Validating inputs for PATCH requests in ASP.NET WebAPI

In ASP.NET WebAPI (with its recent OData additions) there is good support for HTTP PATCH requests via the Delta<T> class. I won’t bother reproducing a tutorial here since there’s already a good one online.

The only problem with the PATCH support and tutorial is that there is no guidance on how to validate the model once you’ve accepted the partially updated data. So here’s what I came up with to validate the model once we’ve called Patch:

public HttpResponseMessage Patch(Guid id, Delta<TenantData> data)
{
    var tenant = this.TenantRepository.Get(id);
    if (tenant == null) return Request.CreateResponse(HttpStatusCode.NotFound);

    data.Patch(tenant);

    // this is where we do the validation on the model after we've
    // merged in the patch values
    var svc = this.Configuration.Services;
    var validator = svc.GetBodyModelValidator();
    var ad = svc.GetActionSelector().SelectAction(this.ControllerContext);
    var ac = new HttpActionContext(this.ControllerContext, ad);
    var mp = svc.GetModelMetadataProvider();
    if (!validator.Validate(tenant, typeof(TenantData), mp, ac, "data"))
    {
        // validation failed, so return our error and pass along the 
        // ModelState from the action context (which is a different
        // instance than this.ModelState)
        return Request.CreateErrorResponse(HttpStatusCode.BadRequest, 
                                           ac.ModelState);
    }

    this.TenantRepository.SaveChanges();

    return Request.CreateResponse(HttpStatusCode.OK, tenant);
}

In essence, I needed to manually trigger validation that normally happens during model binding. I wish there was a nice API built-in for this, but alas there is not.