How Import data from Excel file to sql server in c# windows form

You may use the code from the OleDB-SQL Utility[^].

See also there examples of connection strings and SQL queries to Excel. For example:

SELECT NULL AS ImportID, * FROM [Sheet1$]

Here is the main function to import using SqlBulkCopy:

static void OleDbToSqlServer(string oleDbConnectionString, string oleDbSQL,
    string sqlConnectionString, string sqlTableName)
{

    oleDbConnectionString = ExpandConnectionStringFileName(oleDbConnectionString);

    OleDbConnection oleDbConnection = new OleDbConnection(oleDbConnectionString);
    try
    {
        oleDbConnection.Open();
        OleDbCommand command = new OleDbCommand(oleDbSQL, oleDbConnection);
        try
        {
            OleDbDataReader reader = command.ExecuteReader();
            try
            {
                SqlConnection sqlConnection = new SqlConnection(sqlConnectionString);
                try
                {
                    sqlConnection.Open();
                    using (SqlBulkCopy bulkCopy = new SqlBulkCopy(sqlConnection))
                    {
                        bulkCopy.DestinationTableName = sqlTableName;
                        bulkCopy.WriteToServer(reader);
                    }
                }
                finally
                {
                    sqlConnection.Close();
                }
            }
            finally
            {
                reader.Close();
            }
        }
        finally
        {
            command.Dispose();
        }
    }
    finally
    {
        oleDbConnection.Close();
    }
}

look for this error that appear when connection is open
Could not find installable ISAM.