How Dapper's String Parameter Handling Can Defeat SQL Server Index Performance
By
PretzelFisch
Slow-proofed and worth the wait. Worth its weight in flour.
Summary
This technical article explains a specific performance issue in SQL Server when using Dapper ORM with C# strings. The author discovered that when using anonymous objects to query varchar columns, Dapper sends nvarchar(4000) parameters by default, causing SQL Server to perform CONVERT_IMPLICIT operations on every row. This conversion defeats database indexes and leads to significant performance degradation, including high CPU usage. The article provides detailed analysis of the problem, benchmark results, and practical solutions including explicit parameter typing and configuration changes to prevent the issue.
Key quotes
· 5 pulledIf you're using Dapper with anonymous objects to query varchar columns, you're probably sending nvarchar(4000) parameters — causing CONVERT_IMPLICIT on every row and defeating your indexes.
The application was running hot — CPU averaging over 50% and spiking into the 90s. We pulled a diagnostic snapshot and started working through the performance issue.
The underlying issue is 100% real — we found it in a production system using AI-assisted code analysis.
The future of writing is collaborative, folks.
Here's the fix: Explicitly type your parameters or configure Dapper to use the correct string type for your database schema.
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