Implementation of implicit imports during evaluation instead of parsing

ref/net46/Microsoft.Build/Microsoft.Build.cs

@@ -14,6 +14,12 @@ protected ElementLocation() { }
         public override int GetHashCode() { throw null; }
         public override string ToString() { throw null; }
     }
+    public enum ImplicitImportLocation
+    {
+        Bottom = 2,
+        None = 0,
+        Top = 1,
+    }
     [System.Diagnostics.DebuggerDisplayAttribute("ProjectChooseElement (#Children={Count} HasOtherwise={OtherwiseElement != null})")]
     public partial class ProjectChooseElement : Microsoft.Build.Construction.ProjectElementContainer
     {
@@ -39,7 +45,6 @@ internal ProjectElement() { }
         public virtual string Condition { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
         public virtual Microsoft.Build.Construction.ElementLocation ConditionLocation { get { throw null; } }
         public Microsoft.Build.Construction.ProjectRootElement ContainingProject { get { throw null; } }
-        public bool IsImplicit { get { throw null; } }
         public string Label { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
         public Microsoft.Build.Construction.ElementLocation LabelLocation { get { throw null; } }
         public Microsoft.Build.Construction.ElementLocation Location { get { throw null; } }
@@ -84,8 +89,11 @@ public override void CopyFrom(Microsoft.Build.Construction.ProjectElement elemen
     public partial class ProjectImportElement : Microsoft.Build.Construction.ProjectElement
     {
         internal ProjectImportElement() { }
+        public Microsoft.Build.Construction.ImplicitImportLocation ImplicitImportLocation { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public string Project { get { throw null; } set { } }
         public Microsoft.Build.Construction.ElementLocation ProjectLocation { get { throw null; } }
+        public string Sdk { get { throw null; } set { } }
+        public Microsoft.Build.Construction.ElementLocation SdkLocation { get { throw null; } }
         protected override Microsoft.Build.Construction.ProjectElement CreateNewInstance(Microsoft.Build.Construction.ProjectRootElement owner) { throw null; }
     }
     [System.Diagnostics.DebuggerDisplayAttribute("#Imports={Count} Condition={Condition} Label={Label}")]
@@ -253,6 +261,8 @@ internal ProjectRootElement() { }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectPropertyGroupElement> PropertyGroups { get { throw null; } }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectPropertyGroupElement> PropertyGroupsReversed { get { throw null; } }
         public string RawXml { get { throw null; } }
+        public string Sdk { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
+        public Microsoft.Build.Construction.ElementLocation SdkLocation { get { throw null; } }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectTargetElement> Targets { get { throw null; } }
         public System.DateTime TimeLastChanged { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } }
         public string ToolsVersion { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }

ref/netstandard1.3/Microsoft.Build/Microsoft.Build.cs

@@ -14,6 +14,12 @@ protected ElementLocation() { }
         public override int GetHashCode() { throw null; }
         public override string ToString() { throw null; }
     }
+    public enum ImplicitImportLocation
+    {
+        Bottom = 2,
+        None = 0,
+        Top = 1,
+    }
     [System.Diagnostics.DebuggerDisplayAttribute("ProjectChooseElement (#Children={Count} HasOtherwise={OtherwiseElement != null})")]
     public partial class ProjectChooseElement : Microsoft.Build.Construction.ProjectElementContainer
     {
@@ -39,7 +45,6 @@ internal ProjectElement() { }
         public virtual string Condition { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
         public virtual Microsoft.Build.Construction.ElementLocation ConditionLocation { get { throw null; } }
         public Microsoft.Build.Construction.ProjectRootElement ContainingProject { get { throw null; } }
-        public bool IsImplicit { get { throw null; } }
         public string Label { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
         public Microsoft.Build.Construction.ElementLocation LabelLocation { get { throw null; } }
         public Microsoft.Build.Construction.ElementLocation Location { get { throw null; } }
@@ -84,8 +89,11 @@ public override void CopyFrom(Microsoft.Build.Construction.ProjectElement elemen
     public partial class ProjectImportElement : Microsoft.Build.Construction.ProjectElement
     {
         internal ProjectImportElement() { }
+        public Microsoft.Build.Construction.ImplicitImportLocation ImplicitImportLocation { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public string Project { get { throw null; } set { } }
         public Microsoft.Build.Construction.ElementLocation ProjectLocation { get { throw null; } }
+        public string Sdk { get { throw null; } set { } }
+        public Microsoft.Build.Construction.ElementLocation SdkLocation { get { throw null; } }
         protected override Microsoft.Build.Construction.ProjectElement CreateNewInstance(Microsoft.Build.Construction.ProjectRootElement owner) { throw null; }
     }
     [System.Diagnostics.DebuggerDisplayAttribute("#Imports={Count} Condition={Condition} Label={Label}")]
@@ -253,6 +261,8 @@ internal ProjectRootElement() { }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectPropertyGroupElement> PropertyGroups { get { throw null; } }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectPropertyGroupElement> PropertyGroupsReversed { get { throw null; } }
         public string RawXml { get { throw null; } }
+        public string Sdk { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }
+        public Microsoft.Build.Construction.ElementLocation SdkLocation { get { throw null; } }
         public System.Collections.Generic.ICollection<Microsoft.Build.Construction.ProjectTargetElement> Targets { get { throw null; } }
         public System.DateTime TimeLastChanged { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } }
         public string ToolsVersion { [System.Diagnostics.DebuggerStepThroughAttribute]get { throw null; } [System.Diagnostics.DebuggerStepThroughAttribute]set { } }

src/Shared/XMakeAttributes.cs

@@ -40,7 +40,6 @@ internal static class XMakeAttributes
         internal const string taskName = "TaskName";
         internal const string continueOnError = "ContinueOnError";
         internal const string project = "Project";
-        internal const string @implicit = "_Implicit";
         internal const string taskParameter = "TaskParameter";
         internal const string itemName = "ItemName";
         internal const string propertyName = "PropertyName";

src/XMakeBuildEngine/Construction/ImplicitImportLocation.cs

@@ -0,0 +1,24 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.Build.Construction
+{
+    /// <summary>
+    /// Represents the location of an implicit import.
+    /// </summary>
+    public enum ImplicitImportLocation
+    {
+        /// <summary>
+        /// The import is not implicitly added and is explicitly added in a user-specified location.
+        /// </summary>
+        None,
+        /// <summary>
+        /// The import was implicitly added at the top of the project.
+        /// </summary>
+        Top,
+        /// <summary>
+        /// The import was implicitly added at the bottom of the project.
+        /// </summary>
+        Bottom
+    }
+}

src/XMakeBuildEngine/Construction/ProjectElement.cs

@@ -292,11 +292,6 @@ public ElementLocation Location
             get { return XmlElement.Location; }
         }
 
-        /// <summary>
-        /// Gets the Implicit state of the element: true if the element was not in the read XML.
-        /// </summary>
-        public bool IsImplicit => XmlElement.HasAttribute(XMakeAttributes.@implicit);
-
         /// <summary>
         /// Gets the name of the associated element. 
         /// Useful for display in some circumstances.

src/XMakeBuildEngine/Construction/ProjectElementContainer.cs

@@ -455,11 +455,6 @@ internal void UpdateElementValue(ProjectElement child)
         /// </remarks>
         internal void AddToXml(ProjectElement child)
         {
-            if (child.IsImplicit)
-            {
-                return;
-            }
-
             if (child.ExpressedAsAttribute)
             {
                 // todo children represented as attributes need to be placed in order too
@@ -485,7 +480,7 @@ internal void AddToXml(ProjectElement child)
                 // If none is found, then the node being added is inserted as the only node of its kind
 
                 ProjectElement referenceSibling;
-                Predicate<ProjectElement> siblingIsExplicitElement = _ => _.ExpressedAsAttribute == false && _.IsImplicit == false;
+                Predicate<ProjectElement> siblingIsExplicitElement = _ => _.ExpressedAsAttribute == false;
 
                 if (TrySearchLeftSiblings(child.PreviousSibling, siblingIsExplicitElement, out referenceSibling))
                 {
@@ -567,11 +562,6 @@ private string GetElementIndentation(XmlElementWithLocation xmlElement)
 
         internal void RemoveFromXml(ProjectElement child)
         {
-            if (child.IsImplicit)
-            {
-                return;
-            }
-
             if (child.ExpressedAsAttribute)
             {
                 XmlElement.RemoveAttribute(child.XmlElement.Name);
@@ -630,10 +620,7 @@ private void AddInitialChild(ProjectElement child)
 
             _count++;
 
-            if (!child.IsImplicit)
-            {
-                MarkDirty("Add child element named '{0}'", child.ElementName);
-            }
+            MarkDirty("Add child element named '{0}'", child.ElementName);
         }
 
         /// <summary>

src/XMakeBuildEngine/Construction/ProjectImportElement.cs

@@ -61,6 +61,38 @@ public string Project
         /// </summary>
         public ElementLocation ProjectLocation => XmlElement.GetAttributeLocation(XMakeAttributes.project);
 
+        /// <summary>
+        /// Gets or sets the SDK that contains the import.
+        /// </summary>
+        public string Sdk
+        {
+            get
+            {
+                return
+                    FileUtilities.FixFilePath(ProjectXmlUtilities.GetAttributeValue(XmlElement, XMakeAttributes.sdk));
+            }
+
+            set
+            {
+                ErrorUtilities.VerifyThrowArgumentLength(value, XMakeAttributes.sdk);
+
+                ProjectXmlUtilities.SetOrRemoveAttribute(XmlElement, XMakeAttributes.sdk, value);
+                MarkDirty("Set Import Sdk {0}", value);
+            }
+        }
+
+        /// <summary>
+        /// Location of the Sdk attribute
+        /// </summary>
+        public ElementLocation SdkLocation => XmlElement.GetAttributeLocation(XMakeAttributes.sdk);
+
+        /// <summary>
+        /// Gets the <see cref="ImplicitImportLocation"/> of the import.  This indicates if the import was implicitly
+        /// added because of the <see cref="ProjectRootElement.Sdk"/> attribute and the location where the project was
+        /// imported.
+        /// </summary>
+        public ImplicitImportLocation ImplicitImportLocation { get; internal set; } = ImplicitImportLocation.None;
+
         /// <summary>
         /// Creates an unparented ProjectImportElement, wrapping an unparented XmlElement.
         /// Validates the project value.
@@ -77,6 +109,21 @@ internal static ProjectImportElement CreateDisconnected(string project, ProjectR
             return import;
         }
 
+        /// <summary>
+        /// Creates an implicit ProjectImportElement as if it was in the project.
+        /// </summary>
+        /// <returns></returns>
+        internal static ProjectImportElement CreateImplicit(string project, ProjectRootElement containingProject, ImplicitImportLocation implicitImportLocation, string sdkName)
+        {
+            ProjectImportElement import = CreateDisconnected(project, containingProject);
+
+            import.ImplicitImportLocation = implicitImportLocation;
+
+            import.Sdk = sdkName;
+
+            return import;
+        }
+
         /// <summary>
         /// Overridden to verify that the potential parent and siblings
         /// are acceptable. Throws InvalidOperationException if they are not.

src/XMakeBuildEngine/Construction/ProjectRootElement.cs

@@ -157,11 +157,6 @@ public class ProjectRootElement : ProjectElementContainer
         /// </summary>
         private BuildEventContext _buildEventContext;
 
-        /// <summary>
-        /// Xpath expression that will find any element with the implicit attribute
-        /// </summary>
-        private static readonly string ImplicitAttributeXpath = $"//*[@{XMakeAttributes.@implicit}]";
-
         /// <summary>
         /// Initialize a ProjectRootElement instance from a XmlReader.
         /// May throw InvalidProjectFileException.
@@ -654,6 +649,28 @@ public string InitialTargets
             }
         }
 
+        /// <summary>
+        /// Gets or sets a semicolon delimited list of software development kits (SDK) that the project uses.
+        /// If  a value is specified, an Sdk.props is simplicity imported at the top of the project and an
+        /// Sdk.targets is simplicity imported at the bottom from the specified SDK.
+        /// If the value is null or empty, removes the attribute.
+        /// </summary>
+        public string Sdk
+        {
+            [DebuggerStepThrough]
+            get
+            {
+                return ProjectXmlUtilities.GetAttributeValue(XmlElement, XMakeAttributes.sdk);
+            }
+
+            [DebuggerStepThrough]
+            set
+            {
+                ProjectXmlUtilities.SetOrRemoveAttribute(XmlElement, XMakeAttributes.sdk, value);
+                MarkDirty("Set project Sdk to '{0}'", value);
+            }
+        }
+
         /// <summary>
         /// Gets or sets the value of TreatAsLocalProperty. If there is no tag, returns empty string.
         /// If the value being set is null or empty, removes the attribute.
@@ -710,10 +727,8 @@ public string RawXml
                 {
                     using (ProjectWriter projectWriter = new ProjectWriter(stringWriter))
                     {
-                        var xmlWithNoImplicits = RemoveImplicits();
-
-                        projectWriter.Initialize(xmlWithNoImplicits);
-                        xmlWithNoImplicits.Save(projectWriter);
+                        projectWriter.Initialize(XmlDocument);
+                        XmlDocument.Save(projectWriter);
                     }
 
                     return stringWriter.ToString();
@@ -848,6 +863,14 @@ public ElementLocation InitialTargetsLocation
             get { return XmlElement.GetAttributeLocation(XMakeAttributes.initialTargets); }
         }
 
+        /// <summary>
+        /// Location of the Sdk attribute, if any
+        /// </summary>
+        public ElementLocation SdkLocation
+        {
+            get { return XmlElement.GetAttributeLocation(XMakeAttributes.sdk); }
+        }
+
         /// <summary>
         /// Location of the TreatAsLocalProperty attribute, if any
         /// </summary>
@@ -1752,10 +1775,8 @@ public void Save(Encoding saveEncoding)
                 {
                     using (ProjectWriter projectWriter = new ProjectWriter(_projectFileLocation.File, saveEncoding))
                     {
-                        var xmlWithNoImplicits = RemoveImplicits();
-
-                        projectWriter.Initialize(xmlWithNoImplicits);
-                        xmlWithNoImplicits.Save(projectWriter);
+                        projectWriter.Initialize(XmlDocument);
+                        XmlDocument.Save(projectWriter);
                     }
 
                     _encoding = saveEncoding;
@@ -1784,26 +1805,6 @@ public void Save(Encoding saveEncoding)
 #endif
         }
 
-        private XmlDocument RemoveImplicits()
-        {
-            if (XmlDocument.SelectSingleNode(ImplicitAttributeXpath) == null)
-            {
-                return XmlDocument;
-            }
-
-            var xmlWithNoImplicits = (XmlDocument) XmlDocument.CloneNode(deep: true);
-
-            var implicitElements =
-                xmlWithNoImplicits.SelectNodes(ImplicitAttributeXpath);
-
-            foreach (XmlNode implicitElement in implicitElements)
-            {
-                implicitElement.ParentNode.RemoveChild(implicitElement);
-            }
-
-            return xmlWithNoImplicits;
-        }
-
         /// <summary>
         /// Save the project to the file system, if dirty or the path is different.
         /// Creates any necessary directories.
@@ -1837,10 +1838,8 @@ public void Save(TextWriter writer)
         {
             using (ProjectWriter projectWriter = new ProjectWriter(writer))
             {
-                var xmlWithNoImplicits = RemoveImplicits();
-
-                projectWriter.Initialize(xmlWithNoImplicits);
-                xmlWithNoImplicits.Save(projectWriter);
+                projectWriter.Initialize(XmlDocument);
+                XmlDocument.Save(projectWriter);
             }
 
             _versionOnDisk = Version;