public async Task<ActionResult> GetPostCountInCategories(int[] categoryIds)
{
    var categories = await categoryService
        .GetCategoriesWithPostCount(categoryIds)
        .Select(c => new { c.CategoryId, c.PostCount })
        .ToDictionaryAsync(k => k.CategoryId, v => v.PostCount);
                         
    return Json(categories);
}

字典的序列化/反序列化不支持类型“System.Collections.Generic.Dictionary`2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]”，键必须为字符串或对象。