Added an option to configure a code actions folder

src/OmniSharp.Abstractions/Options/OmniSharpOptions.cs

@@ -4,13 +4,8 @@ namespace OmniSharp.Options
 {
     public class OmniSharpOptions
     {
-        public FormattingOptions FormattingOptions { get; }
+        public RoslynExtensionsOptions RoslynExtensionsOptions { get; } = new RoslynExtensionsOptions();
 
-        public OmniSharpOptions() : this(new FormattingOptions()) { }
-
-        public OmniSharpOptions(FormattingOptions options)
-        {
-            FormattingOptions = options ?? throw new ArgumentNullException(nameof(options));
-        }
+        public FormattingOptions FormattingOptions { get; } = new FormattingOptions();
     }
 }

src/OmniSharp.Abstractions/Options/RoslynExtensionsOptions.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+
+namespace OmniSharp.Options
+{
+    public class RoslynExtensionsOptions
+    {
+        public string[] LocationPaths { get; set; }
+
+        public IEnumerable<string> GetNormalizedLocationPaths(IOmniSharpEnvironment env)
+        {
+            if (LocationPaths == null || LocationPaths.Length == 0) return Enumerable.Empty<string>();
+
+            var normalizePaths = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            foreach (var locationPath in LocationPaths)
+            {
+                if (Path.IsPathRooted(locationPath))
+                {
+                    normalizePaths.Add(locationPath);
+                }
+                else
+                {
+                    normalizePaths.Add(Path.Combine(env.TargetDirectory, locationPath));
+                }
+            }
+
+            return normalizePaths;
+        }
+    }
+}

src/OmniSharp.Abstractions/Services/IAssemblyLoader.cs

@@ -7,6 +7,8 @@ namespace OmniSharp.Services
     public interface IAssemblyLoader
     {
         Assembly Load(AssemblyName name);
+
+        IReadOnlyList<Assembly> LoadAllFrom(string folderPath);
     }
 
     public static class IAssemblyLoaderExtensions

src/OmniSharp.Host/Services/AssemblyLoader.cs

@@ -1,4 +1,7 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.IO;
 using System.Reflection;
 using Microsoft.Extensions.Logging;
 using OmniSharp.Services;
@@ -11,7 +14,7 @@ internal class AssemblyLoader : IAssemblyLoader
 
         public AssemblyLoader(ILoggerFactory loggerFactory)
         {
-            this._logger = loggerFactory.CreateLogger<AssemblyLoader>();
+            _logger = loggerFactory.CreateLogger<AssemblyLoader>();
         }
 
         public Assembly Load(AssemblyName name)
@@ -30,5 +33,43 @@ public Assembly Load(AssemblyName name)
             _logger.LogTrace($"Assembly loaded: {name}");
             return result;
         }
+
+        public IReadOnlyList<Assembly> LoadAllFrom(string folderPath)
+        {
+            if (string.IsNullOrWhiteSpace(folderPath)) return new Assembly[0];
+
+            var assemblies = new List<Assembly>();
+            foreach (var filePath in Directory.EnumerateFiles(folderPath, "*.dll"))
+            {
+                var assembly = LoadFromPath(filePath);
+                if (assembly != null)
+                {
+                    assemblies.Add(assembly);
+                }
+            }
+
+            return assemblies;
+        }
+
+        private Assembly LoadFromPath(string assemblyPath)
+        {
+            Assembly assembly = null;
+
+            try
+            {
+#if NET46
+                assembly = Assembly.LoadFrom(assemblyPath);
+#else
+                assembly = System.Runtime.Loader.AssemblyLoadContext.Default.LoadFromAssemblyPath(assemblyPath);
+#endif
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, $"Failed to load assembly from path: {assemblyPath}");
+            }
+
+            _logger.LogTrace($"Assembly loaded from path: {assemblyPath}");
+            return assembly;
+        }
     }
 }

src/OmniSharp.Roslyn.CSharp/CodeActions/ExternalCodeActionProvider.cs

@@ -0,0 +1,15 @@
+using System.Composition;
+using OmniSharp.Services;
+
+namespace OmniSharp.Roslyn.CSharp.Services.CodeActions
+{
+    [Export(typeof(ICodeActionProvider))]
+    public class ExternalCodeActionProvider : AbstractCodeActionProvider
+    {
+        [ImportingConstructor]
+        public ExternalCodeActionProvider(ExternalFeaturesHostServicesProvider featuresHostServicesProvider)
+            : base("ExternalCodeActions", featuresHostServicesProvider.Assemblies)
+        {
+        }
+    }
+}

src/OmniSharp.Roslyn.CSharp/Services/ExternalFeaturesHostServicesProvider.cs

@@ -0,0 +1,34 @@
+using System.Collections.Immutable;
+using System.Composition;
+using System.Reflection;
+using OmniSharp.Options;
+using OmniSharp.Services;
+using System.Linq;
+
+namespace OmniSharp.Roslyn.CSharp.Services
+{
+    [Export(typeof(IHostServicesProvider))]
+    [Export(typeof(ExternalFeaturesHostServicesProvider))]
+    [Shared]
+    public class ExternalFeaturesHostServicesProvider : IHostServicesProvider
+    {
+        public ImmutableArray<Assembly> Assemblies { get; }
+
+        [ImportingConstructor]
+        public ExternalFeaturesHostServicesProvider(IAssemblyLoader loader, OmniSharpOptions options, IOmniSharpEnvironment env)
+        {
+            var builder = ImmutableArray.CreateBuilder<Assembly>();
+
+            var roslynExtensionsLocations = options.RoslynExtensionsOptions.GetNormalizedLocationPaths(env);
+            if (roslynExtensionsLocations?.Any() == true)
+            {
+                foreach (var roslynExtensionsLocation in roslynExtensionsLocations)
+                {
+                    builder.AddRange(loader.LoadAllFrom(roslynExtensionsLocation));
+                }
+            }
+
+            Assemblies = builder.ToImmutable();
+        }
+    }
+}

src/OmniSharp.Roslyn.CSharp/Services/RoslynFeaturesHostServicesProvider.cs

@@ -1,12 +1,15 @@
 using System.Collections.Immutable;
 using System.Composition;
 using System.Reflection;
+using OmniSharp.Options;
 using OmniSharp.Services;
+using System.Linq;
 
 namespace OmniSharp.Roslyn.CSharp.Services
 {
     [Export(typeof(IHostServicesProvider))]
     [Export(typeof(RoslynFeaturesHostServicesProvider))]
+    [Shared]
     public class RoslynFeaturesHostServicesProvider : IHostServicesProvider
     {
         public ImmutableArray<Assembly> Assemblies { get; }
@@ -21,7 +24,8 @@ public RoslynFeaturesHostServicesProvider(IAssemblyLoader loader)
 
             builder.AddRange(loader.Load(Features, CSharpFeatures));
 
-            this.Assemblies = builder.ToImmutable();
+
+            Assemblies = builder.ToImmutable();
         }
     }
 }

src/OmniSharp/app.config

@@ -3,4 +3,20 @@
   <startup>
     <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6" />
   </startup>
+  <runtime>
+    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
+      <dependentAssembly>
+        <assemblyIdentity name="Microsoft.CodeAnalysis" publicKeyToken="31bf3856ad364e35" culture="neutral"/>
+        <bindingRedirect oldVersion="0.0.0.0-2.0.0.0" newVersion="2.0.0.0"/>
+      </dependentAssembly>
+      <dependentAssembly>
+        <assemblyIdentity name="Microsoft.CodeAnalysis.CSharp" publicKeyToken="31bf3856ad364e35" culture="neutral"/>
+        <bindingRedirect oldVersion="0.0.0.0-2.0.0.0" newVersion="2.0.0.0"/>
+      </dependentAssembly>
+      <dependentAssembly>
+        <assemblyIdentity name="Microsoft.CodeAnalysis.Workspaces" publicKeyToken="31bf3856ad364e35" culture="neutral"/>
+        <bindingRedirect oldVersion="0.0.0.0-2.0.0.0" newVersion="2.0.0.0"/>
+      </dependentAssembly>
+    </assemblyBinding>
+  </runtime>
 </configuration>