Upload CSV file to SQL server

1st off, You don't need programming stuff. You can directly upload CSV files into SQL Database with SQL management tools. However, if you really need do it through programming, Just read below.

Personally, I think this approach is the most efficient and easiest way to do through programming.

In general, you can achieve it in two steps

1st step is to read the CSV file and hold the records as a DataTable.
2nd step is store the retrieved DataTable into SQL Database Table as a Bulk Entry

This is a function that returns CSV File Data as a DataTable. Call and Keep it in the memory and you can do whatever you want with it.

This function is going to return CSV Read file into DataTable.

private static DataTable GetDataTabletFromCSVFile(string csv_file_path)
    {
        DataTable csvData = new DataTable();
        try
        {
          using(TextFieldParser csvReader = new TextFieldParser(csv_file_path))
             {
                csvReader.SetDelimiters(new string[] { "," });
                csvReader.HasFieldsEnclosedInQuotes = true;
                string[] colFields = csvReader.ReadFields();
                foreach (string column in colFields)
                {
                    DataColumn datecolumn = new DataColumn(column);
                    datecolumn.AllowDBNull = true;
                    csvData.Columns.Add(datecolumn);
                }
                while (!csvReader.EndOfData)
                {
                    string[] fieldData = csvReader.ReadFields();
                    //Making empty value as null
                    for (int i = 0; i < fieldData.Length; i++)
                    {
                        if (fieldData[i] == "")
                        {
                            fieldData[i] = null;
                        }
                    }
                    csvData.Rows.Add(fieldData);
                }
            }
        }
        catch (Exception ex)
        {
           return null;
        }
        return csvData;
    }
  }

SQLBulkCopy - Use this function to insert the Retrieved DataTable into Sql Table

static void InsertDataIntoSQLServerUsingSQLBulkCopy(DataTable csvFileData)
{
    using(SqlConnection dbConnection = new SqlConnection("Data Source=ProductHost;Initial Catalog=yourDB;Integrated Security=SSPI;"))
    {
         dbConnection.Open();
         using (SqlBulkCopy s = new SqlBulkCopy(dbConnection))
         {
             s.DestinationTableName = "Your table name";
             foreach (var column in csvFileData.Columns)
                 s.ColumnMappings.Add(column.ToString(), column.ToString());
             s.WriteToServer(csvFileData);
         }
     }

Source

The best way I found to import large CSV files into SQL Server is by using SqlBulkCopy along with IDataReader implementation. The good thing about it is that you're not reading the entire file into memory (which is the case using DataTable approach) and you can control the size of the batch which gets sent to SQL Server. The bad thing about it is that you have to implement IDataReader which is one of the longest MS interfaces I've seen.

I wrote a nuget package that does the trick for you. It uses the awesome CsvHelper package so there's very little config required. The simplest scenario would look like this:

//Instantiate the reader, providing the list of columns which matches 1 to 1 the data table structure.
var dataReader = new CsvDataReader(filePath,
    new List<TypeCode>(5)
    {
        TypeCode.String,
        TypeCode.Decimal,
        TypeCode.String,
        TypeCode.Boolean,
        TypeCode.DateTime
    });

bulkCopyUtility.BulkCopy("TableName", dataReader);

There are also additional configuration options for more complex scenarios (flexible column mapping, additional static column values which are not present in the csv file, value transformation). If you're interested, the project is on Github and available as a nuget package.

For reference, here's how to use SqlBulkCopy with IDataReader:

public void BulkCopy(string tableName, IDataReader dataReader, Action<SqlBulkCopy>  configureSqlBulkCopy)
{
    using (SqlConnection dbConnection = new SqlConnection(connectionString))
    {
        dbConnection.Open();

        using (SqlBulkCopy bulkCopy = new SqlBulkCopy(dbConnection))
        {
            bulkCopy.BatchSize = 3000; //Data will be sent to SQL Server in batches of this size
            bulkCopy.EnableStreaming = true;
            bulkCopy.DestinationTableName = tableName;

            //This will ensure mapping based on names rather than column position
            foreach (DataColumn column in dataReader.GetSchemaTable().Columns)
            {
                bulkCopy.ColumnMappings.Add(column.ColumnName, column.ColumnName);
            }

            //If additional, custom configuration is required, invoke the action
            configureSqlBulkCopy?.Invoke(bulkCopy);

            try
            {
                // Write from the source to the destination.
                bulkCopy.WriteToServer(dataReader);
            }
            finally
            {
                dataReader.Close();
            }
        }
    }
}