Validate an non-Input model in ASPcore

[amira133]

I have an action-method that have an object type Input like this:

public async Task<IActionResult> DoSomeThing([FromBody]object input, bool options)

{
    if (options == true)
    {
        var castedInput = (A) input;
        if (TryValidateModel(castedInput))
        {
            // do some thing
        }
        else
        {
            //return validation Errors;
            //forexample:return Error("Error1")
            //??!??!!??!?!?!?!??!
        }
    }
    else
    {
        var castedInput = (B)input;
        if (TryValidateModel(castedInput))
        {
            // do some thing
        }
        else
        {
            //return validation Errors;
            //forexample:return Error("You must fill this parameter")
            //??!??!!??!?!?!?!??!

        }
    }
}

In this method first I cast Input to my ViewModel then validate it. now I want to return my Validation Errors that I set on annotations of my model.
How can I do this?

My Viewmodels:

public class A
{
    [Required(ErrorMessage = "Error1")]
    string Phone;
    .
    .
    .
}

public class B
{
    [Required(ErrorMessage = "You must fill this parameter")]
    string Name;
    .
    .
    .  
}