[HttpPost]
public async Task<ActionResult<Todo PostTodo(Todo todo)
{
  if (string.IsNullOrEmpty(todo.Name))
  {
    return Ok("名称不能为空");
  }
  context.Todo.Add(todo);
  await context.SaveChangesAsync();

  return CreatedAtAction("GetTodo", new { id = todo.Id }, todo);
}

namespace App001.Models
{
  /// <summary>
  /// 待办事项
  /// </summary>
  public class Todo
  {
    /// <summary>
    /// ID
    /// </summary>
    public Guid Id { get; set; }
    /// <summary>
    /// 名称
    /// </summary>
    [Required(ErrorMessage = "名称不能为空")]
    public string Name { get; set; }
  }
}

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "|df184e36-4e11844dfd38a626.",
  "errors": {
    "Name": [
      "名称不能为空"
    ]
  }
}

[Required(ErrorMessage = "名称不能为空")]

public class ModelValidateActionFilterAttribute : ActionFilterAttribute
{
  public override void OnActionExecuting(ActionExecutingContext context)
  {
    if (!context.ModelState.IsValid)
    {
      //获取验证失败的模型字段
      var errors = context.ModelState
        .Where(e => e.Value.Errors.Count > 0)
        .Select(e => e.Value.Errors.First().ErrorMessage)
        .ToList();

      var str = string.Join("|", errors);

      //设置返回内容
      var result = new
      {
        Code = 10000,
        Msg = "未通过数据验证。",
        FullMsg = str
      };

      context.Result = new BadRequestObjectResult(result);
    }

  }
}

//关闭默认模型验证
services.Configure<ApiBehaviorOptions>(opt => opt.SuppressModelStateInvalidFilter = true);
services.AddControllers(opt =>
{
  //添加过滤器
  opt.Filters.Add(typeof(ModelValidateActionFilterAttribute));
}).AddNewtonsoftJson(opt =>
{
  //json字符串大小写原样输出
  opt.SerializerSettings.ContractResolver = new DefaultContractResolver();
});

{
  "Code": 10000,
  "Msg": "未通过数据验证。",
  "FullMsg": "名称不能为空。"
}

services.Configure<ApiBehaviorOptions>(opt =>
{
  opt.InvalidModelStateResponseFactory = actionContext =>
  {
    //获取验证失败的模型字段 
    var errors = actionContext.ModelState
      .Where(e => e.Value.Errors.Count > 0)
      .Select(e => e.Value.Errors.First().ErrorMessage)
      .ToList();

    var str = string.Join("|", errors);

    //设置返回内容
    var result = new
    {
      Code = 10000,
      Msg = "未通过数据验证。",
      FullMsg = str
    };

    return new BadRequestObjectResult(result);
  };
});