logic.cs

public interface IsEmailAlreadyInUse
{
  Task<bool> Execute(string emailAddress);
}

public interface CreateUser
{
  Task Execute(string username, string email);
}
 
internal class SqlRepository : IsEmailAlreadyInUse
{
  Task<bool> IsEmailAlreadyInUse.Execute(string emailAddress)
  {
      // Implementation goes here
  }
  
  Task CreateUser.Execute(string username, string emailAddress)
  {
    // Implementation goes here
  }
}

public class CreateUserCommand
{
  private readonly IsEmailAlreadyInUse isEmailAlreadyInUse;
  private readonly CreateUser createUser;
  
  // ctor
  
  public async Task<bool> Execute(string username, string emailAddress)
  {
    if (await this.isEmailAlreadyInUse(emailAddress)) return false;
    
    await this.createUser(username, emailAddress);
    return true;
  }
}