C#: DbType.String versus DbType.AnsiString

Question

I have taken over some C# code.

The code is hitting a database with some SQL which uses parameters.

All of the string parameters are typed as

Answer 1

AnsiString
A variable-length stream of non-Unicode characters ranging between 1 and 8,000 characters.

String
A type representing Unicode character strings.

In database:

nchar and nvarchar is unicode

char and varchar is non-unicode

Answer 2

See how to solve the problem in C# with Dapper:

SqlMapper.AddTypeMap(typeof(string), DbType.AnsiString);
SqlMapper.AddTypeMap(typeof(String), DbType.AnsiString);

Answer 3

You would use ansistring instead of string to avoid implicit conversions in your SQL Server database.

i.e. when you pass a string variable into MSSQL it appears as nvarchar(max). Given the fact a well designed database in MSSQL may use varchars as opposed to nvarchars by default (unless there is a business requirement for non latin character sets).

A string variable in this case will cause an implicit conversion in the database. This can then render the engine unable to use certain indexes and perform full table scans (one of the roots of all evil for db performance)

Answer 4

Why would you use DbType.AnsiString instead of DbType.String?

Short answer: When your SqlParameter is pointing to a varchar (or char) database column you would use DbType.AnsiString. When you point to a nvarchar column, use DbType.String because its Unicode.

Longer answer: If these are not mapped accurately (e.g. pointing DbType.String to a varchar(nn) column, you will see CONVERT_IMPLICIT warnings in your query plans as described by Pinal Dave here: https://blog.sqlauthority.com/2018/06/11/sql-server-how-to-fix-convert_implicit-warnings/

In my case the SqlParameter.Size property also seemed to be important: to completely eliminate CONVERT_IMPLICIT warnings I had to map DbType and Size correctly. The default Size if omitted "is inferred from the actual size of the specified parameter value"(1) which will usually mismatch the real column size definition and that raises a warning as well.

1. https://docs.microsoft.com/en-us/dotnet/api/system.data.sqlclient.sqlparameter.size?view=netframework-4.7.2