namespace Indexers
{
struct PhoneNumber
{
public PhoneNumber(string text)
{
this.number = text;
}

public string Text
{
get { return this.number; }
}

public override int GetHashCode()
{
return this.number.GetHashCode();
}

public override bool Equals(object other)
{
return (other is PhoneNumber) && Equals((PhoneNumber)other);
}

public bool Equals(PhoneNumber other)
{
return this.number == other.number;
}

private string number;
}
}

如红色部份的代码,other是object类型, 为什么可以用other is PhoneNumber呢?