ADO.NET: Safe to specify -1 for SqlParameter.Size for all VarChar parameters?

Question

We have an existing body of C# code that calls parameterized ad-hoc SQL Server queries in many places. We never specify SqlParameter.Size, and it\'s documented that in this

Answer 1

It is not ideal in that it is best to specify a parameter that matches the datatype of the column(s) involved.

You would need to check your query plans to see if they still look reasonable.

Trying the following test

CREATE TABLE #T
(
X VARCHAR(10) PRIMARY KEY
)


DECLARE @A VARCHAR(MAX) = CAST('A' AS VARCHAR(MAX))

SELECT *
FROM #T 
WHERE X = @A

Gives a plan like

SQL Server adds a compute scalar to the plan which calls the internal function GetRangeWithMismatchedTypes and still manages to perform an index seek (more details on implicit conversions here).

A counter example where it does matter is shown in the article Why Doesn’t Partition Elimination Work?. The behaviour described in that article also applies for a varchar(max) parameter against a table partitioned on a varchar(n) column.