Question

然而,我花了很多时间,使C#类推论达到我所希望的目的。我有一个非常具体的情况,即我拥有大量变数。

ConcurrentDictionary<T, IDictionary<U, V> >

如果T,U,V可以是一些任意的,如长期、隐蔽或任何。

我想写出与这些变量有关的方法,特别是审查这些变量的图表。

因此,我写了一种方法。

public static IOrderedEnumerable<Tuple<int,int>> GetDictionaryHistogram<T, U, V, W>(T dictionary) where T : ConcurrentDictionary<U, IDictionary<V, W>>
{
    return dictionary.Select(p => p.Value.Count)
                     .GroupBy(p => p)
                     .Select(p => new Tuple<int, int>(p.Key, p.Count()))
                     .OrderBy(p => p.Item1);
}

But when I try to call it, C# gives me an error that it cannot infer the types. For example on a variable of type

ConcurrentDictionary<int,IDictionary<int, int> > foo;

我发现错误:

Error 118 The type arguments for method Auditor.AuditorHelpers.GetDictionaryHistogram(T) cannot be inferred from the usage. Try specifying the type arguments explicitly.

我有什么错误?

Answer 1

arguments to ual paranote category。在constraints上从未作过任何推论,因为制约因素不属于一种方法的signature的一部分。

In your case type inference must always fail; type inference cannot possibly infer types for U and V because they do not appear in a formal parameter type.

For about a dozen people telling me that I am wrong to believe that this rule is sensible, see the comments to my article on the subject.

Answer 2

The type parameter T is unnecessary. You should just write

public static IOrderedEnumerable<Tuple<int,int>> GetDictionaryHistogram<U, V, W>(
    ConcurrentDictionary<U, IDictionary<V, W>> dictionary)

并且应当处以罚款。

你的制约因素类型,即参数必须是某种类型,只在少数情况下才有用。最常见的是避免采用接口的价值类型的盒式。比较这两种方法:

public static DoSomething(ISomeInterface thing) {}
public static DoSomething<T>(T thing) where T : ISomeInterface {}

当第一种方法被称作具有价值类型论点时,该论点被打上箱子,并投到接口类型上。

当第二种方法被称作具有价值类型时,一般类型参数被替换为价值类型,没有发生任何箱子。

Answer 3

I think the compiler is saying that it can t figure out the dependencies between your types.

Try doing:

public static IOrderedEnumerable<Tuple<int,int>> GetDictionaryHistogram<U, V, W>(ConcurrentDictionary<U, IDictionary<V, W>> dictionary)
{
    return dictionary.Select(p => p.Value.Count)
                     .GroupBy(p => p)
                     .Select(p => new Tuple<int, int>(p.Key, p.Count()))
                     .OrderBy(p => p.Item1);
}

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