我想，你可能会用分组来完成任务。

var groups = test.GroupBy(x => x.Email);
foreach (var group in groups)
{
  var email = group.Key; // Exactly the property from GroupBy
  var goods = group.Select(x => x.Good); // Donut, Bagel, Cake...
  var body = string.Join(", ", goods);

  // Now you can send the email with body containing "Donut, Bagel. Cake"
}

group变数的每个执行严修都包含Key属性和test集合中群组项目的集合。
因此，可以将group与Select之类的方法或foreach运算符一起使用。

List<test> test=new List<test>();
test.Add(new test("Bakery","bakery@store.com","Donut"));
test.Add(new test("Bakery","bakery@store.com","Bagel"));
test.Add(new test("Bakery","bakery@store.com","Cake"));
test.Add(new test("Produce","produce@store.com","Apple"));
test.Add(new test("Dairy","dairy@store.com","Milk"));
test.Add(new test("Dairy","dairy@store.com","Yogurt"));

foreach (var group in test.GroupBy(x=> x.Department))
{
    var department = group.Key;
    var email = group.First().Email;
    var body = string.Join(", ", group.Select(x => x.Product));

    SendMail(email, department, body);
}

如果要 * 分组 *（项目），让我们使用GroupBy（按Department和Address）。首先，让我们准备数据：

var emails = test
  .GroupBy(item => (department : item.Department, address : item.Address),
           item => item.Item)
  .Select(group => (department : group.Key.department,
                    address    : group.Key.address,
                    body       : string.Join(", ", group.Distinct().OrderBy(item => item))));

然后我们就可以发送邮件了：

foreach (var letter in emails)
  SendEmail(letter.address, letter.deparment, letter.body);

您确定部门为Bakery的所有tests都有相同的电子邮件地址吗？或者可能是（例如，由于键入错误）某些Bakeries有不同的电子邮件地址？
如果所有Bakeries都有相同的地址，那么您可以使用Enumerable.GroupBy的重载之一来创建具有相同部门的测试组。然后，对于每个组，获取该组中第一个测试的电子邮件（或者任何其他测试，反正它们都是相同的）。使用带有参数resultselector的重载来指定您的结果

class Test
{
    public string Department {get; set;}
    public string EmailAddress {get; set;}
    ... // other properties
}

IEnumerable<Test> myTests = ...

var departmentsEmailAddresses = myTests.GroupBy(

    // make groups of test that have the same value for Department
    test => test.Department,

    // parameter resultSelector: for every Department name and all Tests that have
    // this value for Department name, make one new object:
    (department, testsWithThisValueForDepartment) => new
    {
        // if desired: remember the department name
        Department = department,

        // to get the e-mail address, take the first test.
        // BTW: all tests in this group will have the same e-mail address
        Email = testsWithThisValueForDepartment.Select(test => test.EmailAddress)
                                               .FirstOrDefault(),
    })

换句话说：对于所有的测试，将属性Department的值相同的测试分组。每组中至少有一个测试。从每组中，取该组中所有测试将具有的Department的公共值。此外，取该组中的测试，并选择EmailAddress的值。仅对组中的第一个测试执行此操作。
结果将是[Department, Email]的组合序列