C# 可为空的引用类型：此函数能否在不覆盖空检查的情况下接受具有可为空值和不可为空值的字典？

Question

I am trying to write a function that will convert a Dictionary's TryGet into a "lookup" object in C# 9.0. The (simplified version of) code I have so far looks like this:

class C1
{
    [Test]
    public void METHOD()
    {
        var lookupValue1 = Lookup(new Dictionary<string, string?>(), "a");
        var lookupValue2 = Lookup(new Dictionary<string, string>(), "a");
    }

    public LookupValue<TV> Lookup<TK, TV>(Dictionary<TK, TV?> d, TK key) 
        where TK : notnull
        where TV : notnull
    {
        if (d.TryGetValue(key, out var result))
        {
            return new LookupValue<TV>(result);
        }
        else
        {
            return new LookupValue<TV>(default);
        }
    }

    public record LookupValue<T>(T? Value) where T : notnull;
}

But this code won't compile, specifically, this line: var lookupValue2 = Lookup(new Dictionary<string, string>(), "a");

I know I can override the null reference checking by using a ! operator: var lookupValue2 = Lookup(new Dictionary<string, string>()!, "a"); but this is supposed to be a public API and I son't want the users of the API to have to do this. Also, I know I can create two methods named differently in the same class or methods named the same in separate classes, but I'd like to avoid that if possible.

Is there any way to make this Lookup function accept a dictionary with a nullable value type as well as one with non-nullable value type and preserve the signature of the LookupValue record?

Answer 1

You've written a function which demands a Dictionary whose values are nullable. In other words, the value for a key might be null, and it's allowed to set a null value for a key. So there's no reasonable way for such a function to accept a Dictionary whose values are non-nullable.

Instead, you could consider making the function accept a dictionary with any kind of value type. You can do this by removing the notnull constraint on TV and removing the nullable annotation in the type argument to the dictionary type.

SharpLab

class C1
{
    [Test]
    public void METHOD()
    {
        var lookupValue1 = Lookup(new Dictionary<string, string?>(), "a");
        var lookupValue2 = Lookup(new Dictionary<string, string>(), "a");
    }

    public LookupValue<TV?> Lookup<TK, TV>(Dictionary<TK, TV> d, TK key) 
        where TK : notnull
    {
        if (d.TryGetValue(key, out var result))
        {
            return new LookupValue<TV?>(result);
        }
        else
        {
            return new LookupValue<TV?>(default);
        }
    }

    public record LookupValue<T>(T Value);
}