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After starting ArcGIS Explorer, the Add-In Error dialog box displays the following error message:
"Could not load file or assembly 'file:///C:\Users\<User Name>\AppData\Roaming\ESRI\ArcGIS Explorer\AddIns\Cache\<Add-in Name>\<Assembly Name>.dll' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded."
ArcGIS Explorer 900 and 1200 required .NET Framework version 2.0 (SP2) and used MSBuild 2.0 to specifically target this version. The MSBuild toolset version was specified explicitly in the Visual Studio 2008 ArcGIS Explorer project templates by way of the ToolsVersion attribute on the Project element in the project file (<Project ToolsVersion="2.0" ... </Project>). The ArcGIS Explorer 1500 SDK requires .NET Framework version 3.5 Service Pack 1, but when converting an ArcGIS Explorer 900 or 1200 Add-in project from Visual Studio 2008 to Visual Studio 2010, the MSBuild toolset version specified in the project file is left unchanged. After conversion, the resulting add-in actually targets the .NET Framework 4.0 instead of 3.5.
This error can be resolved using the steps below.
<Project ToolsVersion="3.5" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">