在C#中使用csvhelper

date
Sep 29, 2022
slug
10065
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Published
tags
C#
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type
Post
C# CSV - read write CSV data
last modified July 13, 2022 C# CSV tutorial shows how to read and write CSV data in C#. CSV (Comma Separated Values) is a very popular import and export data format used in spreadsheets and databases. Each line in a CSV file is a data record.
C# CSV - read write CSV data
https://zetcode.com/csharp/csv/

C# CSV - read write CSV data

last modified July 13, 2022
C# CSV tutorial shows how to read and write CSV data in C#.

CSV

CSV (Comma Separated Values) is a very popular import and export data format used in spreadsheets and databases. Each line in a CSV file is a data record. Each record consists of one or more fields, separated by commas. While CSV is a very simple data format, there can be many differences, such as different delimiters, new lines, or quoting characters.
In this article, we read and write CSV data with the CsvHelper library.
$ dotnet add package CsvHelper
We need to add the CsvHelper package to our projects.

C# CSV read data by records

In the following example, we read a CSV file by records.
users.csv
FirstName,LastName,Occupation
John,Doe,gardener
Roger,Roe,driver
Lucy,Smith,teacher
We have this users.csv file.
Program.cs
using System.Globalization;
using CsvHelper;
using CsvHelper.Configuration;

var csvConfig = new CsvConfiguration(CultureInfo.CurrentCulture)
{
    HasHeaderRecord = false
};

using var streamReader = File.OpenText("users.csv");
using var csvReader = new CsvReader(streamReader, csvConfig);
string value;
while (csvReader.Read())
{
    for (int i = 0; csvReader.TryGetField<string>(i, out value); i++)
    {
        Console.Write($"{value} ");
    }

    Console.WriteLine();
}
The Read method advances the reader to the next record. We read the fields of the record with TryGetField.
$ dotnet run
FirstName LastName Occupation
John Doe gardener
Roger Roe driver
Lucy Smith teacher

C# CSV read data into objects

In the next example, we read the data into objects with GetRecords.
Program.cs
using System.Globalization;
using CsvHelper;
using CsvHelper.Configuration;

namespace ConsoleApp16_csv_test
{
    internal class Program
    {
        static void Main(string[] args)
        {
            using var streamReader = File.OpenText("users.csv");
            using var csvReader = new CsvReader(streamReader, CultureInfo.CurrentCulture);

            var users = csvReader.GetRecords<User>();

            foreach (var user in users)
            {
                Console.WriteLine(user);
            }
        }
    }    
		record User(string FirstName, String LastName, string Occupation);
}
In the example, we define the User class and read the records of the users.csv file into instances of this class. The GetRecords returns the IEnumerable of the given type.
$ dotnet run
User { FirstName = John, LastName = Doe, Occupation = gardener }
User { FirstName = Roger, LastName = Roe, Occupation = driver }
User { FirstName = Lucy, LastName = Smith, Occupation = teacher }

C# CSV configuration

In the following example, we are going to have a CSV file with semicolon separators and a comment. In order to parse such a different "CSV" file, we need to configure the parser.
users.csv
# this is users.csv file

John;Doe;gardener
Roger;Roe;driver
Lucy;Smith;teacher
We have this users.csv file.
Program.cs
using System.Globalization;
using CsvHelper;
using CsvHelper.Configuration;

var csvConfig = new CsvConfiguration(CultureInfo.CurrentCulture)
{
    HasHeaderRecord = false,
    Comment = '#',
    AllowComments = true,
    Delimiter = ";",
};

using var streamReader = File.OpenText("users.csv");
using var csvReader = new CsvReader(streamReader, csvConfig);

while (csvReader.Read())
{
    var firstName = csvReader.GetField(0);
    var lastName = csvReader.GetField(1);
    var occupation = csvReader.GetField(2);

    Console.WriteLine($"{firstName} {lastName} is {occupation}");
}
The reader is configured with CsvConfiguration.
var csvConfig = new CsvConfiguration(CultureInfo.CurrentCulture)
{
    HasHeaderRecord = false,
    Comment = '#',
    AllowComments = true,
    Delimiter = ";",
};
We tell the reader that there is no header and that the comment character is #. We allow comments in the file and set the comment's character. (Actually, we don't have to set the comment character because by default the # is used.) We set the delimiter to semicolon character. (The blank lines are ignored by default.)
using var csvReader = new CsvReader(streamReader, csvConfig);
The configuration file is passed to the CsvReader.
$ dotnet run
John Doe is gardener
Roger Roe is driver
Lucy Smith is teacher

C# CSV quote fields

In the following example, we show how to quote fields.
Program.cs
using System.Globalization;
using CsvHelper;
using CsvHelper.Configuration;

namespace ConsoleApp16_csv_test
{
    internal class Program
    {
        static void Main(string[] args)
        {
            var csvConfig = new CsvConfiguration(CultureInfo.CurrentCulture)
            {
                ShouldQuote = args => args.Row.Index == 1
            };
            var users = new List<User>
{
    new (1, "John Doe", "gardener", "12/5/1997"),
    new (2, "Lucy Smith", "teacher", "5/12/1983"),
    new (3, "Roger Roe", "driver", "4/2/2001"),
    new (4, "Robert Smith", "cook", "21/11/1976"),
    new (5, "Maria Smith", "accountant", "5/9/1986"),
};
            using var fs = new StreamWriter("users.csv");
            using var csvWriter = new CsvWriter(fs, csvConfig);
            csvWriter.WriteHeader<User>();
            csvWriter.NextRecord();
            csvWriter.WriteRecords(users);
        }
    }
    record User(int Id, string Name, string Occupation, string DateOfBirth);
}
We have a list of users. We decide to quote the second field of each row.
var csvConfig = new CsvConfiguration(CultureInfo.CurrentCulture)
{
    ShouldQuote = args => args.Row.Index == 1
};
In the CsvConfiguration, we set the ShouldQuote property to return true for the second field.
$ dotnet run
$ cat users.csv
Id,"Name",Occupation,DateOfBirth
1,"John Doe",gardener,12/5/1997
2,"Lucy Smith",teacher,5/12/1983
3,"Roger Roe",driver,4/2/2001
4,"Robert Smith",cook,21/11/1976
5,"Maria Smith",accountant,5/9/1986

C# CSV WriteField

A field from a record is written with WriteField.
Program.cs
using System.Globalization;
using System.Text;
using CsvHelper;
using CsvHelper.Configuration;

namespace ConsoleApp16_csv_test
{
    internal class Program
    {
        static void Main(string[] args)
        {
            var users = new List<User>
{
    new ("John", "Doe", "gardener"),
    new ("Roger", "Roe", "driver"),
    new ("Lucy", "Smith", "teacher"),
};
            using var mem = new MemoryStream();
            using var writer = new StreamWriter(mem);
            using var csvWriter = new CsvWriter(writer, CultureInfo.CurrentCulture);

            csvWriter.WriteField("FirstName");
            csvWriter.WriteField("LastName");
            csvWriter.WriteField("Occupation");
            csvWriter.NextRecord();

            foreach (var user in users)
            {
                csvWriter.WriteField(user.FirstName);
                csvWriter.WriteField(user.LastName);
                csvWriter.WriteField(user.Occupation);
                csvWriter.NextRecord();
            }
            writer.Flush();
            var res = Encoding.UTF8.GetString(mem.ToArray());
            Console.WriteLine(res);
        }
    }
    record User(string FirstName, string LastName, string Occupation);
}
In the example, we write CSV data into memory and then to the console.
csvWriter.WriteField("FirstName");
csvWriter.WriteField("LastName");
csvWriter.WriteField("Occupation");
csvWriter.NextRecord();
First, we write the header. The NextRecord method adds a newline.
foreach (var user in users)
{
    csvWriter.WriteField(user.FirstName);
    csvWriter.WriteField(user.LastName);
    csvWriter.WriteField(user.Occupation);
    csvWriter.NextRecord();
}
The WriteField writes the field to the CSV file. A new record is started with NextRecord.
writer.Flush();
To actually write the data, we need to call Flush.
var result = Encoding.UTF8.GetString(mem.ToArray());
Console.WriteLine(result);
We write the data from the memory to the console.
$ dotnet run
FirstName,LastName,Occupation
John,Doe,gardener
Roger,Roe,driver
Lucy,Smith,teacher

C# CSV write data with WriteRecords

In the following example, we write all the records in one shot with WriteRecords.
Program.cs
using System.Globalization;
using CsvHelper;

var users = new List<User>
{
    new ("John", "Doe", "gardener"),
    new ("Lucy", "Smith", "teacher"),
    new ("Roger", "Roe", "writer"),
};

using var writer = new StreamWriter(Console.OpenStandardOutput());
using var csvWriter = new CsvWriter(writer, CultureInfo.CurrentCulture);

csvWriter.WriteHeader<User>();
csvWriter.NextRecord(); // adds new line after header
csvWriter.WriteRecords(users);

record User(string FirstName, string LastName, string Occupation);
In the example, we write the data from the list of user objects into console. The WriteHeader writes the header record from the given members.

C# CSV custom solution

Generally, it is recommended to use an existing library for working with CSV. Despite its perceived simplicity, it is not easy to provide a robust solution. (For instance, fields may be quoted.)
data.csv
John Doe, gardener, 12/5/1997
Jane Doe, teacher, 5/10/1983
Robert Smith, driver, 4/2/2001
Maria Smith, cook, 9/11/1976
This is data.csv file.
Program.cs
using System.Globalization;
using System.Text;
using CsvHelper;
using CsvHelper.Configuration;

namespace ConsoleApp16_csv_test
{
    internal class Program
    {
        static void Main(string[] args)
        {
            var path = "data.csv";
            var lines = File.ReadLines(path, Encoding.UTF8);
            var users = from line in lines
                        let fields = line.Replace(", ", ",").Split(",")
                        select new User(fields[0], fields[1], DateTime.Parse(fields[2]));
            var sorted = from user in users
                         orderby user.DateOfBirth descending
                         select user;
            foreach (var user in sorted)
            {
                Console.WriteLine(user);
            }
        }
    }
    public record User(string Name, string Occupation, DateTime DateOfBirth);
}
The example parses a CSV file using Linq. It also sorts the users by their birthday in descending order.
$ dotnet run
User { Name = Robert Smith, Occupation = driver, DateOfBirth = 4/2/2001 12:00:00 AM }
User { Name = John Doe, Occupation = gardener, DateOfBirth = 12/5/1997 12:00:00 AM }
User { Name = Jane Doe, Occupation = teacher, DateOfBirth = 5/10/1983 12:00:00 AM }
User { Name = Maria Smith, Occupation = cook, DateOfBirth = 9/11/1976 12:00:00 AM }

C# export HTML table into CSV file

In the next example, we scrape an HTML table from a web site and export the data into a CSV file.
For web scraping, we use the AngleSharp library.
Program.cs
using System.Globalization;
using AngleSharp;
using CsvHelper;

var config = Configuration.Default.WithDefaultLoader();
using var context = BrowsingContext.New(config);

var url = "https://nrf.com/resources/top-retailers/top-100-retailers/top-100-retailers-2020";
using var doc = await context.OpenAsync(url);

var htable = doc.GetElementById("stores-list--section-23906");
var trs = htable.QuerySelectorAll("tr").Skip(1);

using var fs = new StreamWriter("data.csv");
using var writer = new CsvWriter(fs, CultureInfo.CurrentCulture);

var rows = new List<Row>();

foreach (var tr in trs)
{
    var tds = tr.QuerySelectorAll("td").Take(3);
    var fields = (from e in tds select e.TextContent).ToArray();
    var row = new Row(fields[0], fields[1], fields[2]);

    rows.Add(row);
}

writer.WriteRecords(rows);

record Row(string Rank, string Company, string Sales);
We scrape data from a table that contains 100 top US retailers for 2020.
var config = Configuration.Default.WithDefaultLoader();
using var context = BrowsingContext.New(config);

var url = "https://nrf.com/resources/top-retailers/top-100-retailers/top-100-retailers-2020";
using var doc = await context.OpenAsync(url);
We set up the AngleSharp context and retrieve a document from the provided link.
var htable = doc.GetElementById("stores-list--section-23906");
var trs = htable.QuerySelectorAll("tr").Skip(1);
We locate the HTML table and select all the rows except for the header.
using var fs = new StreamWriter("data.csv");
using var writer = new CsvWriter(fs, CultureInfo.CurrentCulture);
We set up the CsvWriter.
foreach (var tr in trs)
{
    var tds = tr.QuerySelectorAll("td").Take(3);
    var fields = (from e in tds select e.TextContent).ToArray();
    var row = new Row(fields[0], fields[1], fields[2]);

    rows.Add(row);
}
We get the data from the first three columns of the table.
writer.WriteRecords(rows);
Finally, the records are written to the file with WriteRecords.
In this article, we have read and written CSV data in C# with the CsvHelper library.
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