ASP.NET Core嵌套模型[FromForm]无法绑定问题咨询

有以下两个模型类：

public class TokenRequestModel
{
    [FromHeader(Name = HeaderName.RequestId)]
    public Guid? RequestId { get; set; }

    [FromHeader(Name = HeaderName.Authorization), Required, SwaggerParameter(DocConstants.Authorization, Required = true)]
    public required string Authorization { get; set; }

    [FromRoute(Name = RouteConstants.AuthorizationServerId)]
    public required string AuthorizationServerId { get; set; }
}

public class TokenRequest
{
    [FromForm(Name = "code")]
    public string? Code { get; set; }

    [FromForm(Name = "refresh_token")]
    public string? RefreshToken { get; set; }

    [FromForm(Name = "grant_type")]
    public required string GrantType { get; set; }

    [FromForm(Name = "redirect_uri")]
    public string? RedirectUri { get; set; }
}

当将两个模型分别作为Action参数传入时，绑定可正常完成：

public async Task<ActionResult<TokenResponse>> Exchange(
    TokenRequestModel tokenRequestModel,
    [FromForm]
    TokenRequest tokenRequest)        
{ }

但将TokenRequest作为属性嵌套到TokenRequestModel中后，绑定失败：

public class TokenRequestModel
{
    [FromHeader(Name = HeaderName.RequestId)]
    public Guid? RequestId { get; set; }

    [FromHeader(Name = HeaderName.Authorization)]
    public required string Authorization { get; set; }

    [FromRoute(Name = RouteConstants.AuthorizationServerId)]
    public required string AuthorizationServerId { get; set; }

    [FromForm]
    public TokenRequest? TokenRequest { get; set; }
}

对应的Action方法：

public async Task<ActionResult<TokenResponse>> Exchange(
    TokenRequestModel tokenRequestModel)        
{ }

预期通过URL编码表单数据，使用[FromForm]将数据绑定到TokenRequest属性，但该属性始终为null。此前使用[FromBody]处理JSON数据时绑定正常，询问该场景是否不被支持还是操作有误。

这是ASP.NET Core表单绑定的命名规则导致的问题，并非场景不被支持。

原因分析 #

当使用嵌套对象进行表单绑定时，ASP.NET Core默认会期望表单字段名采用父属性名+子属性名的格式（比如TokenRequest.Code），但当前提交的表单字段是code、grant_type这类平级名称，和绑定规则不匹配，所以TokenRequest属性无法被正确赋值，始终为null。

而使用[FromBody]处理JSON时，JSON的层级结构天然匹配嵌套对象，所以绑定正常；分开作为参数时，[FromForm]直接作用在TokenRequest上，会直接匹配平级的表单字段，因此也能正常绑定。

解决方案 #

有两种可行的处理方式：

1. 修改表单提交的字段名
   提交表单时，将字段名改为嵌套格式，比如：

   • TokenRequest.Code
   • TokenRequest.GrantType
   • TokenRequest.RefreshToken
   • TokenRequest.RedirectUri

2. 手动映射属性
   如果无法修改提交的字段格式，可以在TokenRequestModel中添加与表单字段对应的直接属性，然后手动映射到TokenRequest对象中，示例：

   public class TokenRequestModel
   {
       [FromHeader(Name = HeaderName.RequestId)]
       public Guid? RequestId { get; set; }
   
       [FromHeader(Name = HeaderName.Authorization)]
       public required string Authorization { get; set; }
   
       [FromRoute(Name = RouteConstants.AuthorizationServerId)]
       public required string AuthorizationServerId { get; set; }
   
       [FromForm(Name = "code")]
       public string? Code { get; set; }
   
       [FromForm(Name = "refresh_token")]
       public string? RefreshToken { get; set; }
   
       [FromForm(Name = "grant_type")]
       public required string GrantType { get; set; }
   
       [FromForm(Name = "redirect_uri")]
       public string? RedirectUri { get; set; }
   
       public TokenRequest? TokenRequest => new TokenRequest
       {
           Code = Code,
           RefreshToken = RefreshToken,
           GrantType = GrantType,
           RedirectUri = RedirectUri
       };
   }
   

内容的提问来源于stack exchange，提问作者Mark Good