Sql-server – Conversion failed when converting from a character string to uniqueidentifier – Two GUIDs

The database is trying to convert the value from whatever DateTime.ToString is giving you... do you really want to trust that .NET on your calling machine and SQL Server use exactly the same format? That sounds brittle to me.

Avoid this by not putting the value into the SQL directly in the first place - use a parameterized query. This not only avoids conversion issues, but also (equally importantly) avoids SQL injection attacks.

Sample code:

DateTime start = DateTime.Now;
DateTime end = start.AddDays(3);
string sql = @"
SELECT * FROM Forecast
WHERE City = @City AND Date BETWEEN @StartDate AND @EndDate";

// Don't forget to close this somewhere. Why not create a new connection
// and dispose it?
Con.Open();
using (SqlCommand command = new SqlCommand(sql, Con))
{
    command.Parameters.Add("@City", SqlDbType.NVarChar).Value = city;
    command.Parameters.Add("@StartDate", SqlDbType.DateTime).Value = start;
    command.Parameters.Add("@EndDate", SqlDbType.DateTime).Value = end;
    using (SqlDataReader reader = command.ExecuteReader())
    {
        int i = 0;
        while (reader.Read())
        {
            //do something
            i++;
        }
    }
}

There are many formats supported by SQL Server - see the MSDN Books Online on CAST and CONVERT. Most of those formats are dependent on what settings you have - therefore, these settings might work some times - and sometimes not.

The way to solve this is to use the (slightly adapted) ISO-8601 date format that is supported by SQL Server - this format works always - regardless of your SQL Server language and dateformat settings.

The ISO-8601 format is supported by SQL Server comes in two flavors:

• YYYYMMDD for just dates (no time portion); note here: no dashes!, that's very important! YYYY-MM-DD is NOT independent of the dateformat settings in your SQL Server and will NOT work in all situations!

or:

• YYYY-MM-DDTHH:MM:SS for dates and times - note here: this format has dashes (but they can be omitted), and a fixed T as delimiter between the date and time portion of your DATETIME.

This is valid for SQL Server 2000 and newer.

So in your specific case - use these strings:

insert into table1 values('2012-02-21T18:10:00', '2012-01-01T00:00:00');

and you should be fine (note: you need to use the international 24-hour format rather than 12-hour AM/PM format for this).

Alternatively: if you're on SQL Server 2008 or newer, you could also use the DATETIME2 datatype (instead of plain DATETIME) and your current INSERT would just work without any problems! :-) DATETIME2 is a lot better and a lot less picky on conversions - and it's the recommend date/time data types for SQL Server 2008 or newer anyway.

SELECT
   CAST('02-21-2012 6:10:00 PM' AS DATETIME2),     -- works just fine
   CAST('01-01-2012 12:00:00 AM' AS DATETIME2)   -- works just fine  

Don't ask me why this whole topic is so tricky and somewhat confusing - that's just the way it is. But with the YYYYMMDD format, you should be fine for any version of SQL Server and for any language and dateformat setting in your SQL Server.