Check the ubuntu version by using this command

$ lsb_release -a

Let’s Add the Microsoft package signing key

$ wget https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb

run the command below

$ sudo dpkg -i packages-microsoft-prod.deb

Install .NET core 3.1 on Ubuntu 20.04

$ sudo apt-get install -y apt-transport-https

$ sudo apt-get update

$ sudo apt-get install -y dotnet-sdk-3.1

Verify the installation by using the command below

$ dotnet --info

Create c# console application

$ dotnet new console -o hellowordapplication

run the application

$ cd hellowordapplication

$ dotnet run