Newtonsoft JSON dynamic property name

You can do this with a custom ContractResolver. The resolver can look for a custom attribute which will signal that you want the name of the JSON property to be based on the class of the items in the enumerable. If the item class has another attribute on it specifying its plural name, that name will then be used for the enumerable property, otherwise the item class name itself will be pluralized and used as the enumerable property name. Below is the code you would need.

First let's define some custom attributes:

public class JsonPropertyNameBasedOnItemClassAttribute : Attribute
{
}

public class JsonPluralNameAttribute : Attribute
{
    public string PluralName { get; set; }
    public JsonPluralNameAttribute(string pluralName)
    {
        PluralName = pluralName;
    }
}

And then the resolver:

public class CustomResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty prop = base.CreateProperty(member, memberSerialization);
        if (prop.PropertyType.IsGenericType && member.GetCustomAttribute<JsonPropertyNameBasedOnItemClassAttribute>() != null)
        {
            Type itemType = prop.PropertyType.GetGenericArguments().First();
            JsonPluralNameAttribute att = itemType.GetCustomAttribute<JsonPluralNameAttribute>();
            prop.PropertyName = att != null ? att.PluralName : Pluralize(itemType.Name);
        }
        return prop;
    }

    protected string Pluralize(string name)
    {
        if (name.EndsWith("y") && !name.EndsWith("ay") && !name.EndsWith("ey") && !name.EndsWith("oy") && !name.EndsWith("uy"))
            return name.Substring(0, name.Length - 1) + "ies";

        if (name.EndsWith("s"))
            return name + "es";

        return name + "s";
    }
}

Now you can decorate the variably-named property in your PagedData<T> class with the [JsonPropertyNameBasedOnItemClass] attribute:

public class PagedData<T>
{
    [JsonPropertyNameBasedOnItemClass]
    public IEnumerable<T> Data { get; private set; }
    ...
}

And decorate your DTO classes with the [JsonPluralName] attribute:

[JsonPluralName("Users")]
public class UserDTO
{
    ...
}

[JsonPluralName("Items")]
public class ItemDTO
{
    ...
}

Finally, to serialize, create an instance of JsonSerializerSettings, set the ContractResolver property, and pass the settings to JsonConvert.SerializeObject like so:

JsonSerializerSettings settings = new JsonSerializerSettings
{
    ContractResolver = new CustomResolver()
};

string json = JsonConvert.SerializeObject(pagedData, settings);

Fiddle: https://dotnetfiddle.net/GqKBnx

If you're using Web API (looks like you are), then you can install the custom resolver into the pipeline via the Register method of the WebApiConfig class (in the App_Start folder).

JsonSerializerSettings settings = config.Formatters.JsonFormatter.SerializerSettings;
settings.ContractResolver = new CustomResolver();

Another Approach

Another possible approach uses a custom JsonConverter to handle the serialization of the PagedData class specifically instead using the more general "resolver + attributes" approach presented above. The converter approach requires that there be another property on your PagedData class which specifies the JSON name to use for the enumerable Data property. You could either pass this name in the PagedData constructor or set it separately, as long as you do it before serialization time. The converter will look for that name and use it when writing out JSON for the enumerable property.

Here is the code for the converter:

public class PagedDataConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType.IsGenericType && objectType.GetGenericTypeDefinition() == typeof(PagedData<>);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        Type type = value.GetType();

        var bindingFlags = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public;
        string dataPropertyName = (string)type.GetProperty("DataPropertyName", bindingFlags).GetValue(value);
        if (string.IsNullOrEmpty(dataPropertyName)) 
        {
            dataPropertyName = "Data";
        }

        JObject jo = new JObject();
        jo.Add(dataPropertyName, JArray.FromObject(type.GetProperty("Data").GetValue(value)));
        foreach (PropertyInfo prop in type.GetProperties().Where(p => !p.Name.StartsWith("Data")))
        {
            jo.Add(prop.Name, new JValue(prop.GetValue(value)));
        }
        jo.WriteTo(writer);
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

To use this converter, first add a string property called DataPropertyName to your PagedData class (it can be private if you like), then add a [JsonConverter] attribute to the class to tie it to the converter:

[JsonConverter(typeof(PagedDataConverter))]
public class PagedData<T>
{
    private string DataPropertyName { get; set; }
    public IEnumerable<T> Data { get; private set; }
    ...
}

And that's it. As long as you've set the DataPropertyName property, it will be picked up by the converter on serialization.

Fiddle: https://dotnetfiddle.net/8E8fEE