如果为Mysql 数据库--->providerName="MySql.Data.MySqlClient"（没装mysql dotnet 安装版的话就得在C:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/CONFIG中machine.config加语句了）

在machine.config文件中有这么一段，如果没有name="MySQL Data Provider" 则会出现这种错误。

<system.data>

    <DbProviderFactories>

      <add name="Odbc Data Provider" invariant="System.Data.Odbc" description=".Net Framework Data Provider for Odbc" type="System.Data.Odbc.OdbcFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

      <add name="OleDb Data Provider" invariant="System.Data.OleDb" description=".Net Framework Data Provider for OleDb" type="System.Data.OleDb.OleDbFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

      <add name="OracleClient Data Provider" invariant="System.Data.OracleClient" description=".Net Framework Data Provider for Oracle" type="System.Data.OracleClient.OracleClientFactory, System.Data.OracleClient, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

      <add name="SqlClient Data Provider" invariant="System.Data.SqlClient" description=".Net Framework Data Provider for SqlServer" type="System.Data.SqlClient.SqlClientFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

      <add name="SQL Server CE Data Provider" invariant="Microsoft.SqlServerCe.Client" description=".NET Framework Data Provider for Microsoft SQL Server 2005 Mobile Edition" type="Microsoft.SqlServerCe.Client.SqlCeClientFactory, Microsoft.SqlServerCe.Client, Version=9.0.242.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />

      <add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data, Version=5.0.2.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d" />

    </DbProviderFactories>

</system.data>

如果为Aceess数据库--->providerName="System.Data.OleDb"

如果为Oracle 数据库--->providerName="System.Data.OracleClient"或者providerName="Oracle.DataAccess.Client"

如果为SQLite数据库--->providerName="System.Data.SQLite"

如果为sql     数据库--->providerName="System.Data.SqlClient"