Add operator to query an environment variable

Bonsai.System/IO/GetEnvironmentVariable.cs

@@ -0,0 +1,51 @@
+using System;
+using System.ComponentModel;
+using System.Reactive.Linq;
+
+namespace Bonsai.IO
+{
+    /// <summary>
+    /// Represents an operator that gets the value of an environment variable for the current process.
+    /// </summary>
+    [Description("Returns the value of an environment variable for the current process.")]
+    public class GetEnvironmentVariable : Source<string>
+    {
+        /// <summary>
+        /// Gets or sets the name of the environment variable to query the value of.
+        /// </summary>
+        [Description("The name of the environment variable to query the value of.")]
+        public string Name { get; set; }
+
+        /// <summary>
+        /// Gets the value of the specified environment variable for the current process
+        /// and returns it through an observable sequence.
+        /// </summary>
+        /// <returns>
+        /// A sequence containing the value of the specified environment variable. The
+        /// value will be <see langword="null"/> if the environment variable is not found.
+        /// </returns>
+        public override IObservable<string> Generate()
+        {
+            return Observable.Return(Environment.GetEnvironmentVariable(Name));
+        }
+
+        /// <summary>
+        /// Gets the value of the specified environment variable for the current process
+        /// whenever an observable sequence emits a notification.
+        /// </summary>
+        /// <typeparam name="TSource">
+        /// The type of the elements in the <paramref name="source"/> sequence.
+        /// </typeparam>
+        /// <param name="source">
+        /// The sequence of notifications used to get the value of the environment variable.
+        /// </param>
+        /// <returns>
+        /// A sequence containing the current values of the specified environment variable.
+        /// The value may be <see langword="null"/> if the environment variable is not found.
+        /// </returns>
+        public IObservable<string> Generate<TSource>(IObservable<TSource> source)
+        {
+            return source.Select(_ => Environment.GetEnvironmentVariable(Name));
+        }
+    }
+}