C＃：有一个“可选”参数，默认情况下使用所需参数的值

Question

How can i implement a "optional" parameter to a function such that when endMarker is not given, i will use the value from a required parameter startMarker ? i currently use a nullable type and check if endMarker is null i set it to startMarker

protected void wrapText(string startMarker, string? endMarker = null) { 
    if (endMarker == null)
        endMarker = startMarker; 
}

but the problem now is i get an error saying it cannot cast string? into string

(string)endMarker

how can i cast endMarker to a string so i can use it? or is there a better way of implementing this?

Answer 1

This will work:

protected void wrapText(string startMarker, string endMarker = null) { 
    if (endMarker == null)
        endMarker = startMarker; 
}

In other words: remove the question mark from the string? . System.String is a reference type and can already be null . The Nullable<T> structure can only be used on value types.

Answer 2

You need to overload the method to have a call without the "optional" parameter. Then in this method you just call the normal method passing in the same parameter twice:

protected void wrapText(String startMarker, String endMarker) 
{  
    // do stuff 
} 


protected void wrapText(String startMarker) 
{  
    wrapText(startMarker, startMarker);
} 

Answer 3

Can't you just do something like this:

protected void wrapText(string startMarker, string? endMarker = null) { 
    if (endMarker == null)
    {
        endMarker = "foo";
        endMarker = startMarker;
    }
}