RO3 });
    }

these are the error messages

System.Collections.Generic.List.Add(AnonymousType#1)' has some invalid arguments

Argument '1': cannot convert from 'AnonymousType#2' to 'AnonymousType#1'

whats the reason behind that??

Answer 1

Yes, it's important.

Two anonymous type initializers use the same auto-generated type if the property names and types are the same, in the same order.

The order becomes relevant when hashing; it would have been possible for the type to be generated with a consistent order for calculating a hash value, but it seems simpler to just include the property order as part of what makes a type unique.

See section 7.5.10.6 of the C# 3 spec for details. In particular:

Within the same program, two anonymous object initializers that specify a sequence of properties of the same names and compile-time types in the same order will produce instances of the same anonymous type.