C#: DbType.String versus DbType.AnsiString

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)

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