编写通用扩展方法时类型推断有问题

I really hate sometimes how IDictionary<TKey, TValue> [key] will throw an exception if the key doesn't exist in the dictionary.

我真的很讨厌如果字典中不存在密钥,IDictionary [key]将如何抛出异常。 ,tvalue>

Of course there is TryGetValue(), but that seems to have been optimized for performance and not usability.

当然有TryGetValue(),但似乎已针对性能而非可用性进行了优化。

So I thought, oh I'll just make an extension method for it - which I did :

所以我想,哦,我只是为它做一个扩展方法 - 我做了:

public static class CollectionExtensions
{
    public static TType GetValueOrDefault<TKeyType, TValue, TType>(this IDictionary<TKeyType, TType> dictionary, TKeyType key)
    {
        TType value = default(TType);

        // attempt to get the value of the key from the dictionary
        // if the key doesn't exist just return null
        if (dictionary.TryGetValue(key, out value))
        {
            return value;
        }
        else
        {
            return default(TType);
        }
    }    
}

This works fine EXCEPT I cannot seem to get type inference working.

这工作正常除了我似乎无法得到类型推断工作。

Obviously I want to be able to do the following :

显然我希望能够做到以下几点:

var extraDataLookup = new Dictionary<string, string>();
extraDataLookup["zipcode"] = model.Zipcode; 

and then be able to access the value :

然后能够访问该值:

var zipcode = extraDataLookup.GetValueOrDefault("zipcode");
var foo = extraDataLookup.GetValueOrDefault("foo"); // should be null

I've looked at a few things about type inference, inlucing Jon Skeet's article and even sourcecode to System.Linq.Enumerable in reflector but seem to be missing something.

我已经看了几个关于类型推断的事情,将Jon Skeet的文章甚至源代码都反映到了反射器中的System.Linq.Enumerable,但似乎缺少了一些东西。

This works :

这有效:

extraDataLookup.GetValueOrDefault<string, string,string> ("foo") 

but this doesn't

但事实并非如此

extraDataLookup.GetValueOrDefault ("foo") 

What should I be doing.

我该怎么办

PS. I'm only looking for solutions to the generic type inference problem, not any other suggestions. Thanks.

PS。我只是在寻找泛型类型推理问题的解决方案,而不是任何其他建议。谢谢。

1 个解决方案

#1

You seem to be defining your extension method with three generic types when you only need two. "TValue" and "TType" mean the same thing, don't they? Try this:

当您只需要两个泛型类型时,您似乎正在使用三种泛型类型定义扩展方法。 “TValue”和“TType”意思相同,不是吗?试试这个:

public static TValue GetValueOrDefault<TKey, TValue>(
    this IDictionary<TKey, TValue> dictionary, TKey key)
{
    TValue value;
    // attempt to get the value of the key from the dictionary
    dictionary.TryGetValue(key, out value);
    return value;
}