Conditional member serialization based on query parameter?

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One possibility would be to introduce a custom attribute JsonConditionalIncludeAttribute that can be applied to properties and fields:

[System.AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = true, Inherited = true)]
public class JsonConditionalIncludeAttribute : System.Attribute
{
    public JsonConditionalIncludeAttribute(string filterName)
    {
        this.FilterName = filterName;
    }

    public string FilterName { get; private set; }
}

Next, subclass DefaultContractResolver, override CreateProperty, and return null for properties that have at least one [JsonConditionalInclude] applied, none of which match the a filter supplied to the contract resolver:

public class JsonConditionalIncludeContractResolver : DefaultContractResolver
{
    public JsonConditionalIncludeContractResolver(string filterName)
    {
        this.FilterName = filterName;
    }

    public string FilterName { get; set; }

    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var property = base.CreateProperty(member, memberSerialization);
        // Properties without JsonConditionalIncludeAttribute applied are serialized unconditionally.
        // Properties with JsonConditionalIncludeAttribute are serialized only if one of the attributes
        // has a matching filter name.
        var attrs = property.AttributeProvider.GetAttributes(typeof(JsonConditionalIncludeAttribute), true);
        if (attrs.Count > 0 && !attrs.Cast<JsonConditionalIncludeAttribute>().Any(a => a.FilterName == FilterName))
            return null;
        return property;
    }
}

Finally, when serializing your class to JSON, set JsonSerializerSettings.ContractResolver equal to your custom contract resolver, initializing the FilterName from your web request, for instance:

public class TestClass
{
    public string Property1 { get; set; }

    [JsonConditionalInclude("summary")]
    [JsonConditionalInclude("title")]
    public string Property2 { get; set; }

    [JsonConditionalInclude("summary")]
    public string Property3 { get; set; }

    [JsonConditionalInclude("title")]
    [JsonConditionalInclude("citation")]
    public string Property4 { get; set; }

    [JsonConditionalInclude("citation")]
    public string Field1;

    public static void Test()
    {
        var test = new TestClass { Property1 = "a", Property2 = "b", Property3 = "c", Property4 = "d", Field1 = "e" };
        Test(test, "summary"); // Prints "a", "b" and "c"
        Test(test, "title");   // Prints "a", "b" and "d".
        Test(test, "citation");// Prints "e", "a" and "d"
        Test(test, null);      // Prints "e", "a", "b", "c" and "d".
    }

    public static string Test(TestClass test, string webRequestFormat)
    {
        var settings = new JsonSerializerSettings { ContractResolver = new JsonConditionalIncludeContractResolver(webRequestFormat) };

        var json = JsonConvert.SerializeObject(test, Formatting.Indented, settings);

        Debug.WriteLine(json);
        return json;
    }
}

The contract resolver will apply to all classes being serialized, not just the root class, which looks to be what you want.

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