If you're porting code from .NET 1.x to .NET 2.0, like I'm doing at the moment, and your .NET 1.x code uses Hashtables(), be aware of the following if you decide to migrate your Hashtables to Dictionary<TKey, TValue> variables.
In .NET 1.x, you'd do:
Hashtable myData = new Hashtable();
//... fill it
SomeType value = (SomeType)myData[key];
if(value==null)
{
// key not there, handle...
}
now, if you port this to .NET 2.0, you normally end up with this code:
Dictionary<KeyType, SomeType> myData = new Dictionary<KeyType, SomeType>();
//... fill it
SomeType value = myData[key];
if(value==null)
{
// key not there, handle...
}
Though, this doesn't work. It compiles ok, but something changed in .NET 2.0: if key isn't found, you no longer get null back but you get a nice exception. So the code above should be reworked to:
Dictionary<KeyType, SomeType> myData = new Dictionary<KeyType, SomeType>();
//... fill it
SomeType value = null;
if(!myData.ContainsKey(key))
{
// key not there, handle...
}
else
{
value = myData[key];
}
The sad part is: often the code to port isn't small, and you first try to get it compiled. If you don't keep the above difference in mind, you'll probably have a lovely time fixing bugs after the tests have run. So you've been warned
UPDATE: Wilco Bauwer posted in the comments another work-around, which is even more preferable: TryGetValue(). Sometimes you overlook the most obvious things! 
Reworking my example above, it will look like:
Dictionary<KeyType, SomeType> myData = new Dictionary<KeyType, SomeType>();
//... fill it
SomeType value = null;
if(!myData.TryGetValue(key, out value))
{
// key not there, handle...
}