Could not load file or assembly 'Microsoft.WindowsAzure.Diagnostics' or one of its dependencies

It looks like this is happening when your application starts. Take a look at your web.config, do you have a trace listener pointing to the Microsoft.WindowsAzure.Diagnostics assembly? That might be the reason why your application is not working.

First, take a look at your assembly references and delete Microsoft.WindowsAzure.Diagnostics should it be present (just to be sure we don't use old versions). Then, add a reference to Microsoft.WindowsAzure.Diagnostics, but make sure this is version 1.7.0.0.

You should find the right version of this assembly in: C:\Program Files\Microsoft SDKs\Windows Azure.NET SDK\2012-06\ref

I know this is an old topic but it's still the top Google result for "Could not load file or assembly Microsoft.WindowsAzure.Diagnostics", so here is what I did:

This was happening to me with the Azure SDK 2.7.1 and Visual Studio 2013. Somewhere between upgrading to Windows 10 and Azure SDK 2.7, something didn't get installed right. I tried reinstalling the Azure SDK, upgrading to Visual Studio 2015 but neither worked. I finally had to change the following line in my app.config:

    <system.diagnostics>    <trace>        <listeners>            <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=2.7.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" name="AzureDiagnostics">                <filter type="" />            </add>        </listeners>    </trace></system.diagnostics>

to

    <system.diagnostics>    <trace>        <listeners>            <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=2.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" name="AzureDiagnostics">                <filter type="" />            </add>        </listeners>    </trace></system.diagnostics>

Notice the change from "2.7.0.0" to "2.5.0.0". For me, Microsoft.WindowsAzure.Diagnostics 2.7.0.0 DLL did not exist. Reverting back to 2.5.0.0 works fine. I'd still like to find the root cause but I have more important things to move on to. Hope this helps!

Adding to Sandrinio's answer (sorry I don't have privilege) I had a similar error where 1.7.0.0 was being referenced but this section in my web.config pointed to version 1.0.0.0

<system.diagnostics><trace>  <listeners>    <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"      name="AzureDiagnostics">      <filter type="" />    </add>  </listeners></trace>

I commented it out and the problem was resolved.