Call one constructor from another

Question

I have two constructors which feed values to readonly fields   public class Sample       public Sample string theIntAsString                int i   int Parse theIntAsString            intField   i             public Sample int theInt    gt   intField   theInt      public int IntProperty      gt   intField       private readonly int  intField      One constructor receives the values directly  and the other does some calculation and obtains the values  then sets the fields   Now here s the catch    I don t want to duplicate the setting code  In this case  just one field is set but of course there may well be more than one  To make the fields readonly  I need to set them from the constructor  so I can t  extract  the shared code to a utility function  I don t know how to call one constructor from another    Any ideas

User · Answer

Error handling and making your code reusable is key  I added string to int validation and it is possible to add other types if needed  Solving this problem with a more reusable solution could be this    public class Sample       public Sample object inputToInt                 intField   objectToInt inputToInt              public int IntProperty   gt   intField       private readonly int  intField     public static int objectToInt object inputToInt        switch  inputToInt                        case int inputInt                  return inputInt              break              case string inputString              if   int TryParse inputString  out int parsedInt                                 throw new InvalidParameterException   The input  inputString  could not be parsed to int                              return parsedInt               default                  throw new InvalidParameterException   Constructor do not support  inputToInt GetType   Name                 break

User · Answer

Before the body of the constructor  use either     base  parameters     this  parameters    Example   public class People  User      public People  int EmpID    base  EmpID                Add more statements here

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If what you want can t be achieved satisfactorily without having the initialization in its own method  e g  because you want to do too much before the initialization code  or wrap it in a try-finally  or whatever  you can have any or all constructors pass the readonly variables by reference to an initialization routine  which will then be able to manipulate them at will   public class Sample       private readonly int  intField      public int IntProperty   gt   intField        private void setupStuff ref int intField  int newValue    gt  intField   newValue       public Sample string theIntAsString                int i   int Parse theIntAsString           setupStuff ref  intField i              public Sample int theInt    gt  setupStuff ref  intField  theInt

User · Answer

Please  please  and pretty please do not try this at home  or work  or anywhere really  This is a way solve to a very very specific problem  and I hope you will not have that  I m posting this since it is technically an answer  and another perspective to look at it  I repeat  do not use it under any condition  Code is to run with LINQPad  void Main          new A 1   Dump         new B 2  -1   Dump             var b2   new B 2  -1       b2 Increment             b2 Dump       class A        public readonly int I   0           public A int i                I   i           class B  A       public int J      public B int i  int j   base i                J   j                 public B int i  bool wtf   base i                       public void Increment                 int i   I   1           var t   typeof B  BaseType          var ctor   t GetConstructors   First                     ctor Invoke this  new object     i              Since constructor is a method  you can call it with reflection  Now you either think with portals  or visualize a picture of a can of worms  sorry about this

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Constructor chaining i e you can use  Base  for Is a relationship and  This  you can use for same class  when you want call multiple Constructor in single call     class BaseClass       public BaseClass   this 10                  public BaseClass int val                    class Program               static void Main string   args                        new BaseClass                ReadLine

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When you inherit a class from a base class  you can invoke the base class constructor by instantiating the derived class  class sample       public int x       public sample int value                x   value           class der   sample       public int a      public int b       public der int value1 int value2    base 50                a   value1          b   value2           class run        public static void Main string   args                der obj   new der 10 20            System Console WriteLine obj x           System Console WriteLine obj a           System Console WriteLine obj b             Output of the sample program is     50 10 20     You can also use this keyword to invoke a constructor from another constructor  class sample       public int x       public sample int value                 x   value             public sample sample obj    this obj x                  class run       public static void Main string   args                 sample s   new sample 20           sample ss   new sample s            System Console WriteLine ss x             The output of this sample program is      20

User · Answer

Yeah  you can call other method before of the call base or this   public class MyException   Exception       public MyException int number    base ConvertToString number                     private static string ConvertToString int number                return number toString

User · Answer

I am improving upon supercat s answer  I guess the following can also be done   class Sample       private readonly int  intField      public int IntProperty               get   return  intField               void setupStuff ref int intField  int newValue                  Do some stuff here based upon the necessary initialized variables          intField   newValue             public Sample string theIntAsString  bool  doStuff   true                  Initialization of some necessary variables                                                               int i   int Parse theIntAsString                                                                                                                                if   doStuff HasValue    doStuff Value    true             setupStuff ref  intField i              public Sample int theInt   this theInt  false     false  param to avoid setupStuff   being called two times               setupStuff ref  intField  theInt

User · Answer

In case you need to run something before calling another constructor not after    public class Sample       static int preprocess string theIntAsString                return preprocess int Parse theIntAsString               static int preprocess int theIntNeedRounding                return theIntNeedRounding 100             public Sample string theIntAsString                 intField   preprocess theIntAsString             public Sample int theIntNeedRounding                 intField   preprocess theIntNeedRounding             public int IntProperty    gt   intField       private readonly int  intField      And ValueTuple can be very helpful if you need to set more than one field

User · Answer

Here is an example that calls another constructor  then checks on the property it has set       public SomeClass int i                I   i             public SomeClass SomeOtherClass soc            this soc J                if  I  0                        I   DoSomethingHere

User · Answer

Like this   public Sample string str    this int Parse str

[c#] Call one constructor from another

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