Retrieve Environment Variable Helper

/// <summary>
/// Retrieves the value of an environment variable from the current
/// process or from the Windows operating system registry key for the
/// current user or local machine.
/// </summary>
/// <param name="variable">The name of an environment variable.</param>
/// <param name="required">Whether or not to throw an exception if the variable is not found. Default is false.</param>
/// <returns>The value of the environment variable, or null if not found.</returns>
/// <exception cref="ArgumentNullException">Thrown if <paramref name="variable"/> is null.</exception>
/// <exception cref="Exception">Thrown if <paramref name="required"/> is true, and the environment variable cannot be found.</exception>
public static string GetEnvVariable(string variable, bool required = false)
{
    if (variable == null)
    {
        throw new ArgumentNullException(nameof(variable), "Environment variable cannot be null");
    }

    var value = Environment.GetEnvironmentVariable(variable, EnvironmentVariableTarget.Process);

    if (value == null)
    {
        value = Environment.GetEnvironmentVariable(variable, EnvironmentVariableTarget.User);
    }

    if (value == null)
    {
        value = Environment.GetEnvironmentVariable(variable, EnvironmentVariableTarget.Machine);
    }

    if (value == null && required)
    {
        throw new Exception("Environment variable could not be retrieved.");
    }

    return value;
}