Is it possible to have conditional parameters in C#?

Question 1

All of these solutions are great as long as you are open to overload your functions. I would personally overload the function; however, if you persist to use only one function then I highly recommend you to use Code Contracts to enforce any logic you desire to any combinations of parameters. Here is an example:

public string XYZ(string dobFormatted, string infodateFormatted, 
        string BSN = "", string insuranceID= "", string lastname= "",
        string postalcode = "", int Homenummer = 0, string Homenummeradd = "",
        string insuranceType = "Both")
{
    Contract.Requires(!string.IsNullOrEmpty(dobFormatted) || insuranceType == "NONE");    
}

You can have as many as Requires() functions as you want. You can enable the static check for Code Contracts and compiler can warn you when you are jeopardizing your constrains.

Visit http://msdn.microsoft.com/en-us/library/dd264808.aspx for more information.

Remember you can use Code Contracts even in your overloaded functions.

Question 2

You can create method signatures with separate overloads in C sharp

public void TestMethod(string stringValue, int intValue)
public void TestMethod(double doubleValue, int intValue)
public void TestMethod(string stringValue, string otherStringValue)   
public void TestMethod(string stringValue, int intValue, string otherStringValue)

These could all be called as separate methods.

Overloads

Also in C sharp you can use optional parameters.

public void TestMethod(int intValue
                      ,int optionalIntValue = 0
                      ,int optionalSecondValue = 4)

where you could call like

TestMethod(1) 
//optionalValue = 0(default) & optionalSecondValue = 4(default)

TestMethod(1, optionalSecondValue:2) 
//optionalValue = 0(default) & optionalSecondValue = 2

TestMethod(1, 12) 
//optionalValue = 12 & optionalSecondValue = 4(default)

Question 3

Because the method parameters share types overloading a single method is going to run into trouble. For those to be the only allowable calls you'll need separate methods for each.

public static string getVecozoURL_BSN(string dobFormatted, string infodateFormatted, string BSN, string insuranceType = "Both")
public static string getVecozoURL_InsurranceID(string dobFormatted, string infodateFormatted, string insuranceID, string insuranceType = "Both")
public static string getVecozoURL_Lastname(string dobFormatted, string infodateFormatted, string lastname, string insuranceType = "Both")
public static string getVecozoURL_Postalcode(string dobFormatted, string infodateFormatted, string postalcode, int Homenummer, string Homenummeradd = "", string insuranceType = "Both")

However since you're concerned with readability/maintainability, it may make more sense to encapsulate the logic in it's own class. Something like:

class VecozoURLFormater
{
    public string DobFormatted { get; private set; }
    public string InfodateFormatted { get; private set; }
    public string InsuranceType { get; private set; }
    public VecozoURLFormater(string dobFormatted, string infodateFormatted, string insuranceType = "Both")
    {
        DobFormatted = dobFormatted;
        InfodateFormatted = infodateFormatted;
        InsuranceType = insuranceType;
    }

    public string FromBSN(string BSN){/*...*/}
    public string FromInsurranceID(string insuranceID){/*...*/}
    public string FromLastname(string lastname){/*...*/}
    public string FromPostalcode(string postalcode, int Homenummer, string Homenummeradd = "") {/*...*/}
}

Question 4

You can redesign your method to take in an object of a class that you define containing all this info, or you can create multiple methods with different parameters, basically either overload it or different methods.

I do not think that you could have conditional parameters based on the value of another parameter, as in if this parameter is there, then don't need the other, or so on...

Question 5

Chaining together overloads of the method is your answer. This will allow everyone to manage it how they'd like.

I have returned the string of the chain in the example below, but you could easily add logic into each method so that you didnt have a nasty if statement at the end.

    public static string getVecozoURL(string dobFormatted, string infodateFormatted, string BSN)
    {
        return getVecozoURL(dobFormatted, infodateFormatted, BSN, "", "", "", 0, "", "Both");
    }

    public static string getVecozoURL(string dobFormatted, string infodateFormatted, string BSN, string insuranceID)
    {
        return getVecozoURL(dobFormatted, infodateFormatted, BSN, insuranceID, "", "", 0, "", "Both");
    }

    public static string getVecozoURL(string dobFormatted, string infodateFormatted, string BSN, string postalcode, int Homenummer)
    {
        return getVecozoURL(dobFormatted, infodateFormatted, BSN, "", "", postalcode, Homenummer, "", "Both");
    }

    public static string getVecozoURL(string dobFormatted, string infodateFormatted,
    string BSN, string insuranceID, string lastname,
    string postalcode, int Homenummer, string Homenummeradd,
    string insuranceType)
    {
        string URL = "";
        if (FCelfproef.BSN(BSN))
        {
            // Do stuff
        }
        else if (!insuranceID.IsEmpty())
        {
            // Do stuff
        }
        else if (postalcode.Length > 4 && Homenummer > 0)
        {
            // Do stuff
        }
        else if (!lastname.IsEmpty())
        {
            // Do stuff
        }
        // Do some other stuff
        return URL;
    }

Question 6

Try this approach:

public enum VecozoOverloadedOptions { BSN, InsuranceID, LastName, PostalCode };

public class VecozoValues
{
    public string DobFormatted;
    public string infodateFormatted;
    public VecozoOverloadedOptions OverloadChoice;
    public string OverloadedValue;
    public int Homenummer;
    public string Homenummeradd;
    public string InsuranceType;
}

...

public static string getVecozoURL(VecozoValues data)
{
   ...

Question 7

This type of problem is often best solved with a fluent syntax:

public class VecozoURLBuilder
{
    private string _dobFormatted;
    private string _infodateFormatted;
    private string _bsn;
    private string _insuranceID;
    ...

    public VecozoURLBuilder DobFormatted(string dobFormatted)
    {
        _dobFormatted = dobFormatted;
        return this;
    }

    public VecozoURLBuilder InfodateFormatted(string infodateFormatted)
    {
        _infodateFormatted= nfodateFormatted;
        return this;
    }
    ...

    public string VecozoURL()
    {
        // create string from dobFormatted  etc.
    }
}

It is then used like this:

var url = new VecozoURLBuilder()
    .DobFormatted("xxx")
    .InfodateFormatted("yyy")
    .VecozoURL();

Obviously, the full thing would have fields for all the required values in the question and fluent methods for all the fields.