Output.txt

mock ApplicationId
mock SecretValue1
mock SecretValue2
SetValueAsync(ApplicationId, mock ApplicationId) called.
SetValueAsync(SecretValue1, mock SecretValue1) called.
SetValueAsync(SecretValue2, mock SecretValue2) called.

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp11
{
    class Program
    {
        static void Main(string[] args)
        {
            var h = new KeyVaultHelper();

            var s = h.GetValueAsync<Secrets>().Result;
            Console.WriteLine(s.ApplicationId);
            Console.WriteLine(s.SecretValue1);
            Console.WriteLine(s.SecretValue2);

            h.SetValueAsync(s).Wait();
        }
    }

    class KeyVaultHelper
    {
        public Task<string> GetValueAsync(string key)
        {
            return Task.FromResult($"mock {key}");
        }

        public Task SetValueAsync(string key, string value)
        {
            Console.WriteLine($"SetValueAsync({key}, {value}) called.");
            return Task.CompletedTask;
        }

        public async Task<T> GetValueAsync<T>()
            where T: new()
        {
            var properties = typeof(T).GetProperties(BindingFlags.Instance | BindingFlags.Public)
                .Where(x => x.CanRead && x.CanWrite)
                .Where(x => x.PropertyType == typeof(string));
            var result = new T();
            foreach (var p in properties)
            {
                p.SetValue(result, await this.GetValueAsync(p.Name));
            }
            return result;
        }

        public async Task SetValueAsync<T>(T value)
        {
            var properties = typeof(T).GetProperties(BindingFlags.Instance | BindingFlags.Public)
                .Where(x => x.CanRead && x.CanWrite)
                .Where(x => x.PropertyType == typeof(string));
            foreach (var p in properties)
            {
                await this.SetValueAsync(p.Name, (string)p.GetValue(value));
            }
        }
    }

    class Secrets
    {
        public string ApplicationId { get; set; }
        public string SecretValue1 { get; set; }
        public string SecretValue2 { get; set; }
    }
}