Could not load file or assembly ‘log4net,or one of its dependencies.

The .NET Assembly loader:

• is unable to find 1.2.0.203
• but did find a 1.2.0.200

This assembly does not match what was requested and therefore you get this error.

In simple words, it can't find the assembly that was referenced. Make sure it can find the right assembly by putting it in the GAC or in the application path. Also see https://docs.microsoft.com/archive/blogs/junfeng/the-located-assemblys-manifest-definition-with-name-xxx-dll-does-not-match-the-assembly-reference.

As marc_s pointed out, this problem usually appears when trying to load different versions of the same assembly. Make sure, that your project uses the same assembly version as the LinqToExcel library, which is also dependent on log4net. Also any other libraries should use the same assembly version. To solve the issue, you can also try to use assembly redirect in your app.config like so:

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="log4net" publicKeyToken="669e0ddf0bb1aa2a" culture="neutral" />
        <bindingRedirect oldVersion="1.2.10.0" newVersion="1.2.11.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>