Hi Howard,

Yes, what you want can be achieved in .Net. This is called Side-By-Side
execution.

I have written 3 sample test projects to demonstrate your situation:
SxSTest, AddinPrj and ClassLibrary1. The SxSTest is a winform application
for simplicity which uses the 1.0.0.0 version of ClassLibrary1. AddinPrj is
the Addin project that uses the 2.0.0.0 version of ClassLibrary1. Below is
the test code:

//ClassLibrary1 version 1.0.0.0
namespace ClassLibrary1
{
    public class Class1
    {
        public static int  StaticMethod(int a, int b)
        {
            return a+b;
        }
    }
}

//ClassLibrary1 version 2.0.0.0
namespace ClassLibrary1
{
    public class Class1
    {
        public static int  StaticMethod(int a, int b)
        {
            return a*b;
        }
    }
}

//AddinPrj project
namespace AddinPrj
{
    public class AddIn
    {
        public static int AddInMethod()
        {
            return ClassLibrary1.Class1.StaticMethod(5, 6);
        }
    }
}

//SxSTest
private void button1_Click(object sender, System.EventArgs e)
{
    MessageBox.Show(ClassLibrary1.Class1.StaticMethod(5, 6).ToString());
}

private void button2_Click(object sender, System.EventArgs e)
{
    MessageBox.Show(AddinPrj.AddIn.AddInMethod().ToString());
}
I placed the 1.0.0.0 and 2.0.0.0 version of ClassLibrary1 in "V1" and "V2"
subdirectory of the Exe bin folder.

During the testing, I found that the Probing privatePath can not be used to
notify the runtime to load the assembies with the same name. We should use
the <codeBase> element to configure the runtime the correct loading path.
For example, in my test scenario, the below app.config file works for me:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
 <runtime>
   <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
     <dependentAssembly>
       <assemblyIdentity name="ClassLibrary1"
publicKeyToken="270aa85ff6943c10" />
       <codeBase version="1.0.0.0" href="V1/ClassLibrary1.dll"/>
       <codeBase version="2.0.0.0" href="V2/ClassLibrary1.dll"/>
     </dependentAssembly>
   </assemblyBinding>
 </runtime>
</configuration>

So you may add the same syntax of <codeBase> to your Asp.net application's
web.config file, the runtime will happily load the 2 different versions of
assembly 'A'. I have tested above code under both .Net1.1 and .Net2.0, if
it still does not work for you, please feel free to tell me, thanks.

Finally, below is the official document regarding how the runtime locates
the assemblies. Please note the "Multiple Assemblies with the Same Name"
section, it means "multiple assemblies with the same name" should use
<codeBase> as the configuration element.
http://msdn2.microsoft.com/en-us/library/15hyw9x3.aspx

Hope this helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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