Reset ProjectInstance after build (#255)

Author: jeffkl

src/Microsoft.Build.Utilities.ProjectCreation.UnitTests/BuildTests.cs

@@ -187,6 +187,22 @@ public void CanRestoreAndBuild()
             buildOutput.MessageEvents.High.ShouldContain(i => i.Message == "Building...", buildOutput.GetConsoleLog());
         }
 
+        [Fact]
+        public void CanRestoreAndBuildMultipleTimes()
+        {
+            ProjectCreator projectCreator = ProjectCreator.Templates.SdkCsproj(
+                path: GetTempFileName(".csproj"),
+                targetFramework: TargetFramework)
+                .Save()
+                .TryBuild(restore: true, "Build", out bool result, out BuildOutput buildOutput);
+
+            result.ShouldBeTrue(buildOutput.GetConsoleLog());
+
+            projectCreator.TryBuild(out result, out buildOutput);
+
+            result.ShouldBeTrue(buildOutput.GetConsoleLog());
+        }
+
         [Fact]
         public void ProjectCollectionLoggersWork()
         {

src/Microsoft.Build.Utilities.ProjectCreation/ProjectCreator.Build.cs

@@ -535,6 +535,8 @@ private void Build(bool restore, string[]? targets, IDictionary<string, string>?
             {
                 targetOutputs = buildResult.ResultsByTarget;
             }
+
+            ResetProjectInstance();
         }
 
         private void Restore(IDictionary<string, string>? globalProperties, BuildOutput buildOutput, out bool result, out IDictionary<string, TargetResult> targetOutputs)

src/Microsoft.Build.Utilities.ProjectCreation/ProjectCreator.Project.cs

@@ -126,5 +126,10 @@ public ProjectCreator TryGetProjectInstance(
 
             return this;
         }
+
+        internal void ResetProjectInstance()
+        {
+            _projectInstance = null;
+        }
     }
 }