AddWithValue is Evil

AddWithValue is the root cause of many SQL Server performance and concurrency problems. Albeit the SqlParameterCollection.AddWithValue method is a slightly more convenient way to add parameters and values to a SqlCommand, the implications are insidious because the high costs of SQL Server resource utilization is not apparent during development. It’s often not until the application is in production and under load that it is realized that queries are slow due to full scans, excessive resource usage, lock escalation, blocking, cache bloat, and many deadlocks.

A best practice in SQL Server development has long been strongly-typed parameters. Not only does this practice thwart SQL injection (barring non-parameterized dynamic SQL statements), performance is generally improved due to caching of queries that differ only by values, avoiding the cost of compiling the query each time it is executed. Importantly, matching parameter definitions with referenced columns promotes efficiency by avoiding implicit conversions that can preclude efficient index use.

To follow best practices, avoid AddWithValue and instead add parameters and values using a method that allows specification of the desired SqlDbType along with the appropriate length, precision, and scale. Although slightly more verbose than AddWithValue, the performance benefits are huge and worth a few keystrokes.

The Problem with Addwithvalue
The nastiness with AddWithValue is that ADO.NET infers the parameter definition from the supplied object value. Parameters in SQL Server are inherently strongly-typed, including the SQL Server data type, length, precision, and scale. Types in .NET don’t always map precisely to SQL Server types, and are sometimes ambiguous, so AddWithValue has to makes guesses about the intended parameter type.

The guesses AddWithValue makes can have huge implications when wrong because SQL Server uses well-defined data type precedence rules when expressions involve unlike data types; the value with the lower precedence is implicitly converted to the higher type. The implicit conversion itself isn’t particularly costly but is a major performance concern when it is the column value rather than the parameter value must be converted, especially in a WHERE or JOIN clause predicate. The implicit column value conversion can prevent indexes on the column from being used with an index seek (i.e. non-sargable expression), resulting in a full scan of every row in the table or index.

Below are some of the most common pitfalls with AddWithValue.

.NET Strings
Strings are arguably the single biggest problem with AddWithValue. AddWithValue infers SqlDbType.NVarChar when a string object is provided because strings in .NET are Unicode. The resultant SQL Server parameter type is nvarchar(n) or nvarchar(MAX), depending on the length of the string value provided. The type is nvarchar(n) when the string length is 4000 characters or less and nvarchar(MAX) when over 4000 characters.

The inferred nvarchar parameter type is especially problematic when the parameter value is compared to varchar columns in queries because nvarchar has a higher precedence. This will prevent indexes on the varchar columns from being used as I mentioned earlier. Because varchar columns are widely used when Unicode characters are not required (saving space on disk and memory needs), resultant performance problems due to AddWithValue are quite common in the wild.

Another issue with strings and AddWithValue is cache bloat and lack of reuse. AddWithValue uses the actual length of the supplied value as the nvarchar parameter length. This results in a separate cached plan for queries that differ only by parameter length. For example, you’ll end up with 20 different cached plans with 20 different @LastName parameter value lengths. Consider the number of plans is greatly exacerbated with many string parameters (e.g. @LastName, @FirstName, @Address) and is especially an issue with large parameterized IN clauses.

A better method to add string parameters is with the SqlParameterCollection.Add method. In addition to the desired name and data type, this overload allows one to specify the length, which should match the column maximum length (-1 for MAX types). The value can be easily assigned to the returned SqlParameter instance Value propery in a one-liner:

command.Parameters.Add("@LastName", SqlDbType.VarChar, 30).Value = "Smith";
command.Parameters.Add("@GenderCode", SqlDbType.Char, 1).Value = "M";.

.NET DateTime
AddWithValue infers SqlDbType.Datetime parameter type when a DateTime object value is provided. Although a .NET DateTime aligns closely with SQL Server datetime2(7), AddWithValue uses datetime for backwards compatibility.

The SQL Server datetime type has a fixed precision of 3 with accuracy to 1/300 fractional seconds so ADO.NET will round the value accordingly and sub-millisecond values will be lost, resulting in a value that ends with 0, 3, or 7 milliseconds. This behavior is compatible with a SQL Server datetime column value but loses accuracy and precision when used with a datetime2 column of precision 3 or greater. Values will be rounded to the lesser datetime precision and sub-millisecond values will be lost.

One can explicitly specify SqlDbType.DateTime2 to avoid loss of accuracy when working with the SQL Server datetime2 type. Note that the parameter Scale property is used to specify datetime precision in ADO.NET rather than the Precision property as one might expect. The default Scale is 7, the same as the default datetime2 precision in SQL Server. Although the value will be automatically be rounded when the target column precision is less, I recommend one match the column precision regardless.

This example uses Add instead of AddWithValue, specifying Scale of 3 to match a target column datetime2(3), which is more precise than datetime with 1 byte less storage.

command.Parameters.Add(new SqlParameter("@InsertTimestamp", SqlDbType.DateTime2) { Scale = 3, Value = DateTime.Now });

.NET Decimal
AddWithValue infers a SqlDbType.Decimal with the same precision and scale as the supplied object value. Varying precision and scale properties can bloat cache similarly to string lengths as described earlier so it’s best to explicitly specify the precision and scale matching the column.

Here’s an example that adds a decimal value without AddWithValue, specifying precision and scale (decimal(18,4)):

command.Parameters.Add( new SqlParameter("@InvoiceTotal", SqlDbType.Decimal) { Precision = 18, Scale = 4, Value = 1234.56m });

Summary
I hope this article helps you understand the importance of the strongly-typed parameter best practice. Note only will this help avoid performance and concurrency problems, the attention to detail will save compute costs in the cloud.