Could i extension the ModelStateDictionary Class in MVC3

in the ModelStateDictionary class there are only AddModelError and Add function , i want extention the class, add the method like AddModeSuccess，AddModelWarning. i have a look at the MVC3 source code and found there a lot of thing need add. i don't want to modify the MVC3 code, i just want to add a extention. how could i do?

    public void Add(KeyValuePair<string, ModelState> item) {
        ((IDictionary<string, ModelState>)_innerDictionary).Add(item);
    }

    public void Add(string key, ModelState value) {
        _innerDictionary.Add(key, value);
    }

    public void AddModelError(string key, Exception exception) {
        GetModelStateForKey(key).Errors.Add(exception);
    }开发者_如何学Python

    public void AddModelError(string key, string errorMessage) {
        GetModelStateForKey(key).Errors.Add(errorMessage);
    }

You could add them as extension methods to the ModelStateDictionary class:

public static class ModelStateExtensions
{
    public static void AddModelSuccess(this ModelStateDictionary modelState, ... some parameters)
    {
        ...
    }

    public static void AddModelWarning(this ModelStateDictionary modelState, ... some parameters)
    {
        ...
    }
}