binding a Guid parameter in asp.net mvc core

[UPDATE]

This has been improved in 2.1.0-preview2. Your code actually works now. You can bind a non-string type from header to your parameters. You only need to set compatibility version in your startup class.

Controller

[HttpGet]
public Task<JsonResult> Get([FromHeader] Guid id)
{
    return new JsonResult(new {id});
}

Startup

Services
  .AddMvc
  .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

Look at the same Github discussion mentioned above: https://github.com/aspnet/Mvc/issues/5859

I have just found out that basically ASP Core only supports binding header values to strings and collections of strings! (whereas binding from route values, query string and body supports any complex type)

You can check the HeaderModelBinderProvider source in Github and see for yourself:

public IModelBinder GetBinder(ModelBinderProviderContext context)
{
    if (context == null)
    {
        throw new ArgumentNullException(nameof(context));
    }

    if (context.BindingInfo.BindingSource != null &&
            context.BindingInfo.BindingSource.CanAcceptDataFrom(BindingSource.Header))
    {
        // We only support strings and collections of strings. Some cases can fail
        // at runtime due to collections we can't modify.
        if (context.Metadata.ModelType == typeof(string) ||
            context.Metadata.ElementType == typeof(string))
        {
            return new HeaderModelBinder();
        }
    }

    return null;
}

I have submitted a new issue, but in the meantime I would suggest you either bind to a string or create your own specific model binder (something that combines [FromHeader] and [ModelBinder] into your own binder)

---

Edit

The sample model binder could look like this:

public class GuidHeaderModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType != typeof(Guid)) return Task.CompletedTask;
        if (!bindingContext.BindingSource.CanAcceptDataFrom(BindingSource.Header)) return Task.CompletedTask;

        var headerName = bindingContext.ModelName;
        var stringValue = bindingContext.HttpContext.Request.Headers[headerName];
        bindingContext.ModelState.SetModelValue(bindingContext.ModelName, stringValue, stringValue);

        // Attempt to parse the guid                
        if (Guid.TryParse(stringValue, out var valueAsGuid))
        {
            bindingContext.Result = ModelBindingResult.Success(valueAsGuid);
        }

        return Task.CompletedTask;
    }
}

And this would be an example using it:

public IActionResult SampleAction(
    [FromHeader(Name = "my-guid")][ModelBinder(BinderType = typeof(GuidHeaderModelBinder))]Guid foo)
{
    return Json(new { foo });
}

Which you can try, for example with jquery in the browser:

$.ajax({
  method: 'GET',
  headers: { 'my-guid': '70e9dfda-4982-4b88-96f9-d7d284a10cb4' }, 
  url: '/home/sampleaction'
});