[FIXED] Why doesn't out of range indexes throw an exception for Regex GroupCollection?

Issue

Ordinarily if you try to access an array member that doesn't exist you get an "IndexOutOfRangeException".

However, for some reason, I don't get that for a Regex GroupCollection.

For example:

    var match = Regex.Match("", "a");
    var test = match.Groups[6];

Does not throw an exception, despite match having only 1 group.

Why does this happen?

Solution

This is by design.

See the GroupCollection documentation:

If groupnum is not the index of a member of the collection, or if groupnum is the index of a capturing group that has not been matched in the input string, the method returns a Group object whose Group.Success property is false and whose Group.Value property is String.Empty.

I bolded the beginning of the sentence since it describes your current scenario: a regex pattern finds no matches in an empty string, the match is empty, and there is no group with ID = 6.

Answered By - Wiktor Stribiżew

Answer Checked By - Pedro (PHPFixing Volunteer)