问题
When I try the following in VB.NET
Dim data = New Dictionary(Of String, Integer)
data.Count(Function(x) x.Value > 0) 'Compile-time error!
I get this compile error with .Net Fiddle:
Too many arguments to 'Public Overloads ReadOnly Property Count As Integer'
Visual Studio gives me this error:
'Public ReadOnly Property Count As Integer' has no parameters and its return type cannot be indexed.
The following does work though:
Enumerable.Where(data, Function(x) x.Value > 0).Count() 'Works!
data.Where(Function(x) x.Value > 0).Count() 'Works!
It seems to not be finding the correct overload.
Oddly, enough the C# version of this works just fine in Visual Studio (but fails in .NET Fiddle - odd... what's going on?):
var data = new Dictionary<string, int>();
data.Count(x => x.Value > 0);
What's the correct way to use the LINQ version of .Count() with a predicate against a dictionary?
回答1:
You need to use AsEnumerable() (see the Remarks section) to pick up the extension methods when the names clash.
data.AsEnumerable().Count(Function(x) x.Value > 0)
回答2:
There is however a reason why in VB.NET it works differently from C#:
When an in-scope instance method has a signature that is compatible with the arguments of a calling statement, the instance method is chosen in preference to any extension method. The instance method has precedence even if the extension method is a better match.
and even
The situation is simpler with properties: if an extension method has the same name as a property of the class it extends, the extension method is not visible and cannot be accessed.
a Dictionary(Of TKey, TValue) Class already has a property named Count so the Count extension is hidden.
Extension Methods (Visual Basic) > Extension Methods, Instance Methods, and Properties
I'll skip the how-part because @Mark's already answerd that.
来源：https://stackoverflow.com/questions/27932993/cant-use-count-from-linq-on-dictionary