Azure Service Fabric and Azure Key Vault Secret Error

I am getting the following when trying to implement Azure Key Vault Secret from a stateless service fabric works just fine from a console app.

System.TypeLoadException
  HResult=0x80131522
  Message=Inheritance security rules violated by type: 'System.Net.Http.WebRequestHandler'. Derived types must either match the security accessibility of the base type or be less accessible.
  Source=Microsoft.Rest.ClientRuntime
  StackTrace:
   at Microsoft.Rest.ServiceClient`1.CreateRootHandler

public async Task<string> GetAccessToken(string authority, string resource, string scope)
        {
            var clientId = MyConfig.Settings.Sections["MyConfigSection"].Parameters["AuthClientId"].Value;
            var clientSecret = MyConfig.Settings.Sections["MyConfigSection"].Parameters["AuthClientSecret"].Value;
            ClientCredential clientCredential = new ClientCredential(clientId, clientSecret);

            var context = new AuthenticationContext(authority, TokenCache.DefaultShared);
            var result = await context.AcquireTokenAsync(resource, clientCredential);

            return result.AccessToken;
        }

        public string GetCRMConnectionString()
        {
            var secretvaultAddress = MyConfig.Settings.Sections["MyConfigSection"].Parameters["SecretVaultUrl"].Value;
            var client = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(GetAccessToken));
            return client.GetSecretAsync(secretvaultAddress).GetAwaiter().GetResult().Value;
        }

WebRequestHandler type(whose instance gets created in your case) is a part of System.Net.Http.WebRequest.dll. If you explore assembly's attributes, you'll find the next one applied to it -

[assembly: AllowPartiallyTrustedCallers]

This attribute makes the assembly being considered as SecurityTransparent. WebRequestHandler is derived from HttpClientHandler defined in another assembly - System.Net.Http.dll. So probably on the environment where you have the code deployed, System.Net.Http.dll is missing AllowPartiallyTrustedCallers which makes it security-critical, meaning that rules get violated - transparent code can not call into security-critical one.

Try to resolve it by either creating a binding rule to a specific System.Net.Http.dll version that has AllowPartiallyTrustedCallers attribute or try to create HttpClient explicitly and pass it then into KeyVaultClient ctr.

Refer to this link for more details and options - Inheritance security rules violated by type: 'System.Net.Http.WebRequestHandler'.