ModelBindingContext.HttpContext.Request.Body already consumed!

[ans-ashkan]

Request.Body is already consumed in CustomModelBinder or in an Action that contains some parameters.

[ModelBinder(typeof(CustomModelBinder))]
public class StreamModel
{
    public MemoryStream Stream
    {
        get;
        set;
    }
}

public class CustomModelBinder : IModelBinder
{
    public async Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var request = bindingContext.HttpContext.Request;
        var ms = new MemoryStream();
        request.Body.CopyTo(ms);
        bindingContext.Result = ModelBindingResult.Success(new StreamModel
        {
            Stream = ms
        });
    }
}

Value of ms.Length always equals to 0.

the action with parameters code:

public class TestController : Controller
{
    [HttpPost]
    public IActionResult Test(string data)
    {
        var ms = new MemoryStream();
        request.Body.CopyTo(ms);
        return OK(ms.Length);
    }
}

Also this always returns 0. but if you drop the paramter string data, it returns the actual body length.

by using the following middleware, it works. but I consider this a workaround.

app.Use(next => context => { context.Request.EnableRewind(); return next(context); });

Shouldn't this happen internally (in mvc)?

(link to stack overflow question: https://stackoverflow.com/questions/48297331/using-request-body-in-custom-modelbinder)
(another related question on stack overflow: https://stackoverflow.com/questions/31389781/read-request-body-twice)

[davidfowl]

Why should it happen in MVC? By default the http request body isn't rewindable.

[ans-ashkan]

Because it's being used by mvc (not me) and I can't care to add a middleware for it?
If the body is available in the action or more important ModelBinder, I consider it to be usable (as it was in non-core asp.net mvc)

[davidfowl]

@ans-ashkan that means buffering the request body by default, which has performance implications. I think it should be opt-in (if we do it at all). You can easily write a filter to allow this for your specific action. It's rare that you read the body twice during a request.

[ans-ashkan]

@davidfowl I agree, performance matters. the option to enable buffered request body can be useful though.
Another question, when/where that read happens (in MVC).
Would you please show me the code for the filter you mentioned?

[mkArtakMSFT]

Hi @ans-ashkan. What you're looking for can be achieved using Resource Filters, as they execute before Model Binding consumes the request body. You can get more details about Resource Filters at: https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/filters.

As for baking this functionality into ASP.NET Core, we don't want to do that as we haven't heard much need of it, given the easy filters-based solution can be implemented.