1.LINQ過濾數據?

var query = from person in context.Peoplewhere person.FirstName.StartsWith("a")select person;
var methodQuery = context.People.Where(p => p.FirstName.StartsWith("a"));

兩種不同的寫法，效果一樣。

• 多條件組合查找

var query = from person in context.Peoplewhere person.FirstName.StartsWith("a") &&person.LastName.EndsWith("b")select person;
var methodQuery = context.People.Where(p => p.FirstName.StartsWith("a") && p.LastName.EndsWith("b"));

• 對查找結果排序升序：

            var query = from person in context.Peoplewhere person.FirstName.StartsWith("a") &&person.LastName.EndsWith("b")orderby person.FirstName,person.LastNameselect person;var methodQuery = context.People.Where(p => p.FirstName.StartsWith("a") && p.LastName.EndsWith("b")).OrderBy(p => p.FirstName).ThenBy(p => p.LastName);

• 對查找結果排序降序：

            var query = from person in context.Peoplewhere person.FirstName.StartsWith("a") &&person.LastName.EndsWith("b")orderby person.FirstName,person.LastNamedescendingselect person;var methodQuery = context.People.Where(p => p.FirstName.StartsWith("a") && p.LastName.EndsWith("b")).OrderByDescending(p => p.FirstName).ThenByDescending(p => p.LastName);

• 延遲加載和貪婪加載(Lazy Loading和eager loading)

如果不確定是否要加載關聯數據時就使用延遲加載，否則就使用貪婪加載。當然這個不是固定的看具體什么場景下使用。默認情況下延遲加載是生效的，可以在創建DbContex后配置選項

context.Configuration.LazyLoadingEnabled = false;

2.插入數據

多種方法，不多說看代碼：

• 方法一

var person = new Person
{
BirthDate = new DateTime(1980, 1, 2),
FirstName = "John",
HeightInFeet = 6.1M,
IsActive = true,
LastName = "Doe",
MiddleName = "M"
};
person.Phones.Add(new Phone { PhoneNumber = "1-222-333-4444" });
person.Phones.Add(new Phone { PhoneNumber = "1-333-4444-5555" });
using (var context = new Context())
{
context.People.Add(person);
context.SaveChanges();
}

方法二：改變Entity的狀態

using (var context = new Context())
{
context.Entry(person2).State = EntityState.Added;
context.SaveChanges();
}

EntityState:Added、Deleted、Detached(DbContex不追蹤Entity狀態)、Modified、Unchanged

3.更新數據

多種方法：

方法一：

using (var context = new Context())
{
var person = context.People.Find(1);
person.FirstName = "New Name";
context.SaveChanges();
}

方法二（建議使用）注意標記部分：

var person2 = new Person
{
PersonId = 1,
BirthDate = new DateTime(1980, 1, 2),
FirstName = "Jonathan",
HeightInFeet = 6.1m,
IsActive = true,
LastName = "Smith",
MiddleName = "M"
};
person2.Phones.Add(new Phone
{
PhoneNumber = "updated 1",
PhoneId = 1,
PersonId = 1
});
person2.Phones.Add(new Phone
{
PhoneNumber = "updated 2",
PhoneId = 2,
PersonId = 1
});
using (var context = new Context())
{
context.Entry(person2).State = EntityState.Modified;
context.SaveChanges();
}

你會發現執行上面代碼只有Person數據更新了，Phone數據沒有更新，這是因為Insert和Update機制不一樣，Update時設置了Entity的EntityState,但是并沒有傳播到子數據的狀態，需要都上面程序做些修改

using (var context = new Context())
{
context.Entry(person2).State = EntityState.Modified;
foreach (var phone in person2.Phones)
{
context.Entry(phone).State = EntityState.Modified;
}
context.SaveChanges();
}

在開發Web程序時可以使用AsNoTracking來提高查詢性能

using (var context = new Context())
{
var query = context.People.Include(p =>
p.Phones).AsNoTracking();
foreach (var person in query)
{
foreach (var phone in person.Phones)
{
}
}
}

• 使用Attach 會改變狀態為Unchanged，并開始追蹤Entity的狀態

var person3 = new Person
{
PersonId = 1,
BirthDate = new DateTime(1980, 1, 2),
FirstName = "Jonathan",
HeightInFeet = 6.1m,
IsActive = true,
LastName = "Smith",
MiddleName = "M"
};
using (var context = new Context())
{
context.People.Attach(person3);
person3.LastName = "Updated";
context.SaveChanges();
}

上面代碼執行將僅僅只會更新LastName列。也可以用以下代碼替代Attach方法

context.Entry(person3).State = EntityState.Unchanged

4.刪除數據

• 方法一：先查詢出數據，再刪除（注意刪除子數據可以使用設置數據庫級聯刪除更方便）

using (var context = new Context())
{
var toDelete = context.People.Find(personId);
toDelete.Phones.ToList().ForEach(phone =>context.Phones.Remove(phone));
context.People.Remove(toDelete);
context.SaveChanges();
}

• 方法二：使用改變狀態

var toDeleteByState = new Person { PersonId = personId };
toDeleteByState.Phones.Add(new Phone
{
PhoneId = phoneId1,
PersonId = personId
});
toDeleteByState.Phones.Add(new Phone
{
PhoneId = phoneId2,
PersonId = personId
});
using (var context = new Context())
{
   context.People.Attach(toDeleteByState);
   foreach (var phone in toDeleteByState.Phones.ToList()){context.Entry(phone).State = EntityState.Deleted;}context.Entry(toDeleteByState).State = EntityState.Deleted;context.SaveChanges();
}

5.查詢本地數據

當需要查詢的數據已經在內存中，而未提交到數據庫時對內存數據進行查詢

var localQuery = context.People.Local.Where(p =>
p.LastName.Contains("o")).ToList();

轉載于:https://www.cnblogs.com/zjmsky/p/4823847.html

總結

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