C# constructor calls constructor with more value than itself

Question

I'm pretty sure that I am duplicating a question, but somehow in my example it doesn't work, please see the code:

class Program
{
    static void Main(string[] args)
    {
        Test test1 = new Test("str", "strrr");
    }

}
class Test
{

    public string testValue, mType;

    public Test(string value, string messageType)
    {
        this.testValue = value;
        this.mType = messageType;

    }

    public Test (string value) : this (value, messageType)
    {
        //want to manipulate value and messageType here
    }


}

messageType in this constructor public Test (string value) : this (value, messageType) says it doesn't exist in the current context. I want to call it in this way because first of all I want my code instantiating the class with two strings, and then I want to provide value only to one parameter constructor but not lose messageType, because I will use it within this constructor public Test (string value). I read about chaining and instantiating constructors but it seems that this thing is opposite to what I read. Sorry not a lot practice yet with programming if this is a simple question, I would like to know how the code should look. What I have read before pointing this question: Call one constructor from another, How call constructor inside other constructor?, http://www.csharp411.com/constructor-chaining/, it doesn't work for me and again sorry if I am duplicating or doing silly things.

Answer 1

So in your example, you have to constructors: one accepting value and type and the other accepting only value. In such case, if the user provides a value only, the type should have a default assumed value. For example, you could do something like:

public Test (string value) : this (value, "Default Message Type")
{

}

What this code does is that it gets the value from the user, i.e. the caller of the constructor, and pass it to the other constructor, along with a default message called "Default Message Type".

Notice that you don't need to put any code in this constructor, since it will call the other constructor which will set both the value and the type.

Hope this helps.

EDIT: From the comments, I understood from that @ArnosGo wants to do some manipulation on value and add some logic that finds the value of messageType, then pass both to the other constructor. Unfortunately, it is not possible to do some manipulation on data before calling the other constructor, but here is a trick to do this:

class Test
{
    public string testValue, mType;

    public Test(string value, string messageType)
    {
        Initialise(value, messageType);
    }

    public Test (string value)
    {
        // Do some manipulation here and found out the value of messageType.
        Initialise(value, messageType);
    }

    protected void Initialise(string value, string messageType)
    {
        this.testValue = value;
        this.mType = messageType;
    }
}

Basically, you are creating a function that does what both constructors do, then the constructors only calculate the values and pass them to that function.

Answer 2

The call to the database and message string comes before creating an object of Test class and then i want to put two values in it, and then manipulate those values inside and provide them to constructor with one parameter. If that explains something.

Based on OP's comment, I think approach with static method will be better
Because values are manipulated only once and properties assigned only once.

public class Test
{
    private static ManipulateValues(string value, string msgtype)
    {
        //Do your manipulating 
        Return manipulatedValue;
    }

    public static Create(string value, string msgtype)
    {
        return new Test(Test.ManipulateValues(value, msgtype), msgtype); 
    }

    public Test(string value, string msgType)
    {
        this.testValue = value;
        this.mType = messageType;
    }

}

Then use it:

string messageType = someValue;
string value = someDatabaseValue;
Test temp = Test.Create(value, messageType)

Answer 3

I think what you want to do is call Test test1 = new Test("Value", "MessageType"); to instantiate an object. Then the constructor with two strings will set these parameters to testValue and mType fields and call the first constructor to make manipulations.

In your code the first constructor does not call the second one, actually the second constructor calls the first one. If you want the first constructor to call the second your constructor signatures should be like this:

public Test(string value, string messageType) : this(value)
public Test(string value)

But here you cannot send messageType parameter to the second constructor. Therefore you should write an initialization method, put the content of the second constructor in this method and call it from both constructors and do not use constructor chaining.

public Test(string value, string messageType)
{
    this.testValue = value;
    this.mType = messageType;

    Initialize();
}
public Test(string value)
{
    this.testValue = value;

    Initialize()
}

public void Initialize()
{
    //This is the code in the second constructor
}