What method in the String class returns only the first N characters?

I’d like to write an extension method to the String class so that if the input string to is longer than the provided length N, only the first N characters are to be displayed.

Here’s how it looks like:

public static string TruncateLongString(this string str, int maxLength)
{
    if (str.Length <= maxLength)
        return str;
    else
        //return the first maxLength characters                
}

What String.*() method can I use to get only the first N characters of str?