在使用ASP.NET MVC中的refresh_token oAuth2时，可能会出现unsupported_grant_type的错误。这意味着您无法使用refresh_token来获取新的访问令牌。要解决此问题，您需要确保以下代码：

授权服务器：确保您的授权服务器配置中支持refresh_token作为授权类型，例如：

public override async Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
{
    // 可以使用如下方法检查client id是否与refresh_token匹配
    // if (context.ClientId == "myClientId")
    // {
    //     context.Validated();
    // }
    // else
    // {
    //     context.SetError("invalid_client", "Client authentication failed.");
    //     return;
    // }

    context.Validated();
    return;
}

public override async Task GrantRefreshToken(OAuthGrantRefreshTokenContext context)
{
    // 检查client是否和access token相匹配
    var clientId = context.Ticket.Properties.Dictionary["as:client_id"];
    if (context.ClientId != clientId)
    {
        context.SetError("invalid_clientId", "Refresh token is issued to a different clientId.");
        return;
    }

    // 更新过期时间, 生成新的access token和refresh token 
    var newIdentity = new ClaimsIdentity(context.Ticket.Identity);
    newIdentity.AddClaim(new Claim("newClaim", "refreshToken"));

    var newTicket = new AuthenticationTicket(newIdentity, context.Ticket.Properties);
    context.Validated(newTicket);
}

客户端：确保您的客户端代码中请求配置包含grant_type为"refresh_token"，例如：

public async Task RefreshToken()
{
    string refreshToken = // 从用户信息或其他来源中获取

    using (var httpClient = new HttpClient())
    {
        var keyValues = new List>
        {
            new KeyValuePair("grant_type", "refresh_token"),
            new KeyValuePair("refresh_token", refreshToken)
        };
        var request = new HttpRequestMessage(HttpMethod.Post, "https://localhost:5001/token");
        request.Content