How can you use optional parameters in C

Question

Note  This question was asked at a time when C  did not yet support optional parameters  i e  before C  4    We re building a web API that s programmatically generated from a C  class  The class has method GetFooBar int a  int b  and the API has a method GetFooBar taking query params like  amp a foo  amp b bar    The classes needs to support optional parameters  which isn t supported in C  the language  What s the best approach

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You can use optional parameters in C  4 0 without any worries  If we have a method like   int MyMetod int param1  int param2  int param3 10  int param4 20          when you call the method  you can skip parameters like this   int variab   MyMethod param3 50  param1 10     C  4 0 implements a feature called  named parameters   you can actually pass parameters by their names  and of course you can pass parameters in whatever order you want

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In C   I would normally use multiple forms of the method   void GetFooBar int a    int defaultBValue   GetFooBar a  defaultBValue     void GetFooBar int a  int b        whatever here     UPDATE   This mentioned above WAS the way that I did default values with C  2 0   The projects I m working on now are using C  4 0 which now directly supports optional parameters   Here is an example I just used in my own code   public EDIDocument ApplyEDIEnvelop EDIVanInfo sender                                      EDIVanInfo receiver                                      EDIDocumentInfo info                                     EDIDocumentType type                                         new EDIDocumentType EDIDocTypes X12 814                                      bool Production   false          My code is here

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Another option is to use the params keyword  public void DoSomething params object   theObjects      foreach object o in theObjects             Something with the Objects            Called like     DoSomething this  that  theOther

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A little late to the party  but I was looking for the answer to this question and ultimately figured out yet another way to do this   Declare the data types for the optional args of your web method to be type XmlNode   If the optional arg is omitted this will be set to null  and if it s present you can get is string value by calling arg Value  i e     WebMethod  public string Foo string arg1  XmlNode optarg2        string arg2           if  optarg2    null                arg2   optarg2 Value                etc     What s also decent about this approach is the  NET generated home page for the ws still shows the argument list  though you do lose the handy text entry boxes for testing

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Another option is to use the params keyword  public void DoSomething params object   theObjects      foreach object o in theObjects             Something with the Objects            Called like     DoSomething this  that  theOther

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Instead of default parameters  why not just construct a dictionary class from the querystring passed    an implementation that is almost identical to the way asp net forms work with querystrings    i e  Request QueryString  a     This will decouple the leaf class from the factory   boilerplate code     You also might want to check out Web Services with ASP NET  Web services are a web api generated automatically via attributes on C  classes

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I have a web service to write that takes 7 parameters  Each is an optional query attribute to a sql statement wrapped by this web service  So two workarounds to non-optional params come to mind    both pretty poor   method1 param1  param2  param 3  param 4  param 5  param 6  param7  method1 param1  param2  param3  param 4  param5  param 6  method 1 param1  param2  param3  param4  param5  param7     start to see the picture  This way lies madness  Way too many combinations   Now for a simpler way that looks awkward but should work  method1 param1  bool useParam1  param2  bool useParam2  etc      That s one method call  values for all parameters are required  and it will handle each case inside it  It s also clear how to use it from the interface   It s a hack  but it will work

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You can use optional parameters in C  4 0 without any worries  If we have a method like   int MyMetod int param1  int param2  int param3 10  int param4 20          when you call the method  you can skip parameters like this   int variab   MyMethod param3 50  param1 10     C  4 0 implements a feature called  named parameters   you can actually pass parameters by their names  and of course you can pass parameters in whatever order you want

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In C   I would normally use multiple forms of the method   void GetFooBar int a    int defaultBValue   GetFooBar a  defaultBValue     void GetFooBar int a  int b        whatever here     UPDATE   This mentioned above WAS the way that I did default values with C  2 0   The projects I m working on now are using C  4 0 which now directly supports optional parameters   Here is an example I just used in my own code   public EDIDocument ApplyEDIEnvelop EDIVanInfo sender                                      EDIVanInfo receiver                                      EDIDocumentInfo info                                     EDIDocumentType type                                         new EDIDocumentType EDIDocTypes X12 814                                      bool Production   false          My code is here

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You could use method overloading      GetFooBar   GetFooBar int a  GetFooBar int a  int b    It depends on the method signatures  the example I gave is missing the  int b  only method because it would have the same signature as the  int a  method   You could use Nullable types      GetFooBar int  a  int  b    You could then check  using a HasValue  to see if a parameter has been set   Another option would be to use a  params  parameter    GetFooBar params object   args    If you wanted to go with named parameters would would need to create a type to handle them  although I think there is already something like this for web apps

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Instead of default parameters  why not just construct a dictionary class from the querystring passed    an implementation that is almost identical to the way asp net forms work with querystrings    i e  Request QueryString  a     This will decouple the leaf class from the factory   boilerplate code     You also might want to check out Web Services with ASP NET  Web services are a web api generated automatically via attributes on C  classes

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For just in case if someone wants to pass a callback  or delegate  as an optional parameter  can do it this way   Optional Callback parameter   public static bool IsOnlyOneElement this IList lst  Action callbackOnTrue    Action   null    Action callbackOnFalse    Action   null          var isOnlyOne   lst Count    1      if  isOnlyOne  amp  amp  callbackOnTrue    null  callbackOnTrue        if   isOnlyOne  amp  amp  callbackOnFalse    null  callbackOnFalse        return isOnlyOne

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From this site   http   www tek-tips com viewthread cfm qid 1500861 amp page 1  C  does allow the use of the  Optional  attribute  from VB  though not functional in C    So you can have a method like this   using System Runtime InteropServices  public void Foo int a  int b   Optional  int c              In our API wrapper  we detect optional parameters  ParameterInfo p IsOptional  and set a default value  The goal is to mark parameters as optional without resorting to kludges like having  optional  in the parameter name

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From this site   http   www tek-tips com viewthread cfm qid 1500861 amp page 1  C  does allow the use of the  Optional  attribute  from VB  though not functional in C    So you can have a method like this   using System Runtime InteropServices  public void Foo int a  int b   Optional  int c              In our API wrapper  we detect optional parameters  ParameterInfo p IsOptional  and set a default value  The goal is to mark parameters as optional without resorting to kludges like having  optional  in the parameter name

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Surprised no one mentioned C  4 0 optional parameters that work like this   public void SomeMethod int a  int b   0         some code     Edit  I know that at the time the question was asked  C  4 0 didn t exist  But this question still ranks  1 in Google for  C  optional arguments  so I thought - this answer worth being here  Sorry

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I agree with stephenbayer  But since it is a webservice  it is easier for end-user to use just one form of the webmethod  than using multiple versions of the same method  I think in this situation Nullable Types are perfect for optional parameters   public void Foo int a  int b  int  c      if c HasValue             do something with a b and c       else            do something with a and b only

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Surprised no one mentioned C  4 0 optional parameters that work like this   public void SomeMethod int a  int b   0         some code     Edit  I know that at the time the question was asked  C  4 0 didn t exist  But this question still ranks  1 in Google for  C  optional arguments  so I thought - this answer worth being here  Sorry

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You can overload your method  One method contains one parameter GetFooBar int a  and the other contain both parameters  GetFooBar int a  int b

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Another option is to use the params keyword  public void DoSomething params object   theObjects      foreach object o in theObjects             Something with the Objects            Called like     DoSomething this  that  theOther

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The typical way this is handled in C  as stephen mentioned is to overload the method   By creating multiple versions of the method with different parameters you effectively create optional parameters   In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method

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In the case when default values aren t available the way to add an optional parameter is to use  NET OptionalAttribute class - https   docs microsoft com en-us dotnet api system runtime interopservices optionalattribute view netframework-4 8   Example of the code is below   namespace OptionalParameterWithOptionalAttribute       class Program               static void Main string   args                          Calling the helper method Hello only with required parameters             Hello  Vardenis    Pavardenis                  Calling the helper method Hello with required and optional parameters             Hello  Vardenis    Pavardenis    Palanga                      public static void Hello string firstName  string secondName                System Runtime InteropServices OptionalAttribute  string  fromCity                        string result   firstName         secondName              if  fromCity    null                                result      from     fromCity                            Console WriteLine  Hello     result

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The typical way this is handled in C  as stephen mentioned is to overload the method   By creating multiple versions of the method with different parameters you effectively create optional parameters   In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method

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An easy way which allows you to omit any parameters in any position  is taking advantage of nullable types as follows   public void PrintValues int  a   null  int  b   null  float  c   null  string s             if a HasValue          Console Write a       else         Console Write  -         if b HasValue          Console Write b       else         Console Write  -         if c HasValue          Console Write c       else         Console Write  -         if string IsNullOrEmpty s      Different check for strings         Console Write s       else         Console Write  -        Strings are already nullable types so they don t need the     Once you have this method  the following calls are all valid   PrintValues  1  2  2 2f   PrintValues  1  c  1 2f   PrintValues b 100   PrintValues  c  1 2f  s   hello    PrintValues      When you define a method that way you have the freedom to set just the parameters you want by naming them  See the following link for more information on named and optional parameters   Named and Optional Arguments  C  Programming Guide    MSDN

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You could use method overloading      GetFooBar   GetFooBar int a  GetFooBar int a  int b    It depends on the method signatures  the example I gave is missing the  int b  only method because it would have the same signature as the  int a  method   You could use Nullable types      GetFooBar int  a  int  b    You could then check  using a HasValue  to see if a parameter has been set   Another option would be to use a  params  parameter    GetFooBar params object   args    If you wanted to go with named parameters would would need to create a type to handle them  although I think there is already something like this for web apps

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From this site   http   www tek-tips com viewthread cfm qid 1500861 amp page 1  C  does allow the use of the  Optional  attribute  from VB  though not functional in C    So you can have a method like this   using System Runtime InteropServices  public void Foo int a  int b   Optional  int c              In our API wrapper  we detect optional parameters  ParameterInfo p IsOptional  and set a default value  The goal is to mark parameters as optional without resorting to kludges like having  optional  in the parameter name

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Hello Optional World  If you want the runtime to supply a default parameter value  you have to use reflection to make the call  Not as nice as the other suggestions for this question  but compatible with VB NET   using System  using System Runtime InteropServices  using System Reflection   namespace ConsoleApplication1       class Class1               public static void sayHelloTo               Optional              DefaultParameterValue  world    string whom                        Console WriteLine  Hello     whom                       STAThread          static void Main string   args                        MethodInfo mi   typeof Class1  GetMethod  sayHelloTo                mi Invoke null  new Object     Missing Value

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Instead of default parameters  why not just construct a dictionary class from the querystring passed    an implementation that is almost identical to the way asp net forms work with querystrings    i e  Request QueryString  a     This will decouple the leaf class from the factory   boilerplate code     You also might want to check out Web Services with ASP NET  Web services are a web api generated automatically via attributes on C  classes

User · Answer

I agree with stephenbayer  But since it is a webservice  it is easier for end-user to use just one form of the webmethod  than using multiple versions of the same method  I think in this situation Nullable Types are perfect for optional parameters   public void Foo int a  int b  int  c      if c HasValue             do something with a b and c       else            do something with a and b only

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Hello Optional World  If you want the runtime to supply a default parameter value  you have to use reflection to make the call  Not as nice as the other suggestions for this question  but compatible with VB NET   using System  using System Runtime InteropServices  using System Reflection   namespace ConsoleApplication1       class Class1               public static void sayHelloTo               Optional              DefaultParameterValue  world    string whom                        Console WriteLine  Hello     whom                       STAThread          static void Main string   args                        MethodInfo mi   typeof Class1  GetMethod  sayHelloTo                mi Invoke null  new Object     Missing Value

User · Answer

You could use method overloading      GetFooBar   GetFooBar int a  GetFooBar int a  int b    It depends on the method signatures  the example I gave is missing the  int b  only method because it would have the same signature as the  int a  method   You could use Nullable types      GetFooBar int  a  int  b    You could then check  using a HasValue  to see if a parameter has been set   Another option would be to use a  params  parameter    GetFooBar params object   args    If you wanted to go with named parameters would would need to create a type to handle them  although I think there is already something like this for web apps

User · Answer

I had to do this in a VB Net 2 0 Web Service  I ended up specifying the parameters as strings  then converting them to whatever I needed  An optional parameter was specified with an empty string  Not the cleanest solution  but it worked  Just be careful that you catch all the exceptions that can occur

User · Answer

Instead of default parameters  why not just construct a dictionary class from the querystring passed    an implementation that is almost identical to the way asp net forms work with querystrings    i e  Request QueryString  a     This will decouple the leaf class from the factory   boilerplate code     You also might want to check out Web Services with ASP NET  Web services are a web api generated automatically via attributes on C  classes

User · Answer

For a larger number of optional parameters  a single parameter of Dictionary could be used with the ContainsKey method  I like this approach because it allows me to pass a List or a T individually without having to create a whole other method  nice if parameters are to be used as filters  for example    Example  new Dictionary lt string Object gt    would be passed if no optional parameters are desired    public bool Method string ParamA  Dictionary lt string Object gt  AddlParams        if ParamA     Alpha   amp  amp   AddlParams ContainsKey  foo      AddlParams ContainsKey  bar               return true        else           return false

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For a larger number of optional parameters  a single parameter of Dictionary could be used with the ContainsKey method  I like this approach because it allows me to pass a List or a T individually without having to create a whole other method  nice if parameters are to be used as filters  for example    Example  new Dictionary lt string Object gt    would be passed if no optional parameters are desired    public bool Method string ParamA  Dictionary lt string Object gt  AddlParams        if ParamA     Alpha   amp  amp   AddlParams ContainsKey  foo      AddlParams ContainsKey  bar               return true        else           return false

User · Answer

I have a web service to write that takes 7 parameters  Each is an optional query attribute to a sql statement wrapped by this web service  So two workarounds to non-optional params come to mind    both pretty poor   method1 param1  param2  param 3  param 4  param 5  param 6  param7  method1 param1  param2  param3  param 4  param5  param 6  method 1 param1  param2  param3  param4  param5  param7     start to see the picture  This way lies madness  Way too many combinations   Now for a simpler way that looks awkward but should work  method1 param1  bool useParam1  param2  bool useParam2  etc      That s one method call  values for all parameters are required  and it will handle each case inside it  It s also clear how to use it from the interface   It s a hack  but it will work

User · Answer

In C   I would normally use multiple forms of the method   void GetFooBar int a    int defaultBValue   GetFooBar a  defaultBValue     void GetFooBar int a  int b        whatever here     UPDATE   This mentioned above WAS the way that I did default values with C  2 0   The projects I m working on now are using C  4 0 which now directly supports optional parameters   Here is an example I just used in my own code   public EDIDocument ApplyEDIEnvelop EDIVanInfo sender                                      EDIVanInfo receiver                                      EDIDocumentInfo info                                     EDIDocumentType type                                         new EDIDocumentType EDIDocTypes X12 814                                      bool Production   false          My code is here

User · Answer

You can overload your method  One method contains one parameter GetFooBar int a  and the other contain both parameters  GetFooBar int a  int b

User · Answer

In the case when default values aren t available the way to add an optional parameter is to use  NET OptionalAttribute class - https   docs microsoft com en-us dotnet api system runtime interopservices optionalattribute view netframework-4 8   Example of the code is below   namespace OptionalParameterWithOptionalAttribute       class Program               static void Main string   args                          Calling the helper method Hello only with required parameters             Hello  Vardenis    Pavardenis                  Calling the helper method Hello with required and optional parameters             Hello  Vardenis    Pavardenis    Palanga                      public static void Hello string firstName  string secondName                System Runtime InteropServices OptionalAttribute  string  fromCity                        string result   firstName         secondName              if  fromCity    null                                result      from     fromCity                            Console WriteLine  Hello     result

User · Answer

An easy way which allows you to omit any parameters in any position  is taking advantage of nullable types as follows   public void PrintValues int  a   null  int  b   null  float  c   null  string s             if a HasValue          Console Write a       else         Console Write  -         if b HasValue          Console Write b       else         Console Write  -         if c HasValue          Console Write c       else         Console Write  -         if string IsNullOrEmpty s      Different check for strings         Console Write s       else         Console Write  -        Strings are already nullable types so they don t need the     Once you have this method  the following calls are all valid   PrintValues  1  2  2 2f   PrintValues  1  c  1 2f   PrintValues b 100   PrintValues  c  1 2f  s   hello    PrintValues      When you define a method that way you have the freedom to set just the parameters you want by naming them  See the following link for more information on named and optional parameters   Named and Optional Arguments  C  Programming Guide    MSDN

User · Answer

The typical way this is handled in C  as stephen mentioned is to overload the method   By creating multiple versions of the method with different parameters you effectively create optional parameters   In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method

User · Answer

I had to do this in a VB Net 2 0 Web Service  I ended up specifying the parameters as strings  then converting them to whatever I needed  An optional parameter was specified with an empty string  Not the cleanest solution  but it worked  Just be careful that you catch all the exceptions that can occur

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Using overloads or using C  4 0 or above    private void GetVal string sName  int sRoll        if  sRoll  gt  0              do some work           private void GetVal string sName         GetVal  testing   0

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A little late to the party  but I was looking for the answer to this question and ultimately figured out yet another way to do this   Declare the data types for the optional args of your web method to be type XmlNode   If the optional arg is omitted this will be set to null  and if it s present you can get is string value by calling arg Value  i e     WebMethod  public string Foo string arg1  XmlNode optarg2        string arg2           if  optarg2    null                arg2   optarg2 Value                etc     What s also decent about this approach is the  NET generated home page for the ws still shows the argument list  though you do lose the handy text entry boxes for testing

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You can try this too Type 1     public void YourMethod int a 0  int b   0                    some code          Type 2      public void YourMethod int  a  int  b                                some code

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The typical way this is handled in C  as stephen mentioned is to overload the method   By creating multiple versions of the method with different parameters you effectively create optional parameters   In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method

User · Answer

You can try this too Type 1     public void YourMethod int a 0  int b   0                    some code          Type 2      public void YourMethod int  a  int  b                                some code

User · Answer

Hello Optional World  If you want the runtime to supply a default parameter value  you have to use reflection to make the call  Not as nice as the other suggestions for this question  but compatible with VB NET   using System  using System Runtime InteropServices  using System Reflection   namespace ConsoleApplication1       class Class1               public static void sayHelloTo               Optional              DefaultParameterValue  world    string whom                        Console WriteLine  Hello     whom                       STAThread          static void Main string   args                        MethodInfo mi   typeof Class1  GetMethod  sayHelloTo                mi Invoke null  new Object     Missing Value

User · Answer

From this site   http   www tek-tips com viewthread cfm qid 1500861 amp page 1  C  does allow the use of the  Optional  attribute  from VB  though not functional in C    So you can have a method like this   using System Runtime InteropServices  public void Foo int a  int b   Optional  int c              In our API wrapper  we detect optional parameters  ParameterInfo p IsOptional  and set a default value  The goal is to mark parameters as optional without resorting to kludges like having  optional  in the parameter name

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optional parameters are nothing but default parameters  i suggest you give both of them default parameters  GetFooBar int a 0  int b 0  if you don t have any overloaded method  will result in a 0  b 0 if you don t pass any values if you pass 1 value  will result in  passed value for a  0 and if you pass 2 values 1st will be assigned to a and second to b   hope that answers your question

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optional parameters are for methods  if you need optional arguments for a class and you are    using c  4 0  use optional arguments in the constructor of the class  a solution i prefer  since it s closer to what is done with methods  so easier to remember  here s an example   class myClass       public myClass int myInt   1  string myString                               wow  this is cool  i can have a default string                    do something here if needed          using c  versions previous to c 4 0  you should use constructor chaining  using the  this keyword   where simpler constructors lead to a  master constructor   example   class myClass       public myClass                this is the default constructor            public myClass int myInt            this myInt   whatever                    do something here if needed           public myClass string myString            this 0  myString                   do something here if needed           public myClass int myInt  string myString                   do something here if needed - this is the master constructor

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I agree with stephenbayer  But since it is a webservice  it is easier for end-user to use just one form of the webmethod  than using multiple versions of the same method  I think in this situation Nullable Types are perfect for optional parameters   public void Foo int a  int b  int  c      if c HasValue             do something with a b and c       else            do something with a and b only

User · Answer

You could use method overloading      GetFooBar   GetFooBar int a  GetFooBar int a  int b    It depends on the method signatures  the example I gave is missing the  int b  only method because it would have the same signature as the  int a  method   You could use Nullable types      GetFooBar int  a  int  b    You could then check  using a HasValue  to see if a parameter has been set   Another option would be to use a  params  parameter    GetFooBar params object   args    If you wanted to go with named parameters would would need to create a type to handle them  although I think there is already something like this for web apps

User · Answer

optional parameters are nothing but default parameters  i suggest you give both of them default parameters  GetFooBar int a 0  int b 0  if you don t have any overloaded method  will result in a 0  b 0 if you don t pass any values if you pass 1 value  will result in  passed value for a  0 and if you pass 2 values 1st will be assigned to a and second to b   hope that answers your question

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Using overloads or using C  4 0 or above    private void GetVal string sName  int sRoll        if  sRoll  gt  0              do some work           private void GetVal string sName         GetVal  testing   0

User · Answer

Hello Optional World  If you want the runtime to supply a default parameter value  you have to use reflection to make the call  Not as nice as the other suggestions for this question  but compatible with VB NET   using System  using System Runtime InteropServices  using System Reflection   namespace ConsoleApplication1       class Class1               public static void sayHelloTo               Optional              DefaultParameterValue  world    string whom                        Console WriteLine  Hello     whom                       STAThread          static void Main string   args                        MethodInfo mi   typeof Class1  GetMethod  sayHelloTo                mi Invoke null  new Object     Missing Value

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In C   I would normally use multiple forms of the method   void GetFooBar int a    int defaultBValue   GetFooBar a  defaultBValue     void GetFooBar int a  int b        whatever here     UPDATE   This mentioned above WAS the way that I did default values with C  2 0   The projects I m working on now are using C  4 0 which now directly supports optional parameters   Here is an example I just used in my own code   public EDIDocument ApplyEDIEnvelop EDIVanInfo sender                                      EDIVanInfo receiver                                      EDIDocumentInfo info                                     EDIDocumentType type                                         new EDIDocumentType EDIDocTypes X12 814                                      bool Production   false          My code is here

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You can use default  public void OptionalParameters int requerid  int optinal   default

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Another option is to use the params keyword  public void DoSomething params object   theObjects      foreach object o in theObjects             Something with the Objects            Called like     DoSomething this  that  theOther

User · Answer

optional parameters are for methods  if you need optional arguments for a class and you are    using c  4 0  use optional arguments in the constructor of the class  a solution i prefer  since it s closer to what is done with methods  so easier to remember  here s an example   class myClass       public myClass int myInt   1  string myString                               wow  this is cool  i can have a default string                    do something here if needed          using c  versions previous to c 4 0  you should use constructor chaining  using the  this keyword   where simpler constructors lead to a  master constructor   example   class myClass       public myClass                this is the default constructor            public myClass int myInt            this myInt   whatever                    do something here if needed           public myClass string myString            this 0  myString                   do something here if needed           public myClass int myInt  string myString                   do something here if needed - this is the master constructor

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I agree with stephenbayer  But since it is a webservice  it is easier for end-user to use just one form of the webmethod  than using multiple versions of the same method  I think in this situation Nullable Types are perfect for optional parameters   public void Foo int a  int b  int  c      if c HasValue             do something with a b and c       else            do something with a and b only

User · Answer

For just in case if someone wants to pass a callback  or delegate  as an optional parameter  can do it this way   Optional Callback parameter   public static bool IsOnlyOneElement this IList lst  Action callbackOnTrue    Action   null    Action callbackOnFalse    Action   null          var isOnlyOne   lst Count    1      if  isOnlyOne  amp  amp  callbackOnTrue    null  callbackOnTrue        if   isOnlyOne  amp  amp  callbackOnFalse    null  callbackOnFalse        return isOnlyOne

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You can use default  public void OptionalParameters int requerid  int optinal   default

[c#] How can you use optional parameters in C#?

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