Why is my SqlCommand returning a string when it should be an int?

问题

I have a query that should always be returning a single int. I have now logged it returning a string entirely unrelated to what it should be.

We've been getting some random FormatExceptions that we've tracked down to several database queries. After some additional logging, I found that, this morning, the query below returned the string "gladiator". Website.PkID is an int column and works most of the time, but some times it fails miserably and returns either an int that's waaaay out there (bigger than any valid WebsiteID) or a random string.

This particular query is hit once per session start. It's not using a shared connection, so I'm having trouble understanding how it could get such a mixed-up result. Could there be some kind of corruption in the connection pools?

I don't think the problem is isolated to this query. I've seen similar FormatExceptions (because of an unexpected result) coming from LINQ queries as well. We've also spotted some of these errors around the same times:

A transport-level error has occurred when sending the request to the server. (provider: TCP Provider, error: 0 - An existing connection was forcibly closed by the remote host.

Could it be a connection issue? Or maybe we're getting result sets mixed up between the db server and the web server? This has really got me scratching my head.

Offending query:

public static int GetActiveWebSiteID(string storeID, string statusID)
{
    int retval;

    string sql = @"SELECT isnull(MAX(PkID),0) FROM WebSite 
                   WHERE StoreID = @StoreID 
                   AND WebSiteStatusID = @WebSiteStatusID";

    SqlConnection conn = new SqlConnection(Settings.ConnString);
    SqlCommand cmd = new SqlCommand(sql, conn);
    cmd.CommandType = CommandType.Text;
    cmd.Parameters.AddWithValue("@StoreID", (object)storeID ?? DBNull.Value);
    cmd.Parameters.AddWithValue("@WebSiteStatusID", (object)statusID ?? DBNull.Value);

    conn.Open();
    using(conn)
    {
        var scalar = cmd.ExecuteScalar(); // <-- This value returned here should only ever be an int, but randomly is a string

        retval = Convert.ToInt32(scalar);
    }
    return retval;
}

The above query has worked fine for years until recently. We now have a bunch of additional LINQ queries in the app (not sure if that makes a difference). We're running .Net 3.5.

回答1:

After a few months of ignoring this issue, it started to reach a critical mass as traffic gradually increased. We increased logging and found numerous definitive instances where, under heavy load, completely different result sets would get returned to unrelated queries.

We watched the queries in Profiler and were able to see that the bad results were always associated with the same spid, and that each bad result was always one query behind the actual sql statement being queried. It was like a result set got missed and whatever result set was next in the spid (from another connection in the same pool) was returned. Crazy.

Through trial and error, we eventually tracked down a handful of SqlCommand or LINQ queries whose SqlConnection wasn't closed immediately after use. Instead, through some sloppy programming originating from a misunderstanding of LINQ connections, the DataContext objects were disposed (and connections closed) only at the end of a request rather than immediately.

Once we refactored these methods to immediately close the connection with a C# "using" block (freeing up that pool for the next request), we received no more errors. While we still don't know the underlying reason that a connection pool would get so mixed up, we were able to cease all errors of this type. This problem was resolved in conjunction with another similar error I posted, found here: What Causes "Internal Connection Fatal Errors"?

回答2:

The ExecuteScalar() function's return type is object, and you declare your result variable with the var keyword. That's not really a good combination, because you're putting a lot of pressure on the system to get the type inference right.

回答3:

I've recently seen a case where code was switching connection strings unexpectedly. For diagnostic purposes, please hard code the connection string and see if the problem goes away.

Also, for sanity's sake, please use nested using blocks like:

using(SqlConnection conn = new SqlConnection("hard-coded connection string"))
{
    using (SqlCommand cmd = new SqlCommand(sql, conn))
    {
        // more init
        object scalar = cmd.ExecuteScalar();

        // process result
    }
 }

It wouldn't surprise me to find there are two database instances, and in one, PkID is an int, in another it's varchar.

Take a look with SQL Profiler to see if you can catch the return of "gladiator". In the other case I was working with, SQL Profiler showed nothing at all, indicating that the actual query was going to a different database.

回答4:

I think that you were thinking about sqlCommand.ExecuteNonQuery that returns the number of rows affected within an int value...

This is the definition of the ExecuteScalar method:

public override object ExecuteScalar()
Member of System.Data.SqlClient.SqlCommand

Summary:

Executes the query, and returns the first column of the first row in the result set returned by the query. Additional columns or rows are ignored.

Returns:

The first column of the first row in the result set, or a null reference (Nothing in Visual Basic) if the result set is empty.

So, I think that the common way of returning that column is as a string representation of the column value.

回答5:

the Field "PkID" is a varchar/char in the "WebSite" Table.

If the "ISNULL" part of the query is true then it will return an integer (0), else it will return an string with the value of "PkID"

回答6:

There could be more than one WebSite table. Can you qualify the table with schema name:

SELECT isnull(MAX(PkID),0) FROM YourSchema.WebSite WHERE StoreID = @StoreID AND WebSiteStatusID = @WebSiteStatusID

回答7:

Is there any commonality to when it fails to return an int?

Since your query only ever returns a single column in a single row, what do you get if you use a more typesafe ExecuteReader and take the first column's value?

Is it always returning a row? If the WHERE clause causes it to return no rows (say your parameters are not what you think they are), your ISNULL doesn't come into effect - there are no rows at all, and ExecuteScalar is supposed to return a NULL.

回答8:

I think neither your posted query nor LINQ are the problem.

Are you really sure you're looking at the right source? How is the method called? How is logging done?

Select isn't broken.

回答9:

I'm assuming that Settings.ConnString is read from Web.Config or the registry and is re-used other static routines. Could it be possible that there is a timing issue where a second method is executed prior to the call to cmd.ExecuteScalar() in your routine which modifies cmd.CommandText on the connection?

Hope this helps,

Bill

来源：https://stackoverflow.com/questions/882784/why-is-my-sqlcommand-returning-a-string-when-it-should-be-an-int

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