Philip Hendry's Blog

Camel Cased JSON Error Responses in ASP.NET WebAPI

Dec 7, 2019 • ASP.NET • 2 min read

In my previous post ASP.NET Web Api Complex Model Binding and Validation I had described how to use Request.CreateErrorResponse() in either a controller action or a filter to convert the errors held in ModelState into JSON that can be passed back to the client.

What I hadn’t taken into account was the camel casing in the result that the framework had failed to fix. For example, given a class structure like this:

public class Person
    public Address Address { get; set; }
public class Address
    [Required] public String AddressLine1 { get; set; }
    [Required] public String Town { get; set; }
    [Required] public String PostCode { get; set; }

And if I don’t submit those fields to the PUT method then I’ll get back the following errors:

    "Message": "The request is invalid.",
    "ModelState": {
        "person.Address.AddressLine1": [
            "The AddressLine1 field is required."
        "person.Address.Town": [
            "The Town field is required."
        "person.Address.PostCode": [
            "The PostCode field is required."

The problem here is that the naming strategy registered with NewtonSoft has kicked in after the ModelState has concatenated the field names and therefore only the first letter has been lowercased. In other words, where I see person.Address.PostCode I actually should see person.address.postCode.

Like any good developer, before reading the docs, I took to google and within a couple of minutes found a stackoverflow post which had the answer!

The first place to change was in the WebApiConfig.Register:

var contractResolver = HtmlSanitisingContractResolver.Instance;
contractResolver.NamingStrategy = new PiecewiseNamingStrategy(new CamelCaseNamingStrategy())
    OverrideSpecifiedNames = true,
    ProcessDictionaryKeys = true

As an added bonus the stackoverflow post also answered the question of how to resolve my HtmlSanitisingContractResolver from being derived from CamelCaseNamingStrategy. The NamingStrategy is assigned to the HtmlSanitisingContractResolver and has a new one called PiecewiseNamingStrategy which simply splits the name on . and camelcases each individual item before re-joining them:

public class PiecewiseNamingStrategy : NamingStrategy
    readonly NamingStrategy baseStrategy;

    public PiecewiseNamingStrategy(NamingStrategy baseStrategy)
        this.baseStrategy = baseStrategy ?? throw new ArgumentNullException();

    protected override string ResolvePropertyName(string name)
        return String.Join(".", name.Split('.')
            .Select(n => baseStrategy.GetPropertyName(n, false)));

Simple but effective. My next post however will discuss throwing this away in favour of reformatting the error response JSON entirely!

Post by: Philip Hendry