空值检查

Question

I am trying to run the following code but the error occurin says you have error near the @tid. The parameter is supposed to be taking NULL value.

 public static DataTable GetChapterArticlesSummary(long ChapterId, long? TopicId)
{
    DataTable TableArticles = new DataTable();
    try {
        using (SqlConnection connection = ConnectionManager.GetConnection())
        {
            SqlCommand command = new SqlCommand();
            command.CommandText = "Select Article_Name, Id, Privacy_Term from Articles where Chapter_Id=@chapterid and Topic_Id is @topicid";
            command.Parameters.Add("@chapterid", SqlDbType.BigInt).Value = ChapterId;
            if (TopicId != null)
            {
                command.Parameters.Add("@topicid", SqlDbType.BigInt).Value = TopicId;
            }
            else
            {
                command.Parameters.Add("@topicid", SqlDbType.BigInt).Value = DBNull.Value;
            }
            command.Connection = connection;
            SqlDataAdapter Adapter = new SqlDataAdapter();
            Adapter.SelectCommand = command;
            Adapter.Fill(TableArticles);
        }
    }
    catch (SqlException ex)
    { }
    return TableArticles;
}

Answer 1

There are two ways I would handle this:

1. Rewrite the SQL
2. Rewrite the entire code

1. Rewrite the SQL

Change the relevant portion of your SQL to this:

and (T_Id = @tid or @tid is null)

2. Rewrite the entire code

This will result in two different SQL statements depending on the parameter (to the code) value:

SqlCommand command = new SqlCommand();
if (TId != null)
{
    command.CommandText = "Select Article_Name, Id, Privacy_Term from Articles where Id=@id and T_Id = @tid";
    command.Parameters.Add("@tid", SqlDbType.BigInt).Value = TId;
}
else
{
    command.CommandText = "Select Article_Name, Id, Privacy_Term from Articles where Id=@id and T_Id is null";
}
command.Parameters.Add("@id", SqlDbType.BigInt).Value = Id;
command.Connection = connection;
SqlDataAdapter Adapter = new SqlDataAdapter();
Adapter.SelectCommand = command;
Adapter.Fill(TableArticles);

Answer 2

The problem might be in the if statement:

if (TId != null)

In c# a long variable is never null, unless you declare it as long? so please check with the debugger if its value is correct. If TId is not null here, your function will not send DBNull.Value to the database.

Answer 3

try

T_Id = @tid

because you are sending the dbnull.value right. else debug and check the values of parameters.