Linq to SQL: Why decimal field get truncated on Insert?

Question

I have a column defined as decimal(10,6). When I try to save the model with the value 10.12345, in the database I saved as 10.123400. The last digit ("5") is truncated.

Why is the number default to only 4 digits in LINQ (for decimal) and how can I avoid this for all columns in my models? The solution I found was to use DbType="Decimal(10,6)", but I have a lot of columns in this situation and the change should be applied to all, and I don't see it like a good idea.

Is there a way to change this behavior without changing all the decimal columns?

Thanks

Answer 1

You need to use the proper DbType, decimal(10, 6) in this case.

The reason for this is simple - while .NET's decimal is actually a (decimal) floating point (the decimal point can move), MS SQL's isn't. It's a fixed "four left of decimal point, six right of decimal point". When LINQ passes the decimal to MS SQL, it has to use a specific SQL decimal - and the default simply happens to use four for the scale. You could always use a decimal big enough for whatever value you're trying to pass, but that's very impractical - for one, it will pretty much eliminate execution plan caching, because each different decimal(p, s) required will be its own separate query. If you're passing multiple decimals, this means you'll pretty much never get a cached plan; ouch.

In effect, the command doesn't send the value 10.12345 - it sends 10123450 (not entirely true, but just bear with me). Thus, when you're passing the parameter, you must know the scale - you need to send 10 as 10000000, for example. The same applies when you're not using LINQ - using SqlCommand manually has the same "issue", and you have to use a specific precision and scale.

If you're wary of modifying all those columns manually, just write a script to do it for you. But you do need to maintain the proper data types manually, there's no way around it.