fix: share ProjectCollection instance and implement IDisposable

src/ReferenceCop.MSBuild/Providers/MSBuildProjectMetadataProvider.cs

@@ -1,26 +1,37 @@
 namespace ReferenceCop.MSBuild
 {
+    using System;
     using System.Collections.Generic;
     using System.Linq;
     using Microsoft.Build.Evaluation;
 
     /// <summary>
     /// Provides project reference information using MSBuild project evaluation.
     /// </summary>
-    public class MSBuildProjectMetadataProvider : IProjectMetadataProvider
+    public class MSBuildProjectMetadataProvider : IProjectMetadataProvider, IDisposable
     {
         private const string ProjectReferenceNode = "ProjectReference";
         private const string NoWarnMetadata = "NoWarn";
 
+        private readonly ProjectCollection projectCollection;
+        private bool disposed;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MSBuildProjectMetadataProvider"/> class.
+        /// </summary>
+        public MSBuildProjectMetadataProvider()
+        {
+            this.projectCollection = new ProjectCollection();
+        }
+
         /// <summary>
         /// Gets the project references from a project file.
         /// </summary>
         /// <param name="projectFilePath">The path to the project file.</param>
         /// <returns>The collection of project references.</returns>
         public IEnumerable<ProjectReferenceInfo> GetProjectReferences(string projectFilePath)
         {
-            var projectCollection = new ProjectCollection();
-            var project = projectCollection.LoadProject(projectFilePath);
+            var project = this.LoadOrGetProject(projectFilePath);
 
             // Get all ProjectReference items. These are the direct project references.
             var projectReferences = project.GetItems(ProjectReferenceNode);
@@ -47,11 +58,48 @@ public IEnumerable<ProjectReferenceInfo> GetProjectReferences(string projectFile
         /// <returns>The resolved property value.</returns>
         public string GetPropertyValue(string projectFilePath, string propertyName)
         {
-            var projectCollection = new ProjectCollection();
-            var project = projectCollection.LoadProject(projectFilePath);
+            var project = this.LoadOrGetProject(projectFilePath);
             project.ReevaluateIfNecessary();
 
             return project.GetPropertyValue(propertyName);
         }
+
+        /// <inheritdoc />
+        public void Dispose()
+        {
+            this.Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        /// <summary>
+        /// Releases the resources used by the <see cref="MSBuildProjectMetadataProvider"/>.
+        /// </summary>
+        /// <param name="disposing">Whether managed resources should be disposed.</param>
+        protected virtual void Dispose(bool disposing)
+        {
+            if (!this.disposed)
+            {
+                if (disposing)
+                {
+                    this.projectCollection.UnloadAllProjects();
+                    this.projectCollection.Dispose();
+                }
+
+                this.disposed = true;
+            }
+        }
+
+        private Project LoadOrGetProject(string projectFilePath)
+        {
+            var fullPath = System.IO.Path.GetFullPath(projectFilePath);
+            var loaded = this.projectCollection.GetLoadedProjects(fullPath);
+
+            if (loaded.Count > 0)
+            {
+                return loaded.First();
+            }
+
+            return this.projectCollection.LoadProject(fullPath);
+        }
     }
 }

src/ReferenceCop.MSBuild/ReferenceCopTask.cs

@@ -120,6 +120,10 @@ public bool Execute()
                 success = false;
                 this.BuildEngine.LogErrorEvent(ex);
             }
+            finally
+            {
+                (this.projectReferencesProvider as IDisposable)?.Dispose();
+            }
 
             return success;
         }