What happens to C# Dictionary lookup if the key does not exist?

Question

I tried checking for null but the compiler warns that this condition will never occur. What should I be looking for?

Answer 1

A helper class is handy:

public static class DictionaryHelper
{
    public static TVal Get<TKey, TVal>(this Dictionary<TKey, TVal> dictionary, TKey key, TVal defaultVal = default(TVal))
    {
        TVal val;
        if( dictionary.TryGetValue(key, out val) )
        {
            return val;
        }
        return defaultVal;
    }
}

Answer 2

If you're just checking before trying to add a new value, use the ContainsKey method:

if (!openWith.ContainsKey("ht"))
{
    openWith.Add("ht", "hypertrm.exe");
}

If you're checking that the value exists, use the TryGetValue method as described in Jon Skeet's answer.

Answer 3

Consider the option of encapsulating this particular dictionary and provide a method to return the value for that key:

public static class NumbersAdapter
{
    private static readonly Dictionary<string, string> Mapping = new Dictionary<string, string>
    {
        ["1"] = "One",
        ["2"] = "Two",
        ["3"] = "Three"
    };

    public static string GetValue(string key)
    {
        return Mapping.ContainsKey(key) ? Mapping[key] : key;
    }
}

Then you can manage the behaviour of this dictionary.

For example here: if the dictionary doesn't have the key, it returns key that you pass by parameter.