demo.cs

[WebMethod]
public static List<Type> GetTypeList(string categoryId)
{
    var types = new List&ltType&gt
    {
    	new Type(){ Name = "Red", Id=1, CategoryId=5 },
        new Type(){ Name = "Blue", Id=2, CategoryId=5 },
        new Type(){ Name = "Green", Id=3, CategoryId=3 }
    };
    var catId = Convert.ToInt32(categoryId);
    types = types.Select(p => p.CategoryId == catId).ToList();
    return types;
}