!DataSerializationHelper.SerializerSettings.SerializeReadOnlyTypes

Describe the bug

Read-only data cannot be used as dynamic data, because the serializer ignores read-only types. Use-case for read-only data is when the application subject for testing has that kind of data.

Steps To Reproduce

Conceptually in C# Interactive window, showing example of read-only data not being serialized unless SerializeReadOnlyTypes=true;

#r "System.Runtime.Serialization.Json"
using System.Runtime.Serialization;
using System.Runtime.Serialization.Json;

readonly struct Foo1 { public Foo1(int a) { A = a; } public int A { get; } }
var ms1 = new MemoryStream();
new DataContractJsonSerializer(typeof(Foo1), new DataContractJsonSerializerSettings()).WriteObject(ms1, new Foo1(42));

[DataContract]
readonly struct Foo2 { public Foo2(int a) { A = a; } [DataMember]public int A { get; } }
var ms2 = new MemoryStream();
new DataContractJsonSerializer(typeof(Foo2), new DataContractJsonSerializerSettings { SerializeReadOnlyTypes = true }).WriteObject(ms2, new Foo2(42));

//Encoding.UTF8.GetString(ms1.ToArray()) => "{}" // So with default value SerializeReadOnlyTypes=false it will never work
//Encoding.UTF8.GetString(ms2.ToArray()) => "{\"A\":42}"

For DataSerializationHelperTests instead, though I have not managed to build under VS2022 yet;

    public void DataSerializerShouldRoundTripReadOnlyData()
    {
        var source = new ReadOnlyDataWrapper { ReadOnlyData = new ReadOnlyDataType(42) };

        var actual = DataSerializationHelper.Deserialize(
            DataSerializationHelper.Serialize(new object[] { source }));

        Verify(actual.Length == 1);
        Verify(actual[0].GetType() == typeof(ReadOnlyDataWrapper));
        Verify(((ReadOnlyDataWrapper)actual[0]).ReadOnlyData.Equals(source));
    }

    public class ReadOnlyDataWrapper
    {
        public ReadOnlyDataType ReadOnlyData { get; set; }
    }

    // Should serialize fine if DataSerializationHelper.SerializerSettings.SerializeReadOnlyTypes = true
    [DataContract]
    public readonly struct ReadOnlyDataType
    {
        public ReadOnlyDataType(int a)
        {
            A = a;
        }

        [DataMember]
        public int A { get; }
    }

Expected behavior

Dynamic test data with read-only types should not go lost.

Actual behavior

Read-only types in dynamic test data lose their values.

Additional context

Providing PR for your convenience.

[Evangelink]

I will have a deeper look after but we already have a couple of issues related to serialization of data during discovery (see #1462). So far the best option I see would be to use BinaryFormatter but I fear it will be flagged internally for the security issues it raises. I will sync with runtime team to see what they would suggest for this case.

[Evangelink]

Thanks for the contribution and detailed explanations @aliquid. I will make sure to come back to you asap.