[HttpGet]
    public JsonResult AddTextFile(string path)
    {
        if(string.IsNullOrEmpty(path))
        {
            // return error
        }
    }

[HttpGet]
public JsonResult AddTextFile(MyObject obj) {
    if(!ModelState.IsValid) {
        // return error
    }
}

public class MyObject {
    [Required]
    public string Path { get; set; }
}