how to generate qr code in asp.net using c# var filteredNumbers = DynamicWhere(nums, Test); // Compiler error in C#.NET

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var filteredNumbers = DynamicWhere(nums, Test); // Compiler error
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The C# compiler complains:
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Argument 2: cannot convert from 'method group' to 'dynamic'
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The problem is that we ve given it too much latitude. Example 18-25 will work with a wide range of delegate types. It would be happy with Predicate<object>, Predicate<dynamic>, Predicate<int>, Func<object, bool>, Func<dynamic, bool>, or Func<int, bool>. Or you could define a custom delegate type of your own that was equivalent to any of these. The only thing the C# compiler can see is that Dynamic Where expects a dynamic argument, so for all it knows, it could pass any type at all. All it would have to do is pick one that fits the Test method s signature any delegate type with a single argument and a return type of bool would do. But it doesn t have any rule to say which particular delegate type to use by default here. In Example 18-22, the compiler knew what to do because the Where method expected a specific delegate type: Func<int, bool>. Since there was only one possible option, the C# compiler was able to create a delegate of the right kind. But now that it has too much choice, we need to narrow things down again so that it knows what to do. Example 18-27 shows one way to do this, although you could cast to any of the delegate types mentioned earlier.
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var filteredNumbers = DynamicWhere(nums, (Predicate<dynamic>) Test);
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Again, we ve ended up doing extra work just to satisfy the C# type system, which is the opposite of what you d usually expect in the dynamic idiom types are supposed to matter less.
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This is exactly the sort of problem you ll run into if you attempt to treat C# as a dynamic programming language the underlying issue here is that dynamic was designed to solve specific interop problems. It does that job very well, but C# as a whole is not really at home in the dynamic style. So it s not a good idea to attempt to make heavy use of that style in your C# code.
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C# 4.0 s new dynamic keyword makes it much easier to use objects that were designed to be used from dynamic programming languages. In particular, COM automation APIs such as those offered by the Microsoft Office suite are far more natural to use than they have been in previous versions of the language. Interoperating with browser script objects in Silverlight is also easier than before.
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Programmers love a clean slate. The thought of throwing away all the code we ve ever written and starting over can seem alluring, but this typically isn t a viable option for most companies. Many development organizations have made a substantial investment in developing and purchasing COM components and ActiveX controls. Microsoft has made a commitment to ensure that these legacy components are usable from within .NET applications, and (perhaps less important) that .NET components are easily callable from COM. The ability to mix managed .NET code with unmanaged code from the older worlds of Win32 and COM is called interoperability, or as it s usually abbreviated, interop. This chapter describes the support .NET provides for using ActiveX controls and COM components into your application, exposing .NET classes to COM-based applications, and making direct calls to Win32 APIs. You ll also learn about C# pointers and keywords for accessing memory directly, which can be necessary for using some unmanaged APIs.
Importing ActiveX Controls
ActiveX controls are COM components designed to be dropped into a form. They usually have a user interface, although you may come across nonvisual controls. When Microsoft developed the OCX standard, which allowed developers to build ActiveX controls in C++ and use them with VB (and vice versa), the ActiveX control revolution began. That was way back in 1994, and since then thousands of such controls have been developed, sold, and used. They are small, usually easy to work with, and are an effective example of binary reuse. That ActiveX controls are still popular more than a decade and a half after their invention demonstrates just how useful a lot of developers find them. COM objects are quite different from .NET objects under the covers. But Microsoft was well aware of how popular ActiveX controls had become by the time .NET was launched, and so it made sure that the .NET Framework and Visual Studio work hard
to bridge the gap between the COM and .NET worlds. Visual Studio is able to import COM components into any .NET project, and makes it particularly easy to use ActiveX controls from Windows Forms.
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