Add support for NO_COLOR

fixes #52

Author: Matthiee

CommandLineParser/Abstractions/Usage/IEnvironmentVariablesService.cs

@@ -0,0 +1,14 @@
+namespace MatthiWare.CommandLine.Abstractions.Usage
+{
+    /// <summary>
+    /// Environment variables 
+    /// </summary>
+    public interface IEnvironmentVariablesService
+    {
+        /// <summary>
+        /// Inidicates if NO_COLOR environment variable has been set
+        /// https://no-color.org/
+        /// </summary>
+        bool NoColorRequested { get; }
+    }
+}

CommandLineParser/Core/Usage/EnvironmentVariableService.cs

@@ -0,0 +1,14 @@
+using MatthiWare.CommandLine.Abstractions.Usage;
+using System;
+
+namespace MatthiWare.CommandLine.Core.Usage
+{
+    /// <inheritdoc/>
+    public class EnvironmentVariableService : IEnvironmentVariablesService
+    {
+        private const string NoColorId = "NO_COLOR";
+
+        /// <inheritdoc/>
+        public bool NoColorRequested => Environment.GetEnvironmentVariable(NoColorId) != null;
+    }
+}

CommandLineParser/Core/Usage/UsagePrinter.cs

@@ -9,30 +9,55 @@ namespace MatthiWare.CommandLine.Core.Usage
     /// <inheritdoc/>
     public class UsagePrinter : IUsagePrinter
     {
+        private readonly IEnvironmentVariablesService environmentVariablesService;
+
         protected ICommandLineCommandContainer Container { get; }
 
         /// <inheritdoc/>
         public IUsageBuilder Builder { get; }
 
-        /// <inheritdoc/>
+        /// <summary>
+        /// Creates a new CLI output usage printer
+        /// </summary>
+        /// <param name="container"></param>
+        /// <param name="builder"></param>
         public UsagePrinter(ICommandLineCommandContainer container, IUsageBuilder builder)
+            : this(container, builder, new EnvironmentVariableService())
+        { }
+
+        /// <summary>
+        /// Creates a new CLI output usage printer
+        /// </summary>
+        /// <param name="container"></param>
+        /// <param name="builder"></param>
+        /// <param name="environmentVariablesService"></param>
+        public UsagePrinter(ICommandLineCommandContainer container, IUsageBuilder builder, IEnvironmentVariablesService environmentVariablesService)
         {
-            Container = container;
-            Builder = builder;
+            Container = container ?? throw new ArgumentNullException(nameof(container));
+            Builder = builder ?? throw new ArgumentNullException(nameof(builder));
+            this.environmentVariablesService = environmentVariablesService ?? throw new ArgumentNullException(nameof(environmentVariablesService));
         }
 
         /// <inheritdoc/>
         public virtual void PrintErrors(IReadOnlyCollection<Exception> errors)
         {
+            bool canOutputColor = !this.environmentVariablesService.NoColorRequested;
+
             var previousColor = Console.ForegroundColor;
 
-            Console.ForegroundColor = ConsoleColor.Red;
+            if (canOutputColor)
+            {
+                Console.ForegroundColor = ConsoleColor.Red;
+            }
 
             Builder.AddErrors(errors);
 
             Console.Error.WriteLine(Builder.Build());
 
-            Console.ForegroundColor = previousColor;
+            if (canOutputColor)
+            { 
+                Console.ForegroundColor = previousColor;
+            }
 
             Console.WriteLine();
         }