How to execute a query:

public static async Task<string> ExecuteQuery(Schema schema, string querystring, Inputs inputs = null)
        {
            try
            {
                var result = await new DocumentExecuter().ExecuteAsync(_ =>
                {
                    _.Schema = schema;
                    _.Query = querystring;
                    _.Inputs = inputs;
                }).ConfigureAwait(false);

                var json = new DocumentWriter(Formatting.None, null).Write(result);
                return json;
            }
            catch (Exception ex)
            {
                return string.Empty;
                //return new ResponseMessage();
            }
        }

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Root Query Class:

    public class ViewerType : ObjectGraphType
    {
        public ViewerType()
        {
            Name = "Viewer";

            #region "Queries"
            Field<ListGraphType<PrescriptionType>>("Prescriptions",
                resolve: context => new PrescriptionMongoDataAccessLayer().GetPrescriptions(string.Empty));

            Field<PrescriptionType>("OnePrescription",
                arguments: new QueryArguments(new QueryArgument<StringGraphType> {Name = "id"}),
                resolve: context =>
                {
                    var id = context.GetArgument<string>("id");
                    return new PrescriptionMongoDataAccessLayer().GetOnePrescriptions(id);
                });

            Field<ListGraphType<DrugsType>>("Drugs",
                arguments: new QueryArguments(new QueryArgument<StringGraphType>() {Name = "drugName"}),
                resolve: context =>
                {
                    var drugname = context.Arguments["drugName"].ToString();
                    return new DrugsDataAccessLayer().SearchDrugs(drugname);
                });

            Field<LoginResultType>("LogIn",
                arguments: new QueryArguments(new QueryArgument<StringGraphType>() { Name = "username" }, new QueryArgument<StringGraphType> { Name = "password"}),
                resolve: context =>
                {
                    var username = context.Arguments["username"].ToString();
                    var password = context.Arguments["password"].ToString();
                    //database/business logic
                });
        }
}