Chapter84:Lambdaexpressions

Section84.1:LambdaExpressionsasShorthandforDelegate Initialization

publicdelegateintModifyInt(intinput);

ModifyIntmultiplyByTwo=x=>x*2;

TheaboveLambdaexpressionsyntaxisequivalenttothefollowingverbosecode:

publicdelegateintModifyInt(intinput);

ModifyIntmultiplyByTwo=delegate(intx){

returnx*2;

};

Section84.2:LambdaExpressionasanEventHandler

Lambdaexpressionscanbeusedtohandleevents,whichisusefulwhen: The handler is short.

• The handler never needs to be unsubscribed.

Agoodsituationinwhichalambdaeventhandlermightbeusedisgivenbelow:

smtpClient.SendCompleted+=(sender,args)=>Console.WriteLine(“Emailsent”);

Ifunsubscribingaregisteredeventhandleratsomefuturepointinthecodeisnecessary,theeventhandler expressionshouldbesavedtoavariable,andtheregistration/unregistrationdonethroughthatvariable:

EventHandlerhandler=(sender,args)=>Console.WriteLine(“Emailsent”);

smtpClient.SendCompleted+=handler; smtpClient.SendCompleted-=

handler;

The reason that this is done rather than simply retyping the lambda expression verbatim to unsubscribe it (-=) is that the C# compiler won’t necessarily consider the two expressions equal:

EventHandler handlerA=(sender, args) =>Console.WriteLine(“Email sent”);

EventHandler handlerB=(sender, args) =>Console.WriteLine(“Email sent”); Console.WriteLine(handlerA.Equals(handlerB));//Mayreturn”False”

Notethatifadditionalstatementsareaddedtothelambdaexpression,thentherequiredsurroundingcurlybraces maybeaccidentallyomitted,withoutcausingcompile-timeerror.Forexample:

smtpClient.SendCompleted+=(sender,args)=>Console.WriteLine(“Emailsent”); emailSendButton.Enabled=true;

Thiswillcompile,butwillresultinaddingthelambdaexpression(sender,args)=>Console.WriteLine(“Emailsent”);asaneventhandler,andexecutingthestatementemailSendButton.Enabled=true;immediately.Tofix this,thecontentsofthelambdamustbesurroundedincurlybraces.Thiscanbeavoidedbyusingcurlybraces fromthestart,beingcautiouswhenaddingadditionalstatementstoalambda-event-handler,orsurroundingthe lambdainroundbracketsfromthestart:

smtpClient.SendCompleted+=((sender,args)=>Console.WriteLine(“Emailsent”));

//Addinganextrastatementwillresultinacompile-timeerror

Section84.3:LambdaExpressionswithMultipleParameters or No Parameters

Useparenthesesaroundtheexpressiontotheleftofthe=>operatortoindicatemultipleparameters.

delegateintModifyInt(intinput1,intinput2);

ModifyIntmultiplyTwoInts=(x,y)=>x*y;

Similarly,anemptysetofparenthesesindicatesthatthefunctiondoesnotaccept parameters.

delegatestringReturnString();

ReturnStringgetGreeting=()=>”Helloworld.”;

Section84.4:Lambdascanbeemittedbothas`Func`and

`Expression`

AssumingthefollowingPersonclass:

publicclassPerson

{

publicstringName{get;set; }

publicintAge{get;set;}

}

Thefollowing lambda:

p=>p.Age>18

Canbepassedasanargumenttobothmethods:

publicvoidAsFunc(Func<Person,bool>func)

publicvoidAsExpression(Expression<Func<Person,bool>>expr)

BecausethecompileriscapableoftransforminglambdasbothtodelegatesandExpressions.

Obviously,LINQprovidersrelyheavilyonExpressions(exposedmainlythroughtheIQueryable<T>interface)in ordertobeabletoparsequeriesandtranslatethemtostorequeries.

Section84.5:PutMultipleStatementsinaStatementLambda

Unlike anexpressionlambda,astatementlambda cancontainmultiplestatementsseparatedby semicolons.

delegatevoidModifyInt(intinput);

ModifyIntaddOneAndTellMe=x=>

{

intresult=x+1; Console.WriteLine(result);

};

Notethatthestatementsareenclosedinbraces{}.

Rememberthat statement lambdascannot be used tocreate expression trees.

Section 84.6: Lambdasfor both `Func`and `Action`

Typicallylambdasareusedfordefiningsimplefunctions(generallyinthecontextofalinqexpression):

varincremented=myEnumerable.Select(x=>x+1);

Herethereturnisimplicit.

However,itisalsopossibletopassactionsaslambdas:

myObservable.Do(x=>Console.WriteLine(x));

Section84.7:Usinglambdasyntaxtocreateaclosure

Seeremarksfordiscussionofclosures.Supposewehaveaninterface:

publicinterfaceIMachine<TState,TInput>

{

TStateState{get;}

publicvoidInput(TInputinput);

}

andthenthefollowingisexecuted:

IMachine<int,int>machine= …;

Func<int,int>machineClosure=i=>{

machine.Input(i);

returnmachine.State;

};

NowmachineClosurereferstoafunctionfrominttoint,whichbehindthescenesusestheIMachineinstance whichmachinereferstoinordertocarryoutthecomputation.Evenifthereferencemachinegoesoutofscope,as longasthemachineClosureobjectismaintained,theoriginalIMachineinstancewillberetainedaspartofa ‘closure’,automaticallydefinedbythecompiler.

Warning: this can mean that the same function call returns different values at different times (e.g. In this example if themachinekeepsasumofitsinputs).Inlotsofcases,thismaybeunexpectedandistobeavoidedforanycodein a functional style – accidental and unexpected closures can be a source of bugs.

Section84.8:PassingaLambdaExpressionasaParameterto a Method

List<int>l2=l1.FindAll(x=>x>6);

Herex=>x>6isalambdaexpressionactingasapredicatethatmakessurethatonlyelementsabove6are returned.

Section84.9:Basiclambdaexpressions

Func<int,int>add1=i=>i+1;

Func<int,int,int>add=(i,j)=>i+j;

//Behaviourallyequivalentto:

intAdd1(inti)

{

returni+1;

}

intAdd(inti,intj)

{

returni+j;

}

…

Console.WriteLine(add1(42));//43

Console.WriteLine(Add1(42));//43

Console.WriteLine(add(100,250));//350

Console.WriteLine(Add(100,250));//350

Section84.10:BasiclambdaexpressionswithLINQ

//assumesourceis{0,1,2,…,10}

varevens=source.Where(n=>n%2==0);

//evens={0,2,4,…10}

varstrings=source.Select(n=>n.ToString());

//strings={“0″,”1”,…,“10”}

Section84.11:Lambdasyntaxwithstatementblockbody

Func<int,string>doubleThenAddElevenThenQuote=i=>{

vardoubled=2*i;

varaddedEleven=11+doubled;

return$”‘{addedEleven}'”;

};

Section 84.12: Lambda expressions with System.Linq.Expressions

Expression<Func<int,bool>>checkEvenExpression=i=>i%2==0;

//lambdaexpressionisautomaticallyconvertedtoanExpression<Func<int,bool>>