How do I get and set Environment variables in C#?

Use the System.Environment class.

The methods

var value = System.Environment.GetEnvironmentVariable(variable [, Target])

and

System.Environment.SetEnvironmentVariable(variable, value [, Target])

will do the job for you.

The optional parameter Target is an enum of type EnvironmentVariableTarget and it can be one of: Machine, Process, or User. If you omit it, the default target is the current process.

I ran into this while working on a .NET console app to read the PATH environment variable, and found that using System.Environment.GetEnvironmentVariable will expand the environment variables automatically.

I didn't want that to happen...that means folders in the path such as '%SystemRoot%\system32' were being re-written as 'C:\Windows\system32'. To get the un-expanded path, I had to use this:

string keyName = @"SYSTEM\CurrentControlSet\Control\Session Manager\Environment\";
string existingPathFolderVariable = (string)Registry.LocalMachine.OpenSubKey(keyName).GetValue("PATH", "", RegistryValueOptions.DoNotExpandEnvironmentNames);

Worked like a charm for me.

If the purpose of reading environment variable is to override the values in the appsetting.json or any other config file, you can archive it through EnvironmentVariablesExtensions.

var builder = new ConfigurationBuilder()
                .AddJsonFile("appSettings.json")
                .AddEnvironmentVariables(prefix: "ABC_")

var config = builder.Build();

According to this example, Url for the environment is read from the appsettings.json. but when the AddEnvironmentVariables(prefix: "ABC_") line is added to the ConfigurationBuilder the value appsettings.json will be override by in the environement varibale value.

In Visual Studio 2019 -- Right Click on your project, select Properties > Settings, Add a new variable by giving it a name (like ConnectionString), type, and value. Then in your code read it so:

var sConnectionStr = Properties.Settings.Default.ConnectionString;

These variables will be stored in a config file (web.config or app.config) depending upon your type of project. Here's an example of what it would look like:

  <applicationSettings>
    <Testing.Properties.Settings>
      <setting name="ConnectionString" serializeAs="String">
        <value>data source=blah-blah;etc-etc</value>
      </setting>
    </Testing.Properties.Settings>
  </applicationSettings>

Environment variables can also be placed in an application's app.config or web.config file, by their name bounded with percentages (%), and then expanded in code.

• Note that when a value of an environment variable is changed (or a new one is set), Visual Studio should be closed and reopened.

For example, in app.config:

<connectionStrings>
    <add name="myConnectionString" connectionString="%DEV_SQL_SERVER_CONNECTION_STRING%" providerName="System.Data.SqlClient" />
</connectionStrings>

And then in the code:

string connectionStringEnv = ConfigurationManager.AppSettings["myConnectionString"];
string connectionString = System.Environment.ExpandEnvironmentVariables(connectionStringEnv);