WebApi2: Custom parameter binding to bind partial parameters

Question

I have a webApi2 project and an other project, in which I have my Model classes and a BaseModel that is a base for all Models, as following,

public class BaseModel
{
    public string UserId { get; set; }
}

All the other models are derived from my BaseModel.

In webapi I have my CustomerController as following,

public class CustomerController : ApiController
{
    [HttpPost]
    public GetCustomerResponseModel Get(GetCustomerRequestModel requestModel)
    {
        var response = new GetCustomerResponseModel();

        //I need only the UserId coming from the BaseModel is binded from request headers
        var userId = requestModel.UserId;

        //I want all model data except UserId is binded with default model binding
        var customerData = requestModel.CustomerData;
        var someOtherData = requestModel.SomeOtherData;

        return response;
    }

    [HttpPost]
    public AddStockAlertResponseModel AddStockAlert(AddStockAlertRequestModel requestModel)
    {
        var response = new AddStockAlertResponseModel();

        //I need only the UserId coming from the BaseModel is binded from request headers
        var userId = requestModel.UserId;

        //I want all model data except UserId is binded with default model binding
        var stockInfo = requestModel.StockInfo;

        return response;
    }
}

Every request that comes to CustomerController has a "UserId" header in request headers and I need a ModelBinder or ParameterBinder or some functionality that binds only the UserId from request headers without touching the other model parameters. I mean model parameters except UserId are to be binded by default..

I don't want to use AOP or interceptors or aspects.. Is it possible to bind only UserId with an asp.net functionality like model binders, parameter binders, etc.

Answer 1

Following is a quick example using HttpParameterBinding. Here I am creating a custom parameter binding where I let the default FromBody based binding to use the formatters to deserialize the request body and then I get the user id from request headers and set on the the deserialized object. (You might need to add additional validation checks on the following code).

config.ParameterBindingRules.Insert(0, (paramDesc) =>
            {
                if (typeof(BaseModel).IsAssignableFrom(paramDesc.ParameterType))
                {
                    return new BaseModelParamBinding(paramDesc);
                }

                // any other types, let the default parameter binding handle
                return null;
            });

---

public class BaseModelParamBinding : HttpParameterBinding
{
    HttpParameterBinding _defaultFromBodyBinding;
    HttpParameterDescriptor _paramDesc;

    public BaseModelParamBinding(HttpParameterDescriptor paramDesc)
        : base(paramDesc)
    {
        _paramDesc = paramDesc;
        _defaultFromBodyBinding = new FromBodyAttribute().GetBinding(paramDesc);
    }

    public override async Task ExecuteBindingAsync(ModelMetadataProvider metadataProvider,
        HttpActionContext actionContext, CancellationToken cancellationToken)
    {
        await _defaultFromBodyBinding.ExecuteBindingAsync(metadataProvider, actionContext, cancellationToken);

        BaseModel baseModel = actionContext.ActionArguments[_paramDesc.ParameterName] as BaseModel;

        if (baseModel != null)
        {
            baseModel.UserId = actionContext.Request.Headers.GetValues("UserId").First();
        }
    }
}