Why Doesn't C# Dictionary Guarantee Element Order, and How Can I Ensure Alphabetical Ordering?

Understanding C# Dictionary Ordering (or Lack Thereof)

Many C# developers mistakenly assume that Dictionary elements are returned in insertion order. This is incorrect. The following example illustrates this:

// Dictionary declaration
private Dictionary<string, string> _Dictionary = new Dictionary<string, string>();

// Add elements
_Dictionary.Add("orange", "1");
_Dictionary.Add("apple", "4");
_Dictionary.Add("cucumber", "6");
_Dictionary["banana"] = "7";
_Dictionary["pineapple"] = "7";

// Iterate and observe the order (non-deterministic)
foreach (KeyValuePair<string, string> kvp in _Dictionary)
{
    Trace.Write(String.Format("{0}={1}", kvp.Key, kvp.Value));
}

The output is unpredictable because Dictionary uses a hash table, which prioritizes efficient key lookups over maintaining insertion order. The order of iteration is not guaranteed and will vary.

Enforcing Alphabetical Order with SortedDictionary

If you need elements sorted alphabetically, use SortedDictionary instead. This class maintains sorted order based on the key's comparer (default is alphabetical for strings).

// SortedDictionary declaration
private SortedDictionary<string, string> _SortedDictionary = new SortedDictionary<string, string>();

// Add elements
_SortedDictionary.Add("orange", "1");
_SortedDictionary.Add("apple", "4");
_SortedDictionary.Add("cucumber", "6");
_SortedDictionary.Add("banana", "7");
_SortedDictionary.Add("pineapple", "7");

// Iterate and observe the alphabetical order
foreach (KeyValuePair<string, string> kvp in _SortedDictionary)
{
    Trace.Write(String.Format("{0}={1}", kvp.Key, kvp.Value));
}

This will consistently produce alphabetically sorted output. Keep in mind that SortedDictionary has a performance overhead compared to Dictionary, so use it only when order is essential.

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