我有一个React Native（Expo运行在Android上的Expo Go上）客户端和一个ASP。NET Core Web API服务器。我的客户端发送内容类型为multipart/form-data的请求，并为每个字段提供正确的值。但是当请求到达服务器时，请求（具有该字段可为空的模型）似乎丢失了字段的值并变为空，尽管发送了一个真实的值。
客户端（React Native移动的应用，使用axios而不是fetch）：

// NationalId, Address, Phone, BirthDate are all strings. BirthDate is possibly undefined.
const formData = new FormData();
formData.append('NationalId', NationalId);
formData.append('Address', Address);
formData.append('Phone', Phone);
if (BirthDate) {
    formData.append('BirthDate', BirthDate);
}
if (Image) {
    formData.append('Image', Image);
}
        
const res = await axios.put(
  `Employees/UpdateSelf/${NationalId}`,
  formData,
  { headers: { 'Content-Type': 'multipart/form-data' } }
);

服务器：

public class UpdateSelfRequest
{
    public string NationalId { get; set; } = string.Empty;
    public string? Phone { get; set; }
    public string? Address { get; set; }
    public DateTimeOffset? BirthDate { get; set; }
    public IFormFile? Image { get; set; }
}

[HttpPut("UpdateSelf/{NationalId}")]
[Authorize(Roles = AuthRoles.Employee)]
[Consumes("multipart/form-data")]
public async Task<IActionResult> UpdateSelf(
    [FromRoute] string NationalId,
    [FromForm] UpdateSelfRequest req)
{
    // -----> Here, req.BirthDate is null despite the client sending a value
    var result = await _service.UpdateSelf(NationalId, req);
    if (!result.Success)
    {
            return Forbid();
    }

    return Ok();
}

一些日志：

Attempting to bind parameter 'req' of type 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest' ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.ComplexObjectModelBinder[44]
      Attempting to bind parameter 'req' of type 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest' using the name '' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[13]
      Attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.NationalId' of type 'System.String' using the name 'NationalId' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[14]
      Done attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.NationalId' of type 'System.String'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[13]
      Attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Phone' of type 'System.String' using the name 'Phone' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[14]
      Done attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Phone' of type 'System.String'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[13]
      Attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Address' of type 'System.String' using the name 'Address' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[14]
      Done attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Address' of type 'System.String'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[13]
      Attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.BirthDate' of type 'System.Nullable`1[System.DateTimeOffset]' using the name 'BirthDate' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[15]
      Could not find a value in the request with name 'BirthDate' for binding property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.BirthDate' of type 'System.Nullable`1[System.DateTimeOffset]'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder[14]
      Done attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.BirthDate' of type 'System.Nullable`1[System.DateTimeOffset]'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.FormFileModelBinder[13]
      Attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Image' of type 'Microsoft.AspNetCore.Http.IFormFile' using the name 'Image' in request data ...
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.FormFileModelBinder[21]
      No files found in the request to bind the model to.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.FormFileModelBinder[21]
      No files found in the request to bind the model to.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.FormFileModelBinder[14]
      Done attempting to bind property 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest.Image' of type 'Microsoft.AspNetCore.Http.IFormFile'.
dbug: Microsoft.AspNetCore.Mvc.ModelBinding.Binders.ComplexObjectModelBinder[45]
      Done attempting to bind parameter 'req' of type 'Capstone.Features.EmployeeModule.Models.UpdateSelfRequest'.