在使用Linq轉化XML,ActiveDirectory,Datatable,Array,List,Dictionary后意識到Linq的強大。VS居然還提供專門的LINQ Explorer，不覺明厲啊~

好用的 Linq Samples and the Sample Query Explorer

http://blog.miniasp.com/post/2008/05/05/About-Linq-Samples-and-the-Sample-Query-Explorer.aspx

該solution是直接使用MDF文件的，自己裝個Northwind，把connection string改下

AttachDBFileName='C:\Program Files (x86)\Microsoft Visual Studio 10.0\Samples\1033\CSharpSamples\LinqSamples\SampleQueries\Data\NORTHWND.MDF';Server='.\SQLEXPRESS';user instance=true;Integrated Security=SSPI;Connection Timeout=60

Data Source=Localhost;Initial Catalog=Northwind;Integrated Security=True

1.查詢Datatbale的值

var q = from dr in ds.Tables[0].AsEnumerable()

select new {Name = dr["Name"].ToString().Trim(),Age=dr["Age"].ToString().Trim()};

2.Join multiple string for group by rows

var employees = new List<Employee>();

            employees.Add(new Employee { Name = "A", Age = 30, ContactNo = "New Contact Number A", Salary = 20000 });

            employees.Add(new Employee { Name = "A", Age = 30, ContactNo = "New Contact Number A", Salary = 20000 });

            employees.Add(new Employee { Name = "B", Age = 31, ContactNo = "New Contact Number B", Salary = 30000 });

            employees.Add(new Employee { Name = "C", Age = 32, ContactNo = "New Contact Number C", Salary = 40000 });

            employees.Add(new Employee { Name = "C", Age = 32, ContactNo = "New Contact Number C", Salary = 40000 });

 var customers = new List<Customer>();

            customers.Add(new Customer { Name = "A", Age = 30, ContactNo = "Old Contact Number A" });

            customers.Add(new Customer { Name = "B", Age = 31, ContactNo = "New Contact Number B" });

  var q2 = from e in employees

                    group e by new { name = e.Name, age = e.Age } into pg

                    select new Customer

                    {

                        Name = pg.Key.name,

                        Age = pg.Key.age,

                        ContactNo = String.Join(",", pg.Select(o => o.ContactNo).ToList())

                    };

3.對List<T>的distinct操作必須定義GetHashCode() 和Equals方法才能成功

不然的話，就只能用group by了

public partial class Person

    {

        public String Name { get; set; }

        public Int AGe { get; set; }

        public override bool Equals(object obj)

        {

            if (obj == null)

                return false;

            if (this.GetType() != obj.GetType())

                return false;

            return Equals(obj as Person);

        }

        private bool Equals(Person p)

        {

            return (this.Name == =p.Name && this.Age == p.Age);

        }

        public override int GetHashCode()

        {

            return this.GetHashCode();

        }

    }

4.Left join

var q4 = from e in employees

                     join c in customers on e.Name equals c.Name into g

                     from lg in g.DefaultIfEmpty()

                     select  new { Name = e.Name, Age = e.Age, ContactNo1 = e.ContactNo, ContactNo2 =(lg==null)?"NULL":lg.ContactNo };

Console.WriteLine("Left Join Results:");

foreach (var c in q4)

{

Console.WriteLine(string.Format("Name:{0} Age:{1} ContactNo1:{2} ContactNo2:{3}", c.Name, c.Age, c.ContactNo1, c.ContactNo2));

}

How to: Perform Left Outer Joins (C# Programming Guide)

http://msdn.microsoft.com/en-us/library/bb397895.aspx

5.Update

我比較意外Method1可以成功。。。

// Method1

            employees.Where(w => w.Name == "A" || w.Name == "C").ToList().ForEach(f => f.Salary = 5000);

            Console.WriteLine("Method1:");

            foreach (Employee  e in employees)

            {

                Console.WriteLine(string.Format("Name:{0} Age:{1} ContactNo:{2} Salary:{3}", e.Name, e.Age, e.ContactNo , e.Salary));

            }

            // Method2

            var updateEmps = employees.Where(w => w.Name == "A" || w.Name == "C");

            foreach(Employee e in updateEmps){

                e.Salary = 6000;

            }

            Console.WriteLine("Method2:");

            foreach (Employee e in employees)

            {

                Console.WriteLine(string.Format("Name:{0} Age:{1} ContactNo:{2} Salary:{3}", e.Name, e.Age, e.ContactNo, e.Salary));

            }

6.Multiple fields join

var q3 = from e in employees

join c in customers on new { e.Name, e.Age } equals new { c.Name, c.Age }

select e;

7.Multiple insert linq result int database

.Net Framework 4.5 似乎有AsDatatable()和CopyToDataTable,如果沒有自己寫一個也可以

Converting a List to Datatable

http://social.msdn.microsoft.com/Forums/vstudio/en-US/6ffcb247-77fb-40b4-bcba-08ba377ab9db/converting-a-list-to-datatable

.NET Framework 4.5 CopyToDataTable Method

http://msdn.microsoft.com/en-us/library/system.data.datatableextensions.copytodatatable(v=vs.110).aspx

            public static DataTable ConvertToDataTable<T>(IList<T> data)

        {

            PropertyDescriptorCollection properties =

               TypeDescriptor.GetProperties(typeof(T));

            DataTable table = new DataTable();

            foreach (PropertyDescriptor prop in properties)

                table.Columns.Add(prop.Name, Nullable.GetUnderlyingType(prop.PropertyType) ?? prop.PropertyType);

            foreach (T item in data)

            {

                DataRow row = table.NewRow();

                foreach (PropertyDescriptor prop in properties)

                    row[prop.Name] = prop.GetValue(item) ?? DBNull.Value;

                table.Rows.Add(row);

            }

            return table;

        }

	   DataTable dt = new DataTable();

           String[] colStrs = new string[] { 。。。};

           String[] colDts = new string[] {。。。};

           String[] colInts = new string[] { 。。。};

           foreach (string col in colStrs) { dt.Columns.Add(new DataColumn(col, typeof(string))); }

           foreach (string col in colDts) { dt.Columns.Add(new DataColumn(col, typeof(DateTime))); }

           foreach (string col in colInts) { dt.Columns.Add(new DataColumn(col, typeof(Int32))); }

            using (SqlBulkCopy bulkCopy = new SqlBulkCopy(connectionString))

                    {

                        var  q = from e in db.TestTable

                                where e.Id > 10

                                select e;

                        DataTable dt2 = ConvertToDataTable( q.ToList());// q.CopyToDataTable();

                        bulkCopy.DestinationTableName = "TestTable";

                        foreach (string col in colStrs) { bulkCopy.ColumnMappings.Add(col, col); }

                        foreach (string col in colDts) { bulkCopy.ColumnMappings.Add(col, col); }

                        foreach (string col in colInts) { bulkCopy.ColumnMappings.Add(col, col); }

                        bulkCopy.WriteToServer(dt2);

                        }

8.   Create XML document

public void LinqToSqlLocalMethodCall02()

{

    XDocument doc = new XDocument(

        new XElement("Customers", from c in db.Customers

                                  where c.Country == "UK" || c.Country == "USA"

                                  select (new XElement("Customer",

                                      new XAttribute("CustomerID", c.CustomerID),

                                      new XAttribute("CompanyName", c.CompanyName),

                                      new XAttribute("InterationalPhone", PhoneNumberConverter(c.Country, c.Phone))

                                      ))));

Console.WriteLine(doc.ToString());

}

9. Sum()

public void LinqToSqlCount03() {

var q = db.Orders.Select(o => o.Freight).Sum();

Console.WriteLine(q);

}

public void LinqToSqlCount04() {

var q = db.Products.Sum(p => p.UnitsOnOrder);

Console.WriteLine(q);

}

10.group by count()

public void LinqToSqlJoin05() {

    var q =

        from c in db.Customers

        join o in db.Orders on c.CustomerID equals o.CustomerID into orders

        select new {c.ContactName, OrderCount = orders.Count()};

ObjectDumper.Write(q);

}

11.union

public void LinqToSqlUnion01() {

    var q = (

             from c in db.Customers

             select c.Phone

            ).Concat(

             from c in db.Customers

             select c.Fax

            ).Concat(

             from e in db.Employees

             select e.HomePhone

            );

ObjectDumper.Write(q);

}

public void LinqToSqlUnion04() {

    var q = (

             from c in db.Customers

             select c.Country

            ).Intersect(

             from e in db.Employees

             select e.Country

            );

ObjectDumper.Write(q);

}

public void LinqToSqlUnion05() {

    var q = (

             from c in db.Customers

             select c.Country

            ).Except(

             from e in db.Employees

             select e.Country

            );

ObjectDumper.Write(q);

}

12.Like

public void LinqToSqlSqlMethods01()

{

var q = from c in db.Customers

where SqlMethods.Like(c.CustomerID, "C%")

select c;

ObjectDumper.Write(q);

}

13.DateDiffDay

public void LinqToSqlSqlMethods02()

{

var q = from o in db.Orders

where SqlMethods.DateDiffDay(o.OrderDate, o.ShippedDate) < 10

select o;

ObjectDumper.Write(q);

}

14.CompiledQuery

public void LinqToSqlCompileQuery01()

{

    //Create compiled query

    var fn = CompiledQuery.Compile((Northwind db2, string city) =>

        from c in db2.Customers

        where c.City == city

        select c);

Console.WriteLine("****** Call compiled query to retrieve customers from London ******");

var LonCusts = fn(db, "London");

ObjectDumper.Write(LonCusts);

Console.WriteLine();

Console.WriteLine("****** Call compiled query to retrieve customers from Seattle ******");

var SeaCusts = fn(db, "Seattle");

ObjectDumper.Write(SeaCusts);

}

15.linq to dictionary

public void LinqToSqlConversion04() {

    var q =

        from p in db.Products

        where p.UnitsInStock <= p.ReorderLevel && !p.Discontinued

        select p;

Dictionary<int, Product> qDictionary = q.ToDictionary(p => p.ProductID);

foreach (int key in qDictionary.Keys) {

        Console.WriteLine("Key {0}:", key);

        ObjectDumper.Write(qDictionary[key]);

        Console.WriteLine();

    }

}

var db = new EmpServiceDataContext();

var q = (from s in db.ChineseCheckInfos

select new { s.EmployerCode ,s.CheckCode,s.CheckName})

.ToDictionary(s=>s.EmployerCode +s.CheckCode,s=>s.CheckName);

Dictionary<string, string> chnCheckNames = q;

16.Direct SQL

public void LinqToSqlDirect01() {

    var products = db.ExecuteQuery<Product>(

        "SELECT [Product List].ProductID, [Product List].ProductName " +

        "FROM Products AS [Product List] " +

        "WHERE [Product List].Discontinued = 0 " +

        "ORDER BY [Product List].ProductName;"

    );

ObjectDumper.Write(products);

}

Console.WriteLine("*** UPDATE ***");

db.ExecuteCommand("UPDATE Products SET UnitPrice = UnitPrice + 1.00");

16. Linq to Store procedure

public void LinqToSqlStoredProc02() {

ISingleResult<CustomersByCityResult> result = db.CustomersByCity("London");

ObjectDumper.Write(result);

}

public void LinqToSqlStoredProc04() {

IMultipleResults result = db.GetCustomerAndOrders("SEVES");

Console.WriteLine("********** Customer Result-set ***********");

    IEnumerable<CustomerResultSet> customer = result.GetResult<CustomerResultSet>();

    ObjectDumper.Write(customer);

    Console.WriteLine();

Console.WriteLine("********** Orders Result-set ***********");

IEnumerable<OrdersResultSet> orders = result.GetResult<OrdersResultSet>();

ObjectDumper.Write(orders);

}

17.Linq to string array compare the length and value

public void Linq5() {

string[] digits = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

var shortDigits = digits.Where((digit, index) => digit.Length < index);

Console.WriteLine("Short digits:");

    foreach (var d in shortDigits)

    {

        Console.WriteLine("The word {0} is shorter than its value.", d);

    }

}

Short digits:

The word five is shorter than its value.

The word six is shorter than its value.

The word seven is shorter than its value.

The word eight is shorter than its value.

The word nine is shorter than its value.

18.Table to Dictionary

connDbConnection_forTemp = new System.Data.OleDb.OleDbConnection();

connDbConnection_forTemp.ConnectionString = conString;

connDbConnection_forTemp.Open();

System.Data.OleDb.OleDbCommand objCommand = new System.Data.OleDb.OleDbCommand();

string sqlStr = "select * from AMS_MailSetting where MailSetting like '" + prefix + "%'";

objCommand.CommandText = sqlStr;

objCommand.Connection = connDbConnection_forTemp;

DataSet ds = new DataSet();

System.Data.OleDb.OleDbDataAdapter objAdaptor = new System.Data.OleDb.OleDbDataAdapter();

objAdaptor.SelectCommand = objCommand;

objAdaptor.Fill(ds);

if (ds.Tables[0].Rows.Count > 0)

{

var q = from dr in ds.Tables[0].AsEnumerable()

select (new KeyValuePair<string, string>(dr["MailSetting"].ToString(), dr["SettingParam"].ToString()));

values = q.ToDictionary(s =>s.Key,s=>s.Value);

// value = ds.Tables[0].Rows[0][0].ToString();

}

19.居然还可以动态列名排序！

public static IQueryable<TEntity> OrderBy<TEntity>(this IQueryable<TEntity> source, string propertyStr) where TEntity : class

{

    ParameterExpression param = Expression.Parameter(typeof(TEntity), "c");

    PropertyInfo property = typeof(TEntity).GetProperty(propertyStr);

    Expression propertyAccessExpression = Expression.MakeMemberAccess(param, property);

    LambdaExpression le = Expression.Lambda(propertyAccessExpression, param);

    Type type = typeof(TEntity);

    MethodCallExpression resultExp = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { type, property.PropertyType }, source.Expression, Expression.Quote(le));

    return source.Provider.CreateQuery<TEntity>(resultExp);

}

887723.html

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強悍的Linq

Linq初体验——Order By 通过属性名动态排序 http://www.cnblogs.com/xxfss2/archive/2010/12/13/1905023.html Linq To Sql进阶系列（六）用object的动态查询与保存log篇 http://www.cnblogs.com/126/archive/2007/09/09/887723.html

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热门专题

Linq初体验——Order By 通过属性名动态排序

http://www.cnblogs.com/xxfss2/archive/2010/12/13/1905023.html

Linq To Sql进阶系列（六）用object的动态查询与保存log篇

http://www.cnblogs.com/126/archive/2007/09/09/887723.html