How do I write to a file in .NET Core 3.1?

Question

Hello,

I just started using VS 2019 and am new to .NET Core.

My File.WriteAllTextAsync call is not recognized.

The error message is:

1>\program.cs(171,42,171,59): error CS0117: 'File' does not contain a definition for 'WriteAllTextAsync'

I did NuGet the System.IO package.

What should I do?

williamj

Answer 1

The following code sample was done by creating a new .NET Core 3.1 Console project, added the following code and using statement for using System.IO.

using System;  
using System.IO;  
using System.Threading.Tasks;  
  
namespace ConsoleAppWriteToFile  
{  
    class Program  
    {  
        static async Task Main(string[] args)  
        {  
            Console.WriteLine("Hello World!");  
  
            await FileOperations.Write("SomeFile.txt", "Line 1\nline2");  
        }  
    }  
  
    public class FileOperations  
    {  
        public static async Task Write(string fileName, string content)  
        {  
            await File.WriteAllTextAsync(fileName, content);  
        }  
    }  
}  
  

If you expand the Dependencies folder to Frameworks to Microsoft.NETCoreApp you will see System.IO is included.

Project file

<Project Sdk="Microsoft.NET.Sdk">  
  
  <PropertyGroup>  
    <OutputType>Exe</OutputType>  
    <TargetFramework>netcoreapp3.1</TargetFramework>  
  </PropertyGroup>  
  
</Project>  
  

Answer 2

I am not sure how to include the System.IO namespace.

Including 'using System.IO;' is shown greyed out.

Answer 3

Try adding the System.IO.FileSystem package and 'using System.IO'.

But the function is usually available by default. What kind of project did you create?

Answer 4

If your class is called File too, then rename it, or use the full name: System.IO.File.WriteAllTextAsync(...).

Answer 5

Are you sure you must use the asynchronous version? Asynchronous is more complicated. The following works for me.

string[] lines = { "First line", "Second line", "Third line" };
File.WriteAllLines("WriteLines.txt", lines);

You do not need to use NuGet for that. You just need using System.IO;.

Are you sure you are using .NET Core 3.1? When you right-click on the project (project not solution) in the Solution Explorer what do you see for the Target framework?