Is there an IDictionary implementation that, on missing key, returns the default value instead of throwing?

Question

Indeed, that won't be efficient at all.

As per comments, in .Net Core 2+ / NetStandard 2.1+ / Net 5, MS added the extension method GetValueOrDefault()

For earlier versions you can write the extension method yourself:

public static TValue GetValueOrDefault<TKey,TValue>
    (this IDictionary<TKey, TValue> dictionary, TKey key)
{
    TValue ret;
    // Ignore return value
    dictionary.TryGetValue(key, out ret);
    return ret;
}

Or with C# 7.1:

public static TValue GetValueOrDefault<TKey,TValue>
    (this IDictionary<TKey, TValue> dictionary, TKey key) =>
    dictionary.TryGetValue(key, out var ret) ? ret : default;

That uses:

• An expression-bodied method (C# 6)
• An out variable (C# 7.0)
• A default literal (C# 7.1)

If you're using .NET Core 2 or above (C# 7.x), the CollectionExtensions class is introduced and you can use the GetValueOrDefault method to get default value if key is not there in a dictionary.

Dictionary<string, string> colorData = new Dictionary<string, string>();
string color = colorData.GetValueOrDefault("colorId", string.Empty);

Carrying these extension methods can help..

public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key)
{
    return dict.GetValueOrDefault(key, default(V));
}

public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key, V defVal)
{
    return dict.GetValueOrDefault(key, () => defVal);
}

public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key, Func<V> defValSelector)
{
    V value;
    return dict.TryGetValue(key, out value) ? value : defValSelector();
}

Collections.Specialized.StringDictionary provides a non-exception result when looking up a missing key's value. It is also case-insensitive by default.

Caveats

It is only valid for its specialized uses, and — being designed before generics — it doesn't have a very good enumerator if you need to review the whole collection.

If you're using .Net Core, you can use the CollectionExtensions.GetValueOrDefault method. This is the same as the implementation provided in the accepted answer.

public static TValue GetValueOrDefault<TKey,TValue> (
   this System.Collections.Generic.IReadOnlyDictionary<TKey,TValue> dictionary,
   TKey key);