WebApi - Binding to Header Values

We have a WebApi project at work where we need to get a value from the header and use it as input to a query.

One way to do this is

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public IActionResult DoSomething()
{
  var headerValues = request.Headers.GetValues("MyHeader");
  var header = headerValues.FirstOrDefault();
}

YUCK.

I wasn’t very keen on this and starting think it must be possible to use WebApi’s model binding. After a bit of searching I found https://blog.bennymichielsen.be/2013/11/17/modelbinding-with-headers-in-asp-net-webapi/ which gave me most of
the details. This left me with model binding working, but not quite in the way I wanted.

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public IActionResult DoSomething([ValueProvider(typeof(HeaderValue))] int custId)
{
  //Do something with custId
}

Better, but not quite what I wanted.

In trying to fill in a few of the missing pieces I had a look at the WebApi code on GitHub and found that in one of their Integration tests Microsoft had pretty much exactly what I was after.

So between the two I was able to put together something I was happy with.

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public class HeaderValueProviderFactory : ValueProviderFactory
{
    /// <summary>
    ///     Gets the value provider.
    /// </summary>
    /// <param name="actionContext">The action context.</param>
    /// <returns></returns>
    public override IValueProvider GetValueProvider(HttpActionContext actionContext)
    {
        return new HeaderValueProvider(actionContext.Request);
    }
}

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/// <summary>
/// Returns the tenant id from the request header.
/// http://blog.bennymichielsen.be/2013/11/17/modelbinding-with-headers-in-asp-net-webapi/
/// also some ideas from the ASP.NET unit tests
/// https://github.com/ASP-NET-MVC/aspnetwebstack/blob/4e40cdef9c8a8226685f95ef03b746bc8322aa92/test/System.Web.Http.Integration.Test/ModelBinding/ModelBindingController.cs#L271
/// </summary>
public class HeaderValueProvider : IValueProvider
{
    private readonly HttpRequestMessage _requestMessage;

    /// <summary>
    /// Initializes a new instance of the <see cref="HeaderValueProvider"/> class.
    /// </summary>
    /// <param name="requestMessage">The request message.</param>
    public HeaderValueProvider(HttpRequestMessage requestMessage)
    {
        _requestMessage = requestMessage;
    }

    /// <summary>
    /// Determines whether the specified prefix contains prefix.
    /// </summary>
    /// <param name="prefix">The prefix.</param>
    /// <returns></returns>
    public bool ContainsPrefix(string prefix)
    {
        return _requestMessage.Headers.Contains(prefix);
    }

    /// <summary>
    /// Gets the value.
    /// </summary>
    /// <param name="key">The key.</param>
    /// <returns></returns>
    public ValueProviderResult GetValue(string key)
    {
        IEnumerable<string> headerValue;
        ValueProviderResult result = null;
        if (_requestMessage.Headers.TryGetValues(key, out headerValue))
        {
            var header = headerValue as IList<string> ?? headerValue.ToList();
            if (header.Any())
            {
                var value = header.First();
                result = new ValueProviderResult(value, value, CultureInfo.InvariantCulture);
            }
        }
        return result;

    }

}
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[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method | AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
public class FromHeaderAttribute : ModelBinderAttribute
{
    /// <summary>
    /// Gets the value providers that will be fed to the model binder.
    /// </summary>
    /// <returns>
    /// A collection of <see cref="T:System.Web.Http.ValueProviders.ValueProviderFactory"/> instances.
    /// </returns>
    /// <param name="configuration">The <see cref="T:System.Web.Http.HttpConfiguration"/> configuration object.</param>
    public override IEnumerable<ValueProviderFactory> GetValueProviderFactories(HttpConfiguration configuration)
    {
        return base.GetValueProviderFactories(configuration).OfType<HeaderValueProviderFactory>();
    }
}
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 public static class WebApiConfig
 {
     public static void Register(HttpConfiguration config)
     {
         // Web API configuration and services
         config.Services.Add(typeof(ValueProviderFactory), new HeaderValueProviderFactory());
     }
}

So finally I can just do

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public IActionResult DoSomething([FromHeader(Name="CustId")] int custId)
{
  // Do something with custId
}

Much nicer.