Data annotation trigger validation on Get method - asp.net-mvc

Question

I have this viewmodel
public class ProductViewModel : BaseViewModel
{
public ProductViewModel()
{
Categories = new List<Categorie>
}
[Required(ErrorMessage = "*")]
public int Code{ get; set; }
[Required(ErrorMessage = "*")]
public string Description{ get; set; }
[Required(ErrorMessage = "*")]
public int CategorieId { get; set; }
public List<Categorie> Categories
}
My controller like this
[HttpGet]
public ActionResult Create(ProductViewModel model)
{
model.Categories = //method to populate the list
return View(model);
}
The problem is, as soon as the view is exhibited, the validation is fired.
Why this is happening?
Thanks in advance for any help.
Update
The view is like this
#using (Html.BeginForm("Create", "Product", FormMethod.Post, new { #class = "form-horizontal", #role = "form" }))
{
<div class="form-group">
<label for="Code" class="col-sm-2 control-label">Code*</label>
<div class="col-sm-2">
#Html.TextBoxFor(x => x.Code, new { #class = "form-control"})
</div>
</div>
<div class="form-group">
<label for="Description" class="col-sm-2 control-label">Desc*</label>
<div class="col-sm-2">
#Html.TextBoxFor(x => x.Description, new { #class = "form-control", maxlength = "50" })
</div>
</div>
<div class="form-group">
<label class="col-sm-2 control-label">Categorie*</label>
<div class="col-sm-4">
#Html.DropDownListFor(x => x.CategorieId, Model.Categories, "Choose...", new { #class = "form-control" })
</div>
</div>

Answer 1

You GET method has a parameter for your model, which means that the DefaultModelBinder initializes and instance of the model and sets its properties based on the route values. Since your not passing any values, all the property values are null because they all have the [Required] attribute, validation fails and ModelState errors are added which is why the errors are displayed when the view is first rendered.
You should not use a model as the parameter in a GET method. Apart from the ugly query string it creates, binding will fail for all properties which are complex objects and collections (look at you query string - it includes &Categories=System.Collections.Generic.List<Categorie> which of course fails and property Categories will be the default empty collection). In addition, you could easily exceed the query string limit and throw an exception.
If you need to pass values to the GET method, for example a value for Code, then you method should be
[HttpGet]
public ActionResult Create(int code)
{
ProductViewModel model = new ProductViewModel
{
Code = code,
Categories = //method to populate the list
};
return View(model);
}