I’m working on a Teams tab app that will only be installed on a Teams Channel. At the present time, the app is not using SSO since the user will already be logged into Teams and be a member of the channel.

Upon request initiated by the user(s), the app will retrieve a file (byte[]) via a REST API call. My goal is to then save that file into the folder in SharePoint that is defined by the TeamsChannelContext.RelativeUrl. By all indications, from reading thru hundreds of articles, I need to use MS Graph to do this.

The main issue I’m having is finding a way to get a valid access token. Just about every article shows code using a clientId (which I assume is either from the TeamsUserContext.Id or perhaps the TeamsChannelContext.Id) and either a client password or client secret, neither of which are available.

After several failed iterations trying to use CredentialCache.DefaultCredentials in a web request, I found an article referring to creating a TokenProvider like…

public class TokenProvider : IAccessTokenProvider
{
    public string token { get; set; }

    public Task<string> GetAuthorizationTokenAsync(Uri uri, Dictionary<string, object>                 additionalAuthenticationContext = default,
        CancellationToken cancellationToken = default)
    {
        return Task.FromResult(token);
    }

    public AllowedHostsValidator AllowedHostsValidator { get; }
}    

…and called with…

        TokenProvider provider = new TokenProvider();
        TokenRequestContext ctx = new TokenRequestContext(new string[] { "User.ReadWrite.All" }, context.User.Id);

I tried to fill the ‘token’ string by creating a Microsoft.TeamsFx.TeamsUserCredential, but could not get the ‘service’ setup for it in Program.cs.

Is what I’m looking to do even possible ?