您不能用相同的路线定义两个动作。动作选择器不考虑其参数类型。因此，为什么不合并此操作；

You can't define two actions with same route. Action Selector doesn't consider their parameter types. So, why don't you merge this actions;

public async Task<IActionResult> postObj([FromBody]EntireData data)
{
    if (data.FirstClass != null)
    {
        //Do something
    }
    if (data.SecondClass != null)
    {
        //Do something
    }
}

public class EntireData
{
    public FirstClass  firstClass { get; set; }

    public SecondClass secondClass { get; set; }
}