How to create a comma separated string from a list of string in C#?

A List of strings can be converted to a comma separated string using the built-in string.Join() method. This method takes a separator and a collection of strings, returning a single concatenated string.

This type of conversion is useful when collecting user data (such as checkbox selections) and converting it to a comma separated string for database queries or API calls.

Syntax

Following is the syntax for string.Join() method −

string.Join(separator, collection);

Parameters

• separator − The string to use as separator between elements

• collection − A collection that contains the strings to concatenate

Return Value

Returns a string that consists of the elements in the collection delimited by the separator string. If the collection is empty, the method returns an empty string.

Using string.Join() with List<string>

Example

using System;
using System.Collections.Generic;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         List<string> fruitsList = new List<string> {
            "banana",
            "apple",
            "mango"
         };
         string fruits = string.Join(",", fruitsList);
         Console.WriteLine(fruits);
      }
   }
}

The output of the above code is −

banana,apple,mango

Using string.Join() with Complex Objects

You can extract properties from a list of complex objects using LINQ's Select() method and then join them −

Example

using System;
using System.Collections.Generic;
using System.Linq;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         var studentsList = new List<Student> {
            new Student {
               Id = 1,
               Name = "John"
            },
            new Student {
               Id = 2,
               Name = "Jack"
            }
         };
         string students = string.Join(",", studentsList.Select(student => student.Name));
         Console.WriteLine(students);
      }
   }
   
   public class Student {
      public int Id { get; set; }
      public string Name { get; set; }
   }
}

The output of the above code is −

John,Jack

Using Different Separators

Example

using System;
using System.Collections.Generic;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         List<string> colors = new List<string> { "Red", "Green", "Blue" };
         
         string commaDelimited = string.Join(",", colors);
         string pipeDelimited = string.Join(" | ", colors);
         string spaceDelimited = string.Join(" ", colors);
         
         Console.WriteLine("Comma: " + commaDelimited);
         Console.WriteLine("Pipe: " + pipeDelimited);
         Console.WriteLine("Space: " + spaceDelimited);
      }
   }
}

The output of the above code is −

Comma: Red,Green,Blue
Pipe: Red | Green | Blue
Space: Red Green Blue

Conclusion

The string.Join() method provides an efficient way to convert a list of strings into a single delimited string. It works with any IEnumerable<string> collection and allows you to specify any separator string, making it ideal for creating CSV data, SQL query parameters, or formatted output.