json deserialize from legacy property names

This can be done with a custom IContractResolver created by extending DefaultContractResolver:

[System.AttributeUsage(System.AttributeTargets.Property | System.AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
public class LegacyDataMemberNamesAttribute : Attribute
{
    public LegacyDataMemberNamesAttribute() : this(new string[0]) { }

    public LegacyDataMemberNamesAttribute(params string[] names) { this.Names = names; }

    public string [] Names { get; set; }
}

public class LegacyPropertyResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        var properties = base.CreateProperties(type, memberSerialization);

        for (int i = 0, n = properties.Count; i < n; i++)
        {
            var property = properties[i];
            if (!property.Writable)
                continue;
            var attrs = property.AttributeProvider.GetAttributes(typeof(LegacyDataMemberNamesAttribute), true);
            if (attrs == null || attrs.Count == 0)
                continue;
            // Little kludgy here: use MemberwiseClone to clone the JsonProperty.
            var clone = property.GetType().GetMethod("MemberwiseClone", BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
            foreach (var name in attrs.Cast<LegacyDataMemberNamesAttribute>().SelectMany(a => a.Names))
            {
                if (properties.Any(p => p.PropertyName == name))
                {
                    Debug.WriteLine("Duplicate LegacyDataMemberNamesAttribute: " + name);
                    continue;
                }
                var newProperty = (JsonProperty)clone.Invoke(property, new object[0]);
                newProperty.Readable = false;
                newProperty.PropertyName = name;
                properties.Add(newProperty);
            }
        }

        return properties;
    }
}

Then add attributes to your type as shown in the question:

[DataContract]
class TestObject
{
    [LegacyDataMemberNames("alpha", "omega")]
    [DataMember(Name = "a")]
    public int A { get; set; }
}

Construct and configure an instance of LegacyPropertyResolver, e.g. as follows:

static IContractResolver legacyResolver = new LegacyPropertyResolver 
{ 
    // Configure as required, e.g. 
    // NamingStrategy = new CamelCaseNamingStrategy() 
};

And then use it in settings:

var settings = new JsonSerializerSettings { ContractResolver = legacyResolver };
var deserialized = JsonConvert.DeserializeObject<TestObject>(jsonString, settings);

Notes:

• This implementation doesn't require that the class have explicit data contract attribute annotation. You could add that restriction, if you prefer.

• You should cache and reuse instances of contract resolvers for best performance.

Demo fiddle here.

A very simple solution using Json.NET is to just provide a legacy property with a setter only.

class TestObject {
    public int A { get; set; }
    public int alpha { set => A = value; }
    public int omega { set => A = value; }
}

You'd probably rather not have these public, in which case you can just mark private and add the JsonProperty attribute.

class TestObject {
    public int A { get; set; }
    [JsonProperty] private int alpha { set => A = value; }
    [JsonProperty] private int omega { set => A = value; }
}