What is the meaning of double "?"

I was in puzzled wheh i saw the code(show below)

publicstringthis[string propertyName]
{
get
{
string result =string.Empty;
propertyName = propertyName ?string.Empty;//Here!!!!
if (propertyName ==string.Empty || propertyName =="Name")
{
if (string.IsNullOrEmpty(this.Name))
{
result ="Name cannot be blank!";
}
}
return result;
}
}

What is meaning of "?"
thanks

[1389 byte] By [stswordman] at [2007-12-24]