How to store and retrieve credentials on Windows using C#

You can use the Windows Credential Management API. This way you will ask the user for the password only once and then store the password in Windows Credentials Manager.

Next time your application starts and it needs to use the password it will read it from Windows Credentials Manager. One can use the Windows Credential Management API directly using P/Invoke (credwrite, CredRead, example here) or via a C# wrapper CredentialManagement.

Sample usage using the NuGet CredentialManagement package:

public class PasswordRepository
{
    private const string PasswordName = "ServerPassword";

    public void SavePassword(string password)
    {
        using (var cred = new Credential())
        {
            cred.Password = password;
            cred.Target = PasswordName;
            cred.Type = CredentialType.Generic;
            cred.PersistanceType = PersistanceType.LocalComputer;
            cred.Save();
        }
    }

    public string GetPassword()
    {
        using (var cred = new Credential())
        {
            cred.Target = PasswordName;
            cred.Load();
            return cred.Password;
        }
    }
}

I don't recommend storing passwords in files on client machines. Even if you encrypt the password, you will probably embed the decryption key in the application code which is not a good idea.