SQL Server Data Type Mappings
SQL Server and the .NET Framework are based on different type systems. For example, the .NET Framework Decimal structure has a maximum scale of 28, whereas the SQL Server decimal and numeric data types have a maximum scale of 38. To maintain data integrity when reading and writing data, the SqlDataReader exposes SQL Server–specific typed accessor methods that return objects of System.Data.SqlTypes as well as accessor methods that return .NET Framework types. Both SQL Server types and .NET Framework types are also represented by enumerations in the DbType and SqlDbType classes, which you can use when specifying SqlParameter data types.
* Use a specific typed accessor if you know the underlying type of the
SQL Server Books Online Reference
For more information about SQL Server data types, see Data Types (Database Engine).
SQL Server Data Types and ADO.NET
SQL Server Binary and Large-Value Data
Data Type Mappings in ADO.NET
Configuring Parameters and Parameter Data Types
ADO.NET Managed Providers and DataSet Developer Center