I am sick of doing blocks of code like this for various bits of code I have:

if (dict.ContainsKey[key]) {  
    dict[key] = value;  
}  
else {  
    dict.Add(key,value);  
}

and for lookups (i.e. key -> list of value)

if (lookup.ContainsKey[key]) {  
    lookup[key].Add(value);  
}  
else {  
    lookup.Add(new List<valuetype>);  
    lookup[key].Add(value);  
}  

Is there another collections lib or extension method I should use to do this in one line of code no matter what the key and value types are?

e.g.

dict.AddOrUpdate(key,value)  
lookup.AddOrUpdate(key,value)