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

Indeed, that won't be efficient at all.

You could always write an extension method:

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 someone using .net core 2 and above (C# 7.X), CollectionExtensions class is introduced and can use 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();
}