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:globe_with_meridians: REST with C++ SDK (Casablanca)

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## HTTP-Clients based on C++ REST SDK & C# ##
This demo consists of three small VS projects:

a) **C++ DLL with exported functions**

b) **C++ Console App** using headers from DLL

c) **C# Console App** using DllImport

The DLL Exports are as follows:

**C++ Client App**

The C++ App uses the *Connector.h* header from DLL to create a *unique_ptr*
of the Connector class.

Connector utilizes internally the <a href="https://casablanca.codeplex.com/" target="_blank">C++ REST SDK (Casablanca)</a> to call a
publicly available JSON Echo service.

To make this example a little bit more realistic the method **GetJson** 
expects a function object. This object (a function pointer) is used to
display the JSON response. There's also a possibility to send additional
headers.

Inside the **GetJson** method the server communication is handled asynchronously
via objects & functions from the <a href="https://msdn.microsoft.com/de-de/library/jj987780.aspx" target="_blank">pplx</a> namespace.

**C# Client App**

The .NET App imposes several restrictions to exported functions. 
Instead of C++ <a href="http://en.cppreference.com/w/cpp/utility/functional/function" target="_blank">function templates</a> I had to use old-style C function
pointers. This is because .NET can only marshal Delegates to function pointers. 
The same happened to the C++ string class. It was replaced by 
raw *wchar_t* pointers. Ugly, but playing with .NET *DllImport* is always rather
unpleasant.

This is how the whole <a href="https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute(v=vs.110).aspx" target="_blank">DllImport soup</a> looks like:

**First**, we declare a delegate and decorate it as an "unmanaged" (that is, non-NET callable) function pointer. Also, we set the
calling convention to *StdCall*. This means that the callee (the DLL function) will be responsible to clean the stack and not the caller (the Client). More info
on argument passing and calling conventions <a href="https://msdn.microsoft.com/en-us/library/984x0h58.aspx" target="_blank">here</a>.

**Second**, we grab the DLL which hopefully exports the desired function. In this case the *Connector.dll* should be located where the executing assembly is. In other cases we'd adjust the first argument after *[DllImport]* Attribute. Also, the same calling convention is applied.
Now, it's important to know that functions from non-NET DLLs are *statically imported externals* and very often demand special treatment of passed arguments.
In this case we have to marshal a pure .NET entity, the <a href="https://msdn.microsoft.com/en-us/library/ms173171.aspx" target="_blank">Delegate</a> to a raw <a href="https://msdn.microsoft.com/en-us/library/7esfatk4%28v=vs.110%29.aspx" target="_blank">function pointer</a> To achieve this we use attribute *[MarshalAs]* and 
decorate the *OnCompletedDelegate* as *unmanaged function pointer*.

**Finally**, the C++ DLL will be able to "call back" our delegate as soon as the JSON Echo Server delivers a response.

*Example, C++ client calling service & passing additional headers*

*Example, C# client calling service*

**IMPORTANT**

The clients need these two libraries to be located in their root folders:

*Connector.dll*

*cpprest120d_2_4.dll* (this is the library from C++ REST SDK and can be installed via <a href="https://www.nuget.org/packages/cpprestsdk/" target="_blank">Nuget</a>)

**LICENSE**

MIT

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