AjaxControlToolkit has methods to access ViewState:

protected V GetPropertyValue<V>(string propertyName, V nullValue)
{
  if (this.ViewState[propertyName] == null)
  {
    return nullValue;
  }

  return (V) this.ViewState[propertyName];
}

protected void SetPropertyValue<V>(string propertyName, V value)
{
  this.ViewState[propertyName] = value;
}

...

public bool EnabledOnClient
{
  get { return base.GetPropertyValue("EnabledOnClient", true); }
  set { base.SetPropertyValue("EnabledOnClient", value); }
}

We find that code unnecessary complex and nonoptimal. Our code to access ViewState looks like this:

public bool EnabledOnClient
{
  get { return ViewState["EnabledOnClient"] as bool? ?? true); }
  set { ViewState["EnabledOnClient"] = value; }
}