CustomGrantType.cs

    public class CustomGrantType : ICustomGrantValidator
    {
        public string GrantType => "wechat_credentials";

        public Task<CustomGrantValidationResult> ValidateAsync(ValidatedTokenRequest request)
        {
            var wechatToken = request.Raw.Get("wechat_token");
            var wechatUnionId = request.Raw.Get("wechat_unionId");

            if (string.IsNullOrWhiteSpace(wechatToken))
            {
                return Task.FromResult<CustomGrantValidationResult>(
                    new CustomGrantValidationResult($"Missing parameter wechat_token"));
            }


            if (string.IsNullOrWhiteSpace(wechatUnionId))
            {
                return Task.FromResult<CustomGrantValidationResult>(
                    new CustomGrantValidationResult($"Missing parameter wechat_unionId"));
            }

            return Task.FromResult(new CustomGrantValidationResult());
        }
    }