新的转换列表方式

var categoriesEntities = new List<CategoryEntity>();
var allCategories = GetCategories();
var postCategoryMappings = _postCategoryMapping.Where(m => m.PostID == postID).ToList();

postCategoryMappings.ForEach(mapping =>
{
　　var category = allCategories.Single(c => c.CategoryID == mapping.CategoryID);
　　var categoryEntity = new CategoryEntity { CategoryID = mapping.CategoryID, CategoryName = category.CategoryName, CategorySlug = category.CategorySlug };
　　categoriesEntities.Add(categoryEntity);
});