sqlparamter如何传递nvarchar(max) 参数

SqlParameter paramSummary =
                new SqlParameter("@DocumentSummary",
                SqlDbType.VarChar, -1);

		

CREATE PROCEDURE GetDocumentSummary
(
    @DocumentID int,
    @DocumentSummary nvarchar(MAX) OUTPUT
)
AS
SET NOCOUNT ON
SELECT  @DocumentSummary=Convert(nvarchar(MAX), DocumentSummary)
FROM    Production.Document
WHERE   DocumentID=@DocumentID

		

static private string GetDocumentSummary(int documentID)
...{
    //Assumes GetConnectionString returns a valid connection string.
    using (SqlConnection connection =
               new SqlConnection(GetConnectionString()))
    ...{
        connection.Open();
        SqlCommand command = connection.CreateCommand();
        try
        ...{
            // Setup the command to execute the stored procedure.
            command.CommandText = "GetDocumentSummary";
            command.CommandType = CommandType.StoredProcedure;

            // Set up the input parameter for the DocumentID.
            SqlParameter paramID =
                new SqlParameter("@DocumentID", SqlDbType.Int);
            paramID.Value = documentID;
            command.Parameters.Add(paramID);

            // Set up the output parameter to retrieve the summary.
            SqlParameter paramSummary =
                new SqlParameter("@DocumentSummary",
                SqlDbType.NVarChar, -1);
            paramSummary.Direction = ParameterDirection.Output;
            command.Parameters.Add(paramSummary);

            // Execute the stored procedure.
            command.ExecuteNonQuery();
            Console.WriteLine((String)(paramSummary.Value));
            return (String)(paramSummary.Value);
        }
        catch (Exception ex)
        ...{
            Console.WriteLine(ex.Message);
            return null;
        }
    }
}