Be sure to call MSBuildLocator.RegisterDefaults only once

Author: qmfrederik

src/NerdBank.GitVersioning.Tests/AssemblyInfoTest.cs

@@ -3,7 +3,6 @@
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.Build.Locator;
 using Nerdbank.GitVersioning.Tasks;
 using Xunit;
 
@@ -13,7 +12,7 @@ public class AssemblyInfoTest
     {
         public AssemblyInfoTest()
         {
-            MSBuildLocator.RegisterDefaults();
+            MSBuildExtensions.LoadMSBuild();
         }
 
         [Fact]

src/NerdBank.GitVersioning.Tests/BuildIntegrationTests.cs

@@ -14,7 +14,6 @@
 using Microsoft.Build.Evaluation;
 using Microsoft.Build.Execution;
 using Microsoft.Build.Framework;
-using Microsoft.Build.Locator;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -47,14 +46,10 @@ public class BuildIntegrationTests : RepoTestBase
     };
     private Random random;
 
-    static BuildIntegrationTests()
-    {
-        MSBuildLocator.RegisterDefaults();
-    }
-
     public BuildIntegrationTests(ITestOutputHelper logger)
         : base(logger)
     {
+        MSBuildExtensions.LoadMSBuild();
 
         int seed = (int)DateTime.Now.Ticks;
         this.random = new Random(seed);

src/NerdBank.GitVersioning.Tests/MSBuildExtensions.cs

@@ -7,12 +7,28 @@
 using Microsoft.Build.Evaluation;
 using Microsoft.Build.Execution;
 using Microsoft.Build.Framework;
+using Microsoft.Build.Locator;
 using Microsoft.Build.Logging;
 using Validation;
 using Xunit.Abstractions;
 
 internal static class MSBuildExtensions
 {
+    private static object loadLock = new object();
+    private static bool loaded = false;
+
+    internal static void LoadMSBuild()
+    {
+        lock (loadLock)
+        {
+            if (!loaded)
+            {
+                MSBuildLocator.RegisterDefaults();
+                loaded = true;
+            }
+        }
+    }
+
     internal static async Task<BuildResult> BuildAsync(this BuildManager buildManager, ITestOutputHelper logger, ProjectCollection projectCollection, ProjectRootElement project, string target, IDictionary<string, string> globalProperties = null, LoggerVerbosity logVerbosity = LoggerVerbosity.Detailed, ILogger[] additionalLoggers = null)
     {
         Requires.NotNull(buildManager, nameof(buildManager));