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Here is a solution I answered previously with a very similar problem:<br />
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<a href="https://www.experts-exchange.com/questions/20790501/How-to-send-info-data-to-a-previous-instance-of-the-application.html" rel="ugc">https://www.experts-exchange.com/questions/20790501/How-to-send-info-data-to-a-previous-instance-of-the-application.html</a><br />
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In general you need to find the old instance, send the new file name to it and quit the new instance. You have the needed classes/methods in the provided link.
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Here is a solution I answered previously with a very similar problem:

https://www.experts-exchange.com/questions/20790501/How-to-send-info-data-to-a-previous-instance-of-the-application.html [https://www.experts-exchange.com/questions/20790501/How-to-send-info-data-to-a-previous-instance-of-the-application.html]

In general you need to find the old instance, send the new file name to it and quit the new instance. You have the needed classes/methods in the provided link.

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I have an application that is mimed to two different file types. &nbsp;Currently when a user double-clicks a file it loads into a new instance of the application. &nbsp;What needs to happen is to have the file load into a new instance of the application only if it is not already running; and if the application is already running, it should be loaded in that existing instance and not open a new instance.
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I have an application that is mimed to two different file types.  Currently when a user double-clicks a file it loads into a new instance of the application.  What needs to happen is to have the file load into a new instance of the application only if it is not already running; and if the application is already running, it should be loaded in that existing instance and not open a new instance.

Mimed files should be opened in a running instance of the application if it exists.

C# is an object-oriented programming language created in conjunction with Microsoft’s .NET framework. Compilation is usually done into the Microsoft Intermediate Language (MSIL), which is then JIT-compiled to native code (and cached) during execution in the Common Language Runtime (CLR).