What is the meaning of `public string this[string columnName]` When implementing IDataErrorInfo interface?

Question

What is the meaning of public string this[string columnName] When implementing IDataErrorInfo interface?

    public string this[string columnName]
    {
        get
        {
            switch (columnName)
            {
                case "Name":
                    return ValidateName();
                case "PhoneNumber":
                    return ValidatePhoneNumber();
                default:
                    return string.Empty;
            }
        }
    }

I don't get it why there are square parentheses and what it does.

---

Answer: Thanks to Hans and Scott now I know that is simply the syntax of an indexer. More information here.

Answer 1

That is a C# indexer, it lets you use your class like

IDataErrorInfo myClass = new MyClass();
string phoneNumberErrorInfo = myClass["PhoneNumber"];`