Accessing to Key Vault from Azure Functions with Managed Identity

appmodule.cs

public class AppModule : Module
{
    public override void Load(IServiceCollection services)
    {
        var azureServiceTokenProvider = new AzureServiceTokenProvider();
        var kv = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback));

        services.AddSingleton<IKeyVaultClient>(kv);
        ...
    }
}

get-secret-function.cs

public class GetSecretFunction : FunctionBase<ILogger>, IGetSecretFunction
{
    private readonly IKeyVaultClient _kv;
    ...

    public GetSecretFunction(AppSettings settings, IMapper mapper, IKeyVaultClient kv)
    {
        this._kv = kv ?? throw new ArgumentNullException(nameof(kv));
        ...
    }

    public override async Task<TOutput> InvokeAsync<TInput, TOutput>(TInput input, FunctionOptionsBase options = null)
    {
        ...
        var secret = await this._kv
                               .GetSecretAsync("https://my-keyvault.vault.azure.net/", "[secret key]")
                               .ConfigureAwait(false);
        ...
    }
}

get-secret.cs

var provider = new AzureServiceTokenProvider();
var kv = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(provider.KeyVaultTokenCallback));

var secret = await kv.GetSecretAsync("https://my-keyvault.vault.azure.net", "[secret key]")
                     .ConfigureAwait(false);