public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }
 
        public IConfiguration Configuration { get; }
 
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            //重写Json非字符串读取到对象字符串属性
            services.AddMvc().AddJsonOptions(opts =>
            {
                var stringConverter = new StringConverter();
                opts.JsonSerializerOptions.Converters.Add(stringConverter);
            });
        }
}

/// <summary>
    /// Json任何类型读取到字符串属性
    /// 因为 System.Text.Json 必须严格遵守类型一致，当非字符串读取到字符属性时报错：
    /// The JSON value could not be converted to System.String.
    /// </summary>
    public class StringConverter : System.Text.Json.Serialization.JsonConverter<string>
    {
        public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
 
            if (reader.TokenType == JsonTokenType.String)
            {
                return reader.GetString();
            }
            else
            {//非字符类型，返回原生内容
                return GetRawPropertyValue(reader);
            }
 
            throw new JsonException();
        }
 
        public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
        {
            writer.WriteStringValue(value);
        }
        /// <summary>
        /// 非字符类型，返回原生内容
        /// </summary>
        /// <param name="jsonReader"></param>
        /// <returns></returns>
        private static string GetRawPropertyValue(Utf8JsonReader jsonReader)
        {
            ReadOnlySpan<byte> utf8Bytes = jsonReader.HasValueSequence ?
            jsonReader.ValueSequence.ToArray() :
            jsonReader.ValueSpan;
            return Encoding.UTF8.GetString(utf8Bytes);
        }
    }

因为ASP.NET Core 5.0 内置的System.Text.Json 转换必须严格规定类型一致，所以当为整数的值转换为字符串时会Json格式转换错误，所以必须用上面的代码处理下，重写字符串转换方法，让所有类型都可以存到对象的字符串属性。