Struct casting behavior different between for and foreach loops

Question

I came across this bizarre situation just now: I was editing some legacy code which looks something like this:

Hashtable hashtable = GetHashtable();

for (int i = 0; i < hashtable.Count; i++)
{
    MyStruct myStruct = (MyStruct)hashtable[i];

    //more code
}

Now when changing this to a foreach loop:

var hashtable = GetHashtable();

foreach (var item in hashtable)
{
    var myStruct = (MyStruct)item;

    //more code
}

I had assumed the behavior would be the same, however, I get System.InvalidCastException: Specified cast is not valid.

What is the reason for this different behavior?

Answer 1

Iterating a Hashtable doesn't iterate its values, rather it iterates key value pairs as a DictionaryEntry object.

Instead try iterating on its .Values collection instead.

foreach (var item in hashtable.Values)
{
    var myStruct = (MyStruct)item;
}

Since you are refactoring old legacy code, if possible, you should also consider using the generic Dictionary<TKey, TValue> instead. It will take advantage of struct value semantics and avoid boxing.

---

If you wanted to iterate on the DictionaryEntry objects, you can do so but will need to cast to it as well as your MyStruct:

foreach (DictionaryEntry entry in hashtable)
{
    var myStruct = (MyStruct)entry.Value;
}

---

Finally, there's the Linq solution, but it may not be applicable for you since this is legacy code; it may not be available:

foreach(var myStruct in hashtable.Values.Cast<MyStruct>())
{

}

Answer 2

Each yielded item in a Hashtable is a DictionaryEntry. Therefore you can also do it like this

foreach (DictionaryEntry de in hashtable)
{
    var myStruct = (MyStruct)de.Value;
    //more code
}