如果您遇到以下情况：

List<ItemBO> items;
List<ItemCategoryBO> categories;

并且您希望获取类别在您的类别列表中的所有项目，则可以使用以下方法：

IEnumerable<ItemBO> result = items.Where(item =>
    categories.Any(category => category.ItemCategory.equals(item.ItemCategory)));

Any()运算符枚举源序列，并在项满足 predicate 给定的测试时返回true。在这种情况下，如果类别列表包含ItemCategoryBO，且其ItemCategory字符串与项的ItemCategory字符串相同，则该运算符返回true。有关此运算符的详细信息，请访问MSDN

尝试使用一些链接

List<ItemBO> itm = new List<ItemBO>;
 //Fill itm with data

 //get selected item from control

 string selectedcategory = cboCatetories.SelectedItem;

 var itms = from BO in itm where itm.ItemCategory = selectedcategory                              select itm;

itms now contains all items in that category

这是我在Linqpad中做的事情

void Main()
{

    var cat1 = new ItemCategoryBO {ItemCategory="c1", Title = "c1"};
    var cat2 = new ItemCategoryBO {ItemCategory="c2", Title = "c2"};

    var item1 = new ItemBO { ItemId = 1, Title = "item1", ItemCategory="c1"};
    var item2 = new ItemBO { ItemId = 1, Title = "item2", ItemCategory="c2"};
    var item3 = new ItemBO { ItemId = 1, Title = "item3", ItemCategory="c2"};
    var item4 = new ItemBO { ItemId = 1, Title = "item4", ItemCategory="c3"};

    var items = new List() {item1, item2, item3, item4};
    var categories = new List() {cat1, cat2};

    var itemsInCategory = from item in items 
    join category in categories on  item.ItemCategory equals category.ItemCategory into itemInCategory
    from categoryItem in itemInCategory
    select new {item.Title, item.ItemCategory};

    itemsInCategory.Dump(); 
}

// Define other methods and classes here
      public class ItemCategoryBO
        {
           public string ItemCategory { get; set; }
           public string Title { get; set; }
        }

        public class ItemBO
        {
           public int ItemId { get; set; }
           public string Title { get; set; }
           public string ItemCategory { get; set; } 
        }

这会传回：

Title, ItemCategory
item1 c1 
item2 c2 
item3 c2

试试这个：

List<ItemBO> items = ...;
ItemCategoryBO category = ...;

List<ItemBO> filteredItems = items
    .Where( i => i.ItemCategory.Equals(category) )
    .FirstOrDefault();

更新以解决OP的更新问题：

如果我有一个包含几个类别的列表，我如何在类别列表中找到包含某个类别的项目列表呢？（在我的示例中，我想返回项目2和3）
我认为你应该分两步来做。首先，得到你独特的项目列表。然后，从你的项目中得到你的类别列表。所以：

// First, get the distinct list of items
List<ItemBO> items = new List<ItemBO>();
foreach ( var category in categories )
{
    foreach ( var item in category.Items )
    {
        if ( !items.Contains(item) )
            items.Add(item);
    }
}

// Second, get the list of items that have the category.
List<ItemBO> filteredItems = items
    .Where( i => i.ItemCategory.Equals(category) )
    .FirstOrDefault();

希望这对您有所帮助：

var result = (Object to search in).Where(m => (Object to compare to).Any(r => r.Equals(m.Key)).ToList();