.NET non-nullable reference type and out parameters

Question

I modified my csproj file to enable null reference types in C#8:

<Nullable>enable</Nullable>

Given the following code:

private static void Method()
{
    var dictionary = new Dictionary<string, string>();
    string value = string.Empty;

    dictionary.TryGetValue("Key", out value);
}

The line with TryGetValue() gives the warning:

CS8600: Converting null literal or possible null value to non-nullable type.

I don't understand why. The signature of TryGetValue() is :

public bool TryGetValue(string key, [MaybeNullWhen(false)] out string value);

The code example has only non-nullable references. Why is it getting this error?

Answer 1

If "Key" isn't found in the dictionary, then a value of null will be assigned to the value variable. However you've declared value as string, meaning that it shouldn't contain null. Therefore the compiler is giving you a warning.

The fact that you've initially assigned string.Empty to value doesn't matter - that will always get overwritten by TryGetValue (and you should get another warning which says that).

You should declare value as a string?, to indicate that its value might be null.

Note that the compiler's pretty smart. If you write:

if (!dictionary.TryGetValue("Key", out string? value))
{
    value = string.Empty;
}

then the compiler knows that value cannot be null, and it won't complain if you then try and call methods on it.

Answer 2

canton7's answer is correct (+1).
This is not an explanation but a workaround:
You could add an extension method to Dictionary<TVey, TValue> like this:

public static bool TryGetValue<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue @default, out TValue @value) where TKey : notnull
{
    var result = dictionary.TryGetValue(key, out var val);
    @value = result ? val : @default;
    return result;
}

Then you can use it like this:

private static void Method()
{
    var dictionary = new Dictionary<string, string>();
    /// populate dictionary here...

    dictionary.TryGetValue("Key", string.Empty, out var value);
}

This should enable you to keep value as a non-nullable string.