StdLink.RequestTransaction Method 

Must always be called before calling into Mathematica from a .NET user-interface action.

Remarks

You should always call RequestTransaction whenever you are preparing to call into Mathematica from a .NET user-interface action, like clicking a button. This only applies for code executing in a .NET runtime launched from Mathematica by InstallNET[], not if you are launching and controlling the kernel from a standalone .NET program.

The only time you do not need to call RequestTransaction before calling into Mathematica is when the call is being made during the handling of a call from Mathematica into .NET. In other words, when Mathematica is currently in the middle of a call into .NET, it is always safe for calls to be made from .NET back into Mathematica (and RequestTransaction does nothing in this circumstance). You need RequestTransaction to guard calls that originate in .NET. User interface actions are typical examples of such calls, as Mathematica might not currently be ready to accept incoming requests from .NET.

What RequestTransaction does is check whether it is safe to call into Mathematica. If it is safe, it returns immediately. If it is not safe, a MathematicaNotReadyException is thrown. You can typically ignore this exception and let it propagate up into the .NET event-dispatching mechanism, where it will be caught by the internals of .NET/Link and cause a MessageBox to be displayed.

Many programmers will never use this method, because they will use the Mathematica function AddEventHandler to establish callbacks from .NET into Mathematica. Event handlers created by AddEventHandler handle all details of the communication with Mathematica for you (and of course they call RequestTransaction internally).

IKernelLink ml = StdLink.Link;
if (ml != null) {
    StdLink.RequestTransaction();
    // Always lock the link before using it.
    lock (ml) {
        ml.Print("You clicked a button");
    }    
}

Note again that this is not the "normal" way you would wire up a Print action to a button click. Instead, you would just write something like this in Mathematica:

(* Mathematica code *)
AddEventHandler[myButton@Click, Print["You clicked a button"]&];

Exceptions

See Also

StdLink Class | Wolfram.NETLink Namespace