Optional parameters in C# and X++
Posted January 30, 2010 
Comments(0) In the upcoming 4.0 version of C# the language now supports optional parameters – just like X++. The following is a valid method declaration in both languages:
{
}
There are a few differences between the languages with regards to optional parameters:
Named parameters
In X++ you can only omit optional parameters starting from the last parameter. C# makes it possible to omit any optional parameter. For example:
will call foo(5, 0, 1000).
prmIsDefault
C# doesn’t provide a way to determine, if a parameter was omitted by the caller, or the caller passed in a value identical to the default value. In X++ the prmIsDefault() method can be used to determine this. However, the use cases for this method are rare – and often fragile. For example, consider this X++ class:
{
int value;
public void init(int _value = 0)
{
if (prmIsDefault(value))
{
value = /* some complex calculation */
}
else
{
value = _value;
}
}
}
{
str myStr;
public void init(int _value = 0)
{
myStr = ‘foo’;
super(_value);
}
}
{
str myStr;
public void init(int _value = 0)
{
myStr = ‘foo’;
if (prmIsDefault(_value))
super();
else
super(_value);
}
}
Which really is terrible, as the implementation of the derived class depends on the implementation of the base class. This is bound to break eventually. Add a few more optional parameters and a few more levels of depth in the class hierarchy, and you’ll have a real nightmare. My recommendation is to only use prmIsDefault() in private methods – as they can’t be overridden. In C# you can easy live without this method, as you can achieve the same in a more robust manner using method overloading:
{
int value;
public void init()
{
int _value = /* some complex calculation */
this.init(_value);
}
public void init(int _value)
{
value = _value;
}
}
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