string connectionString = "Provider=Microsoft.Jet.OleDb.4.0; Data Source=C:\Sample.xls; Extended Properties=Excel 8.0;"

Please note the “Extended Properties” attribute. This attribute helps us to query the excel file.

Exporting Data from Data Table to Excel File.

string connectionString = "Provider=Microsoft.Jet.OleDb.4.0; Data Source=C:\Sample.xls; Extended Properties=Excel 8.0;"
// Establish a connection to the data source.
using(OleDbConnection Connection = new OleDbConnection(connectionString))
{
Connection.Open()
//creating a new Sheet with name sample and three columns with Heading firstname, lastname and email
using(OleDbCommand command = new OleDbCommand())
{
command.Connection = Connection;
command.CommandText = "CREATE TABLE [Sample$](FirstName Char(255), LastName char(255), Email char(255))";
command.ExecuteNonQuery();
}
//Adding records to the Sample Worksheet
using(OleDbCommand command = new OleDbCommand())
{
command.Connection = Connection;
command.CommandText = "INSERT INTO TABLE [Sample$](FirstName,LastName,Email) VALUES('Anuraj','P','anuraj.p@example.com')";
command.ExecuteNonQuery();
command.CommandText = "INSERT INTO TABLE [Sample$](FirstName,LastName,Email) VALUES('sreekumar','vn','sreekumar.vn@example.com')";
command.ExecuteNonQuery();
}
}

Import the Data from Excel

DataTable dt;
string connectionString = "Provider=Microsoft.Jet.OleDb.4.0; Data Source=C:\Sample.xls; Extended Properties=Excel 8.0;"
// Establish a connection to the data source.
using(OleDbConnection Connection = new OleDbConnection(connectionString))
{
Connection.Open()
//reading data from excel to Data Table
using(OleDbCommand command = new OleDbCommand())
{
command.Connection = Connection;
command.CommandText = "SELECT * FROM [Sample]";
using(OleDbDataAdapter adapter =new OleDbDataAdapter())
{
adapter.SelectCommand = command;
adapter.Fill(dt);
}
}
}

You can get more information from Microsoft support site

http://support.microsoft.com/kb/306023 – How to transfer data to an Excel workbook by using Visual C# 2005 or Visual C# .NET