Enable nullable reference types by default

src/Build.OM.UnitTests/AssemblyResources.cs

@@ -3,6 +3,8 @@
 
 using System;
 
+#nullable disable
+
 namespace Microsoft.Build.Shared
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ConstructionEditing_Tests.cs

@@ -15,6 +15,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ElementLocationPublic_Tests.cs

@@ -8,6 +8,8 @@
 using Xunit;
 using Shouldly;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectChooseElement_Tests.cs

@@ -13,6 +13,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectExtensionsElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectFormatting_Tests.cs

@@ -11,6 +11,8 @@
 using Xunit;
 using Xunit.Abstractions;
 
+#nullable disable
+
 namespace Microsoft.Build.Engine.OM.UnitTests.Construction
 {
     public class ProjectFormatting_Tests : IDisposable

src/Build.OM.UnitTests/Construction/ProjectImportElement_Tests.cs

@@ -13,6 +13,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectImportGroupElement_Tests.cs

@@ -10,6 +10,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemDefinitionElement_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Exceptions;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemDefinitionGroupElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemElement_Tests.cs

@@ -12,6 +12,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemGroupElement_tests.cs

@@ -7,6 +7,8 @@
 using Microsoft.Build.Construction;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectMetadataElement_Tests.cs

@@ -12,6 +12,8 @@
 using Microsoft.Build.Evaluation;
 using Xunit.Abstractions;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectOnErrorElement_Tests.cs

@@ -10,6 +10,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectOutputElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectPropertyElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectPropertyGroupElement_Tests.cs

@@ -6,6 +6,8 @@
 using Microsoft.Build.Construction;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectRootElement_Tests.cs

@@ -22,6 +22,8 @@
 using Shouldly;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectTargetElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectTaskElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectUsingTaskElement_Tests.cs

@@ -10,6 +10,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/SolutionFile_Tests.cs

@@ -11,6 +11,8 @@
 using Xunit;
 using System.Text;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/UsingTaskBodyElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/UsingTaskParameterElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/UsingTaskParameterGroup_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/WhiteSpacePreservation_Tests.cs

@@ -12,6 +12,8 @@
 using Microsoft.Build.Shared;
 
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     public class WhitespacePreservation_Tests

src/Build.OM.UnitTests/Definition/DefinitionEditing_Tests.cs

@@ -13,6 +13,8 @@
 using Microsoft.Build.Shared;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/EditingElementsReferencedByOrReferences_Tests.cs

@@ -9,6 +9,8 @@
 using Microsoft.Build.Evaluation;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProjectCollection_Tests.cs

@@ -16,6 +16,8 @@
 using Shouldly;
 using System.Reflection;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProjectItemDefinition_Tests.cs

@@ -14,6 +14,8 @@
 using Microsoft.Build.Shared;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProjectItem_Tests.cs

@@ -15,6 +15,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProjectMetadata_Tests.cs

@@ -11,6 +11,8 @@
 using Microsoft.Build.Shared;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProjectProperty_Tests.cs

@@ -11,6 +11,8 @@
 using Microsoft.Build.Shared;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/Project_Tests.cs

@@ -28,6 +28,8 @@
 using Xunit;
 using Xunit.Abstractions;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Definition/ProtectImports_Tests.cs

@@ -10,6 +10,8 @@
 using Microsoft.Build.Evaluation;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Definition
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectInstance_Tests.cs

@@ -16,6 +16,8 @@
 using Xunit;
 using Xunit.Abstractions;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectItemInstance_Tests.cs

@@ -16,6 +16,8 @@
 using Xunit;
 using System.Linq;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectMetadataInstance_Tests.cs

@@ -5,6 +5,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectOnErrorInstance_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectPropertyInstance_Tests.cs

@@ -6,6 +6,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectTargetInstance_Tests.cs

@@ -9,6 +9,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectTaskInstance_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectTaskOutputItemInstance_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>

src/Build.OM.UnitTests/Instance/ProjectTaskOutputPropertyInstance_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Execution;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Instance
 {
     /// <summary>