Html.NameFor does not correctly generate name when field has two underscores

[stefanodelpero]

Fields with double underscore in name (I've to use a legacy system fields naming) disappear in Request.Form.

So, if I have an object like this:

public class Company
{
  public string Field__Name { get; set; } // 2 or more underscore between Field and Name
}

And a cshtml page with:

<input asp-for="Field__Name" />

The POST form action send the form value to the server but ASP.NET Request.Form didn't have the Field__Name field. Also the field is null in the OnPost parameter object.

Further technical details

• ASP.NET Core 3.1

[Tratcher]

Can you share a Fiddler trace showing the raw form post?

[stefanodelpero]

With this class:

public class Company2
{
  public string Field__Name { get; set; }
  public string OtherField { get; set; }
}

and these values in form:
Field__Name = 111
OtherField = 222

This is the raw form post:

POST https://localhost:44310/ HTTP/1.1
Host: localhost:44310
Connection: keep-alive
Content-Length: 262
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4041.0 Safari/537.36 Edg/81.0.410.1
Content-Type: application/x-www-form-urlencoded
Origin: https://localhost:44310
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: same-origin
Sec-Fetch-Dest: empty
Referer: https://localhost:44310/
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,it;q=0.8
Cookie: .AspNetCore.Antiforgery.VFg60T-jNbs=CfDJ8IGu5-fGa-hMvjpjhbbwAT2wBeUpFLjtv2Zl7tNxsdplZxqaBJMLl7LN5gaZpezeC0d3CfSu7m2vFYeecBzjtCSNPMDCWyY-kKt2zPbggCon8os9n9eVFEuwWTuUHPIwSmpnAEZK8IxIGYkC_U9jxlM; .AspNetCore.Identity.Application=CfDJ8IGu5-fGa-hMvjpjhbbwAT28zMtnMGXxqIiBurU1FfRmocqgJ7_IrrebLKfuimpwt7PLsNcyN7MjIzD9ld9SFbolBX6lCelBeJMHSO3Tcr1RCpr-YDUgDcjVvRytSbEqsU4O2_hSlIU5fttDbN_mo0gSqZzenxU2OAI8GS3WEF-vJcJZT5XRoxlsD1yb9NiXLdQTC9UWX3LDzqn8eFiQ8S00kOxncR1ul0WKaPYvzswFjCi9k8G_fQQ5Clcjf2JflVJhCbt2FSyw36GvhAumda-2i0T51VQTLStTBR3HFEB8CmqXnZXOVvqnriRZWl_rpf5ioBfTiVbGsRU6Fs_dV1YYXD0r8FYtuJOP6K05xeznlxS1Yr1Y1Mg02ZFNEHATYajW2ohmqmdRtPqaWqOf44TmKhnntIB6yqOCKArU3-J63AKQxsfegKCwx2-3zKgBf995ghizKeByyFACtIPNKS6OZd6RH8GqZC2lekDPhaIiUnYSPXDY8LMumiJx6vKng6VfOdHv2_WpDbntkofMBJyzJKyywpX9SqRl2IT08DU1mBZk_Z98_PRPzeSuHWCOXwRMzjWdCN1R_LsOdHOmbS0CgROgYguQih2IPG5LS5POTotcdk4sOcpqzI3A7TvbyvXm5SyMlLuBclKYZpVWWuCM_6mwDsHsvrfGi2LReMZvHjp9IvQ7e3kGME4XW6yr5A

Company2=111&Company2.OtherField=222&__RequestVerificationToken=CfDJ8IGu5-fGa-hMvjpjhbbwAT3u0fwP-rO_dLGaAajzfdlj_bGVAlSfQwFxlqeTQ99vdgvltQEZ6eYM98RAeZer8iJJHZ2DsqwDxl4FGMb_17ftklQTSDFWudJxH8TMkYWrwQ_zMiuKaCG09XreKVF0HctHzxGMSS5N71kK-1r9wYb9xCYyvP1ZCTNC5jQwKey5lg

[stefanodelpero]

This is the HTML generated by ASP.NET Core:

<div class="row">
    <div class="col">
        <div class="form-group">
            <label for="Company2">Field__Name</label>
            <input class="form-control" autofocus type="text" id="Company2" name="Company2" value="" />
            <span class="text-danger field-validation-valid" data-valmsg-for="Company2" data-valmsg-replace="true"></span>
        </div>

        <div class="form-group">
            <label for="Company2_OtherField">OtherField</label>
            <input class="form-control" type="text" id="Company2_OtherField" name="Company2.OtherField" value="" />
            <span class="text-danger field-validation-valid" data-valmsg-for="Company2.OtherField" data-valmsg-replace="true"></span>
        </div>
    </div>
</div>

[Tratcher]

Values in the form POST:
Company2=111
Company2.OtherField=222
__RequestVerificationToken=CfDJ8IGu5...

So this isn't a problem with parsing the form data on the server, it's a problem with generating the correct HTML to submit the form?

Moving back to MVC.

[stefanodelpero]

Yes.
Company2.Field__Name become

<input type="text" id="Company2" name="Company2" value="" />

instead of

<input type="text" id="Company2_Field__Name" name="Company2.Field__Name" value="" />

[javiercn]

@epic2001 thanks for contacting us.

I'm not sure if this is a bug or it is a consequence of the current design. We will look into this and get back to you.

[stefanodelpero]

Ok! Thank you!

[pranavkm]

@stefanodelpero looks like asp-for has some specific code paths for properties that contain __. The background seems to be that __ appear frequently in compiler generated types which appear commonly in lambda expressions and caused MVC to generate incorrect names. aspnet/Mvc#2890, aspnet/Mvc@a045324.

A fix for this could attempt to inspect the declaring type has a compiler generated attribute here:

aspnetcore/src/Mvc/Mvc.ViewFeatures/src/ExpressionHelper.cs

Line 93 in 37da482

if (name.Contains("__"))

. That said, given this is the first we've heard of this bug, and we don't have enough test coverage to verify changing this doesn't affect other esoteric scenarios, I'm hesitant to address this right away. We'll park this in the backlog and reconsider this if we get further feedback.

In the meanwhile, there are a couple of options to workaround this:

a) Use a different property name
b) Explicitly specify the field name:
<input type="text" id="Company2_Field__Name" name="Company2.Field__Name" value="" /> (from your sample)

[mkArtakMSFT]

Hi. Thanks for contacting us.
We're closing this issue as there was not much community interest in this ask for quite a while now.
You can learn more about our triage process and how we handle issues by reading our Triage Process writeup.