Calling the base constructor in C#

Asked 2023-09-21 08:10:21 View 44,204

If I inherit from a base class and want to pass something from the constructor of the inherited class to the constructor of the base class, how do I do that?

For example, if I inherit from the Exception class I want to do something like this:

class MyExceptionClass : Exception
{
     public MyExceptionClass(string message, string extraInfo)
     {
         //This is where it's all falling apart
         base(message);
     }
}

Basically what I want is to be able to pass the string message to the base Exception class.

  • Its also worth noting you can chain constructors in your current class by substituting this for base. - anyone
  • It is here learn.microsoft.com/en-us/dotnet/csharp/language-reference/… - anyone
  • Try: public class MyExceptionClass : Exception { public MyExceptionClass(string message, string extrainfo) : base(message) { //other stuff here } } - anyone
  • @Quibblesome I haven't been able to find an example substituting .this for .base. I'm probably not phrasing it right, but do you have any examples? My question is if you do substitute, do you no longer need :base in the arguments and can just use .base in the constructor? - anyone
  • @DavidMays I mean instead of doing ctor():base(argTheBaseClassNeeds) you can do ctor():this(DateTime.Now) ctor(DateTime dt){ /* some impl */ }. In such as a case I am chaining a constructor in the current class to make the default date time value DateTime.Now. - anyone

Answers

Modify your constructor to the following so that it calls the base class constructor properly:

public class MyExceptionClass : Exception
{
    public MyExceptionClass(string message, string extrainfo) : base(message)
    {
        //other stuff here
    }
}

Note that a constructor is not something that you can call anytime within a method. That's the reason you're getting errors in your call in the constructor body.

Answered   2023-09-21 08:10:21

  • I think you may have missed the point. The problem was about calling a base constructor midway through the overriden constructor. Perhaps the data-type of the base constructor is not the same or you want to do some data moulding before passing it down the chain. How would you accomplish such a feat? - anyone
  • If you need to call the base constructor in the middle of the override, then extract it to an actual method on the base class that you can call explicitly. The assumption with base constructors is that they're absolutely necessary to safely create an object, so the base will be called first, always. - anyone
  • It is just a method you can call any time, IL-wise. C# just happens to put extra restrictions on top of this. - anyone
  • It is worth noting that the base constructor is called before the method block is accessed. msdn.microsoft.com/en-us/library/ms173115.aspx - anyone
  • It is not a good design if you need to call the base class constructor midway during your constructor. The idea of a constructor is that it does all the work needed to do its task. This has the effect that when your derived constructor starts, the base class is already fully initialized and the derived class is free to call any base class function. If your design is such that you want to do something half way your constructor, then apparently this is not initializing the base class ans thus should not be in the constructor of the base class but in a separate, possibly protected function - anyone

Note that you can use static methods within the call to the base constructor.

class MyExceptionClass : Exception
{
     public MyExceptionClass(string message, string extraInfo) : 
         base(ModifyMessage(message, extraInfo))
     {
     }

     private static string ModifyMessage(string message, string extraInfo)
     {
         Trace.WriteLine("message was " + message);
         return message.ToLowerInvariant() + Environment.NewLine + extraInfo;
     }
}

Answered   2023-09-21 08:10:21

  • The Exception class is so locked down that I do find myself doing this a couple times, but also note it's not something you should do if you can avoid it. - anyone
  • There is absolutely nothing harmful about this as long as the intermediate function is morally stateless. Logging is not a good use case, IMO, but normalizing the casing or adding additional seems fine. - anyone
  • If you are in the situation described above. Instead of inheritance you could build your class as a wrapper of the base class as an alternative solution. - anyone

If you need to call the base constructor but not right away because your new (derived) class needs to do some data manipulation, the best solution is to resort to factory method. What you need to do is to mark private your derived constructor, then make a static method in your class that will do all the necessary stuff and later call the constructor and return the object.

public class MyClass : BaseClass
{
    private MyClass(string someString) : base(someString)
    {
        //your code goes in here
    }

    public static MyClass FactoryMethod(string someString)
    {
        //whatever you want to do with your string before passing it in
        return new MyClass(someString);
    }
}

Answered   2023-09-21 08:10:21

  • This could potentially violates the SOLID principles (SRP), because the responsibility of creating the class is encapsulate with whatever other responsibility the class was suppose to take care of. An abstract factory could be used but might add unnecessary complexity to simple code. Of course violation of the SOLIDs is ok if you know the trade off and the toll it is going to put on your architecture (and how to fix any future issues that might arise from your design decision). - anyone

It is true use the base (something) to call the base class constructor, but in case of overloading use the this keyword

public ClassName() : this(par1,par2)
{
// do not call the constructor it is called in the this.
// the base key- word is used to call a inherited constructor   
} 

// Hint used overload as often as needed do not write the same code 2 or more times

Answered   2023-09-21 08:10:21

  • I see what you are trying to explain, and you are right. If you have two constructors in one class, you can reference one from the other by using the "this" keyword similarly to how you use "base" when calling the inherited constructor. However, this isn't what the OP asked for so this isn't really the place to add this. - anyone
  • the keyword 'base' is not valid in this context - anyone
public class MyExceptionClass : Exception
{
    public MyExceptionClass(string message,
      Exception innerException): base(message, innerException)
    {
        //other stuff here
    }
}

You can pass inner exception to one of the constructors.

Answered   2023-09-21 08:10:21

From Framework Design Guidelines and FxCop rules.:

1. Custom Exception should have a name that ends with Exception

    class MyException : Exception

2. Exception should be public

    public class MyException : Exception

3. CA1032: Exception should implements standard constructors.

  • A public parameterless constructor.
  • A public constructor with one string argument.
  • A public constructor with one string and Exception (as it can wrap another Exception).
  • A serialization constructor protected if the type is not sealed and private if the type is sealed. Based on MSDN:

    [Serializable()]
    public class MyException : Exception
    {
      public MyException()
      {
         // Add any type-specific logic, and supply the default message.
      }
    
      public MyException(string message): base(message) 
      {
         // Add any type-specific logic.
      }
      public MyException(string message, Exception innerException): 
         base (message, innerException)
      {
         // Add any type-specific logic for inner exceptions.
      }
      protected MyException(SerializationInfo info, 
         StreamingContext context) : base(info, context)
      {
         // Implement type-specific serialization constructor logic.
      }
    }  
    

or

    [Serializable()]
    public sealed class MyException : Exception
    {
      public MyException()
      {
         // Add any type-specific logic, and supply the default message.
      }

      public MyException(string message): base(message) 
      {
         // Add any type-specific logic.
      }
      public MyException(string message, Exception innerException): 
         base (message, innerException)
      {
         // Add any type-specific logic for inner exceptions.
      }
      private MyException(SerializationInfo info, 
         StreamingContext context) : base(info, context)
      {
         // Implement type-specific serialization constructor logic.
      }
    }  

Answered   2023-09-21 08:10:21

You can also do a conditional check with parameters in the constructor, which allows some flexibility.

public MyClass(object myObject=null): base(myObject ?? new myOtherObject())
{
}

or

public MyClass(object myObject=null): base(myObject==null ? new myOtherObject(): myObject)
{
}

Answered   2023-09-21 08:10:21

  • It is posible to have a condition to use 2 different base constructors? I mean one of them with 2 params and the other with 3 params? - anyone
  • @DavidCon sure you can create multiple constructors with different signatures\params in your base class, use them like above. - anyone
  • I can´t call to diferent base constuctors. I need some think like this: public MyClass(object myObject=null): base(myObject==null ? invokeConstructorBaseA: invokeConstructorBaseB){} Furthermore, ConstructorBaseA has 2 parameters and ConstructorBaseB has 3 parameters. Is there any why of invoke them? - anyone

As per some of the other answers listed here, you can pass parameters into the base class constructor. It is advised to call your base class constructor at the beginning of the constructor for your inherited class.

public class MyException : Exception
{
    public MyException(string message, string extraInfo) : base(message)
    {
    }
}

I note that in your example you never made use of the extraInfo parameter, so I assumed you might want to concatenate the extraInfo string parameter to the Message property of your exception (it seems that this is being ignored in the accepted answer and the code in your question).

This is simply achieved by invoking the base class constructor, and then updating the Message property with the extra info.

public class MyException: Exception
{
    public MyException(string message, string extraInfo) : base($"{message} Extra info: {extraInfo}")
    {
    }
}

Answered   2023-09-21 08:10:21

Example, using this base class you want to derive from:

public abstract class BaseClass
{
    protected BaseClass(int a, int b, int c)
    {
    }
}

The non-compiling pseudo code you want to execute:

public class DerivedClass : BaseClass
{
    private readonly object fatData;
    
    public DerivedClass(int m)
    {
        var fd = new { A = 1 * m, B = 2 * m, C = 3 * m };
        base(fd.A, fd.B, fd.C); // base-constructor call
        this.fatData = fd;
    }
}

2020 version (see below for even more stringent solution)

Using newer C# features, namely out var, you can get rid of the public static factory-method.

I just found out (by accident) that out var parameter of methods called inside base-"call" flow to the constructor body. (maybe it's a C# quirk, see 2023 version for C# 1.0 compatible solution)

Using a static private helper method which produces all required base arguments (plus additional data if needed) to the outside it is just a public plain constructor:

public class DerivedClass : BaseClass
{
    private readonly object fatData;
    
    public DerivedClass(int m)
        : base(PrepareBaseParameters(m, out var b, out var c, out var fatData), b, c)
    {
        this.fatData = fatData;
        Console.WriteLine(new { b, c, fatData }.ToString());
    }

    private static int PrepareBaseParameters(int m, out int b, out int c, out object fatData)
    {
        var fd = new { A = 1 * m, B = 2 * m, C = 3 * m };
        (b, c, fatData) = (fd.B, fd.C, fd); // Tuples not required but nice to use
        return fd.A;
    }
}

2023 update

All you need is an additional private constructor and an accompanying private static factory method which prepares the data for the new private constructor using the same input as for the public ctor:

public class DerivedClass : BaseClass
{
    private readonly FatData fatData;

    public DerivedClass(int m)
        : this(PrepareBaseParameters(m))
    {
        Console.WriteLine(this.fatData.ToString());
    }

    private DerivedClass(FatData fd)
        : base(fd.A, fd.B, fd.C)
    {
        this.fatData = fd;
    }

    private static FatData PrepareBaseParameters(int m)
    {
        // might be any (non-async) code which e.x. calls factory methods 
        var fd = new FatData(A: 1 * m, B: 2 * m, C: 3 * m);
        return fd;
    }

    private readonly record struct FatData(int A, int B, int C);
}

No special C# version needed, the C#10 record struct just for shortness, will work with any C#1.0 class, too.

This version seems to be slightly longer but it is far easier to read and understand.

Answered   2023-09-21 08:10:21

  • there had have been comments - what happended to them? - anyone
public class MyException : Exception
{
    public MyException() { }
    public MyException(string msg) : base(msg) { }
    public MyException(string msg, Exception inner) : base(msg, inner) { }
}

Answered   2023-09-21 08:10:21