[C# feature request] Default value for Dictionary<>

[amshaikh]

It would really be handy if the user of the Dictionary<> class be able to set a default value which should be returned if the key is not found in the dictionary.

eg: (Wished code)

Dictionary<string, int> TotalMarks = new Dictionary<string, int>();
TotalMarks.DefaultValue = 0;
Console.WriteLine(TotalMarks["Tom"]); //should work --no need to check whether the key exists or not.

[amshaikh]

Use the following wrapper:

public class Map<TKey,TValue> : Dictionary<TKey,TValue>
{
    public TValue this [TKey key]
    {
        get
        {
            if (base.ContainsKey(key) || !isDefaultValueSet)
            {
                return base[key];
            }
            else
            {
                return DefaultValue;
            }
        }
        set { base[key] = value; }
    }

    private bool isDefaultValueSet = false;
    private TValue _DefaultValue;
    public TValue DefaultValue
    {
        get { return _DefaultValue; }
        set { _DefaultValue = value; isDefaultValueSet = true; }
    }
}

[YairHalberstadt]

This is a request for dotnet/runtime

[GrabYourPitchforks]

I think @amshaikh provided the solution (though for efficiency the caller should use TryGetValue instead of ContainsKey). Is there anything else to discuss here?

[sharwell]

This functionality is provided through the GetValueOrDefault extension methods:

runtime/src/libraries/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs

Lines 10 to 24 in 839b134

	public static TValue? GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key)
	{
	return dictionary.GetValueOrDefault(key, default!);
	}
	public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
	{
	if (dictionary == null)
	{
	throw new ArgumentNullException(nameof(dictionary));
	}
	TValue value;
	return dictionary.TryGetValue(key, out value) ? value : defaultValue;
	}

[GrabYourPitchforks]

API refs:

• https://apisof.net/catalog/System.Collections.Generic.CollectionExtensions
• https://docs.microsoft.com/dotnet/api/system.collections.generic.collectionextensions