使用内部值类型的接口键入的多维IReadOnlyDictionary

这有点难以用语言表达，所以我将通过示例进行解释。以下代码未编译：

var data = new[] {
        new {Item = "abc", Values = new[] {1,2,3}},
        new {Item = "def", Values = new[] {1,2,3}}};
IReadOnlyDictionary<string, IReadOnlyDictionary<Guid, int>> target;
target = new ReadOnlyDictionary<string, IReadOnlyDictionary<Guid, int>>(
    data.ToDictionary(
            i => i.Item,
            v => new ReadOnlyDictionary<Guid, int>(
                v.Values.ToDictionary(
                    a => Guid.NewGuid(),
                    b => b))));

我得到的错误是：

The best overloaded method match for
    'ReadOnlyDictionary<string,IReadOnlyDictionary<Guid,int>>
        .ReadOnlyDictionary(IDictionary<string,IReadOnlyDictionary<System.Guid,int>>)'
has some invalid arguments

然而，如果我使用一个类而不是内部值的接口来声明target，它会编译：

IReadOnlyDictionary<string, ReadOnlyDictionary<Guid, int>> target;
target = new ReadOnlyDictionary<string, ReadOnlyDictionary<Guid, int>>(
    data.ToDictionary(
            i => i.Item,
            v => new ReadOnlyDictionary<Guid, int>(
                v.Values.ToDictionary(
                    a => Guid.NewGuid(),
                    b => b))));

为什么我不能使用内部字典的接口？

target = new ReadOnlyDictionary<string, IReadOnlyDictionary<Guid, int>>(
    data.ToDictionary(
            i => i.Item,
            v => (IReadOnlyDictionary<Guid, int>)new ReadOnlyDictionary<Guid, int>(
                v.Values.ToDictionary(
                    a => Guid.NewGuid(),
                    b => b))));

target = new ReadOnlyDictionary<string, IReadOnlyDictionary<Guid, int>>(
    data.ToDictionary<string, IReadOnlyDictionary<Guid, int>>(
            i => i.Item,
            v => new ReadOnlyDictionary<Guid, int>(
                v.Values.ToDictionary(
                    a => Guid.NewGuid(),
                    b => b))));