SqlCommand.ExecuteReader Fails to Report Errors - sql

Question

I'm executing a SQL command to create a new record in a database table and get the ID of the created record. However, there's a constraint error generated by the SQL command (uninitialized non-null field) which is not being picked up by the VB code. The code is roughly:-
connection = New SqlConnection(connection_string)
connection.Open()
sql_command = New SqlCommand(command) 'command = the SQL command to execute
sql_command.Connection = connection
sql_command.Parameters.AddRange(sql_parameters.ToArray()) ' sql_parameters is a parameter to the function
reader = sql_command.ExecuteReader()
If reader IsNot Nothing Then
If reader.HasRows Then
While reader.Read
response_handler(reader, data) 'response handler is a callback which populates the data object
End While
End If
reader.Close()
End If
The reader object is non-null but contains no data and no exception is generated. The SQL command is:-
insert into [table] ([column1], [column2], [column3], [column4])
output Inserted.[pk]
values (#1, #2, #3, #4)
Executing the SQL statement using SQL Server Management Studio, I get the error:-
Msg 515, Level 16, State 2, Line 2
Cannot insert the value NULL into column 'somecolumn', table 'tablename'; column does not allow nulls. INSERT fails.
I have also added a handler for the InfoMessage event on the SqlConnection object but that doesn't get called, even when I set FireInfoMessageEventOnUserErrors to true.
Why am I not getting an error and what is the correct way to ensure the error is reported to VB?
I'm using Visual Studio 2008.

Answer 1

The HasRows call returns false if there is an error. That way you will never see the error. Remove both If statements. The null check is redundant, the other one suppresses errors.

Answer 2

The severity level is 16 which does not interrupt the current session. If you want to throw a hard error you could add something like this directly after your insert...
If ##Error <> 0
Begin
Raiserror('Constraint Error Encountered',20,1) With Log;
End
The security context will have to have sysadmin rights in order to perform the RAISERROR WITH LOG, but if you can't do this I'm sure there are other ways to throw a hard error. In any event a warning isn't going to throw an error to your VB code.

Answer 3

I don't know exactly how it works internally, but the property HasRows is set by a private method within the SqlDataReader class called something like TryGetNextResult, which uses a lot of try/catch blocks to set a boolean output parameter, so this method never throws any exception, or even provides any feedback about any errors. What I can't work out is exactly how it realises there will be errors without even attempting the insert, but it does.
With this sample table:
CREATE TABLE T (ID INT IDENTITY, A INT NOT NULL);
I ran:
string sql = #"INSERT T (A) OUTPUT inserted.ID, inserted.A VALUES (1);";
using (var connection = new SqlConnection(connectionString))
using (var command = new SqlCommand(sql, connection))
{
connection.Open();
using (var reader = command.ExecuteReader())
{
Console.WriteLine(reader.HasRows); // True
while (reader.Read())
{
Console.WriteLine(reader.GetInt32(1)); // 1
}
}
}
When I changed this to try and insert null, reader.HasRows was false, but by putting a breakpoint in I was able to test before reader.Read() was called, and this shows that the identity value of the table T was unchanged, so the reader can't be validating inserts by simply rolling back transactions. It is a mystery to me why a SqlDataReader is able to identify that this insert will fail before execution, but yet if the SQL is executed it will still attempt the insert before raising an error.
To further prove the behaviour I ran a similar test with a simple select command, and saw the same behaviour:
static void Main(string[] args)
{
string sql = #" SELECT Date = CAST(D AS DATE)
FROM (VALUES
(1, '20130129'),
(2, '20130130'),
(3, '20130131'),
(4, '20130132')
) t (A, D)
ORDER BY A;";
using (var connection = new SqlConnection(connectionString))
using (var command = new SqlCommand(sql, connection))
{
connection.Open();
using (var reader = command.ExecuteReader())
{
Console.WriteLine(reader.HasRows); // false
while (reader.Read()) // Exception
{
Console.WriteLine(reader.GetString(0));
}
}
}
}
Again this shows, that somehow the SqlDataReader has picked up that there will be an error casting 20130132 to a date. If I remove this row from the SQL the HasRows property becomes true.
I realise this doesn't actually answer your question but it was too long for a comment, hopefully it will help a little, and/or maybe prompt somebody who knows a lot more about c# than me to add their own answer.