Question

I have two generic Dictionaries. Both have the same keys, but their values can be different. I want to compare the 2nd dictionary with the 1st dictionary. If there are differences between their values, I want to store those values in a separate dictionary.

1st Dictionary
------------
key       Value

Barcode   1234566666
Price     20.00


2nd Dictionary
--------------
key       Value

Barcode   1234566666
Price     40.00


3rd Dictionary
--------------
key       Value

Price     40

Can anyone give me the best algorithm to do this? I wrote an algorithm but it has a lot of loops. I am seeking a short and efficient idea, like a solution using LINQ query expressions or LINQ lambda expressions. I am using .NET Framework 3.5 with C#. I found something about the Except() method, but unfortunately I couldn’t understand what is happening on that method. It would be great if anyone could explain the suggested algorithm.

Answer 1

If you’ve already checked that the keys are the same, you can just use:

var dict3 = dict2.Where(entry => dict1[entry.Key] != entry.Value)
                 .ToDictionary(entry => entry.Key, entry => entry.Value);

To explain, this will:

• Iterate over the key/value pairs in dict2
• For each entry, look up the value in dict1 and filter out any entries where the two values are the same
• Form a dictionary from the remaining entries (i.e. the ones where the dict1 value is different) by taking the key and value from each pair just as they appear in dict2.

Note that this avoids relying on the equality of KeyValuePair<TKey, TValue> – it might be okay to rely on that, but personally I find this clearer. (It will also work when you’re using a custom equality comparer for the dictionary keys – although you’d need to pass that to ToDictionary, too.)