Chapter86:Properties

Section86.1:Auto-implementedproperties

Auto-implementedpropertieswereintroducedinC#3.

Anauto-implementedpropertyisdeclaredwithanemptygetterandsetter(accessors):

publicboolIsValid{get;set;}

When an auto-implemented property is written in your code, the compiler creates a private anonymous field that can only be accessed through the property’s accessors.

Theaboveauto-implementedpropertystatementisequivalenttowritingthislengthycode:

privatebool_isValid;

publicboolIsValid

{

get{return_isValid;}

set{_isValid=value;}

}

Auto-implementedpropertiescannothaveanylogicintheiraccessors,forexample:

publicboolIsValid{get;set{PropertyChanged(“IsValid”);}}//Invalidcode

Anauto-implementedpropertycan howeverhavedifferentaccessmodifiersforitsaccessors:

publicboolIsValid{get;privateset;}

C#6allowsauto-implementedpropertiestohavenosetteratall(makingitimmutable,sinceitsvaluecanbeset only inside the constructor or hard coded):

publicboolIsValid{get;}

publicboolIsValid{get;}=true;

Formoreinformationoninitializingauto-implementedproperties,readtheAuto-propertyinitializersdocumentation.

Section86.2:DefaultValuesforProperties

SettingadefaultvaluecanbedonebyusingInitializers(C#6)

publicclassName

{

publicstringFirst{get;set;}=”James”;

publicstringLast{get;set;}=”Smith”;

}

Ifitisreadonlyyoucanreturnvalueslikethis:

publicclassName

{

publicstringFirst=>”James”;

publicstringLast=>”Smith”;

}

Section86.3:PublicGet

Gettersareusedtoexposevaluesfromclasses.

stringname;publicstringName

{

get{returnthis.name;}

}

Section86.4:PublicSet

Settersareusedtoassignvaluestoproperties.

stringname;publicstringName

{

set{this.name=value;}

}

Section86.5:AccessingProperties

classProgram

{

publicstaticvoidMain(string[]args)

{

PersonaPerson=newPerson(“AnnXenaSample”,newDateTime(1984,10,22));

//exampleofaccessingproperties(Id,Name&DOB)

Console.WriteLine(“Idis:             \t{0}\nNameis:\t‘{1}’.\nDOBis:\t{2:yyyy-MM-dd}.\nAgeis:

\t{3}”,aPerson.Id,aPerson.Name,aPerson.DOB,aPerson.GetAgeInYears());

//exampleofsettingproperties

aPerson.Name=”          HansTrimmer     “;

aPerson.DOB=newDateTime(1961,11,11);

//aPerson.Id=5;//thiswon’tcompileasId’sSETmethodisprivate;soonlyaccessible withinthePersonclass.

//aPerson.DOB=DateTime.UtcNow.AddYears(1);//thiswouldthrowaruntimeerrorasthere’s validationtoensuretheDOBisinpast.

//seehowourchangesabovetakeeffect;notethattheNamehasbeentrimmed

Console.WriteLine(“Idis:             \t{0}\nNameis:\t‘{1}’.\nDOBis:\t{2:yyyy-MM-dd}.\nAgeis:

\t{3}”,aPerson.Id,aPerson.Name,aPerson.DOB,aPerson.GetAgeInYears());

Console.WriteLine(“Pressanykeytocontinue”); Console.Read();

}

}

publicclassPerson

{

privatestaticintnextId=0;

privatestringname;

privateDateTimedob;//datesareheldinUTC;i.e.wedisregardtimezones

publicPerson(stringname,DateTimedob)

{

this.Id=++Person.nextId;

this.Name=name;

this.DOB=dob;

}

publicintId

{

get;

privateset;

}

publicstringName

{

get{returnthis.name;} set

{

if(string.IsNullOrWhiteSpace(value))thrownewInvalidNameException(value);

this.name=value.Trim();

}

}

publicDateTimeDOB

{

get{returnthis.dob;} set

{

if(value<DateTime.UtcNow.AddYears(-200)||value>DateTime.UtcNow)thrownew InvalidDobException(value);

this.dob=value;

}

}

publicintGetAgeInYears()

{

DateTimetoday=DateTime.UtcNow;

intoffset=HasHadBirthdayThisYear()?0:-1;

returntoday.Year-this.dob.Year+offset;

}

privateboolHasHadBirthdayThisYear()

{

boolhasHadBirthdayThisYear=true;

DateTimetoday=DateTime.UtcNow;

if(today.Month>this.dob.Month)

{7

hasHadBirthdayThisYear=true;

}

else

{

if(today.Month==this.dob.Month)

{

}

else

{

}

}

hasHadBirthdayThisYear=today.Day>this.dob.Day;

hasHadBirthdayThisYear=false;

returnhasHadBirthdayThisYear;

}

}

publicclassInvalidNameException:ApplicationException

{

conststringInvalidNameExceptionMessage=”‘{0}’isaninvalidname.”; publicInvalidNameException(stringvalue):

base(string.Format(InvalidNameExceptionMessage,value)){}

}

publicclassInvalidDobException:ApplicationException

{

const string InvalidDobExceptionMessage = “‘{0:yyyy-MM-dd}’ is an invalid DOB.                   Thedatemust notbeinthefuture,orover200yearsinthepast.”;

public InvalidDobException(DateTime value): base(string.Format(InvalidDobExceptionMessage,value)){}

}

Section86.6:Read-onlyproperties

Declaration

Acommonmisunderstanding,especiallybeginners,haveisread-onlypropertyistheonemarkedwithreadonly

keyword.That’snotcorrectandinfactfollowingisacompiletimeerror:

publicreadonlystringSomeProp{get;set;}

Apropertyis read-onlywhenit onlyhasa getter.

publicstringSomeProp{get;}

Usingread-onlypropertiestocreateimmutableclasses

publicAddress

{

publicstringZipCode{get;}

publicstringCity{get;}

publicstringStreetAddress{get;}

publicAddress(

stringzipCode,

stringcity,

stringstreetAddress)

{

if(zipCode==null)

throw new ArgumentNullException(nameof(zipCode));

if(city==null)

thrownewArgumentNullException(nameof(city)); if

(streetAddress==null)

thrownewArgumentNullException(nameof(streetAddress));

ZipCode=zipCode;

City=city;

StreetAddress=streetAddress;

}

}

Section86.7:VariousPropertiesinContext

publicclassPerson

{

//Idpropertycanbereadbyotherclasses,butonlysetbythePersonclass

publicintId{get;privateset;}

//Namepropertycanberetrievedorassigned

publicstringName{get;set;}

privateDateTimedob;

//DateofBirthpropertyisstoredinaprivatevariable,butretrievedorassignedthroughthe publicproperty.

publicDateTimeDOB

{

get { return this.dob; }

set{this.dob=value;}

}

//Agepropertycanonlyberetrieved;it’svalueisderivedfromthedateofbirth

publicintAge

{

get

{

intoffset=HasHadBirthdayThisYear()?0 : -1;

returnDateTime.UtcNow.Year-this.dob.Year+offset;

}

}

//thisisnotapropertybutamethod;thoughitcouldberewrittenasapropertyifdesired.

privateboolHasHadBirthdayThisYear()

{

bool hasHadBirthdayThisYear = true;

DateTimetoday=DateTime.UtcNow;

if (today.Month>this.dob.Month)

{

hasHadBirthdayThisYear=true;

}

else

{

if(today.Month==this.dob.Month)

{

hasHadBirthdayThisYear=today.Day>this.dob.Day;

}

else

{

hasHadBirthdayThisYear=false;

}

}

returnhasHadBirthdayThisYear;

}

}