Can't create new MatchCollection -- no constructors defined

Question

So, maybe I'm tired, but why can't I create a new MatchCollection?

I have a method that returns a MatchCollection by calling regex.Matches:

public static MatchCollection GetStringsWithinBiggerString(string sourceString)
{
    Regex regex = new Regex(@"\$\(.+?\)");

    return regex.Matches(sourceString);
}

What I wanted to do was return an empty collection if the parameter was null:

public static MatchCollection GetStringsWithinBiggerString(string sourceString)
{
    if (sourceString == null)
    {
        return new MatchCollection();
    }

    Regex regex = new Regex(@"\$\(.+?\)");

    return regex.Matches(sourceString);
}

But that won't compile because of this line:

return new MatchCollection();

The error:

The type 'System.Text.RegularExpressions.MatchCollection' has no constructors defined.

How can a type have no constructors defined? I thought a default constructor would be created if no constructor was explicitly defined. Is it not possible to create a new instance of MatchCollection for my method to return?

Answer 1

Very appropriate use of the Null Object pattern!

Implement like this:

public static MatchCollection GetStringsWithinBiggerString(string sourceString)
{
    Regex regex = new Regex(@"\$\(.+?\)");

    return regex.Matches(sourceString ?? String.Empty);
}

Answer 2

How can a type have no constructors defined?

It can't. But it can hide all of its constructors by making them non-public - i.e. private, internal, or protected. Moreover, once a constructor is defined, the default constructor becomes inaccessible. Other classes in the same namespace can access internal constructors, but the classes external to the namespace would not be able to instantiate a class directly.

P.S. If you would like to create an empty match collection, you can always make an expression that matches something, and pass it something else:

Regex regex = new Regex(@"foo");
var empty = regex.Matches("bar"); // "foo" does not match "bar"