Getting Started VB Syntax Transformation - Roslyn

Prerequisites

Introduction

This walkthrough builds on concepts and techniques explored in the Getting Started: Syntax Analysis and Getting Started: Semantic Analysis walkthroughs. If you haven't already, it's strongly advised that you complete those walkthroughs before beginning this one.

In this walkthrough, you'll explore techniques for creating and transforming syntax trees. In combination with the techniques you learned in previous Getting Started walkthroughs, you will create your first command-line refactoring!

Immutability and the .NET Compiler Platform

A fundamental tenet of the .NET Compiler Platform is immutability. Because immutable data structures cannot be changed after they are created, they can be safely shared and analyzed by multiple consumers simultaneously without the dangers of one tool affecting another in unpredictable ways. No locks or other concurrency measures needed. This applies to syntax trees, compilations, symbols, semantic models, and every other data structure you'll encounter. Instead of modification, new objects are created based on specified differences to the old ones. You'll apply this concept to syntax trees to create tree transformations!

Creating and "Modifying" Trees

Creating Nodes with Factory Methods

To create SyntaxNodes you must use the SyntaxFactory class factory methods. For each kind of node, token, or trivia there is a factory method which can be used to create an instance of that type. By composing nodes hierarchically in a bottom-up fashion you can create syntax trees.

Example - Creating a SyntaxNode using Factory Methods

This example uses the SyntaxFactory class factory methods to construct a NameSyntax representing the System.Collections.Generic namespace.

NameSyntax is the base class for four types of names that appear in VB:

IdentifierNameSyntax which represents simple single identifier names like System and Microsoft
GenericNameSyntax which represents a generic type or method name such as List(Of Integer)
QualifiedNameSyntax which represents a qualified name of the form <left-name>.<right-identifier-or-generic-name> such as System.IO
GlobalNameSyntax which represents the name of the Global namespace. By composing these names together you can create any name which can appear in the VB language.

Create a new Visual Basic Stand-Alone Code Analysis Tool project.

In Visual Studio, choose File -> New -> Project... to display the New Project dialog.
Under Visual Basic -> Extensibility, choose Stand-Alone Code Analysis Tool.
Name your project "ConstructionVB" and click OK.

Enter the following line at the top of your Module1.vb file:

VB.NET

Option Strict Off

Some readers may run with Option Strict turned On by default at the project level. Turning Option Strict Off in this walkthrough simplifies many of the examples by removing much of the casting required.

Add the following Imports statement to the top of the file to import the factory methods of the SyntaxFactory class so that we can use them later without qualifying them:

VB.NET

Imports Microsoft.CodeAnalysis.VisualBasic.SyntaxFactory

Move your cursor to the line containing the End Sub of your Main method and set a breakpoint there.

In Visual Studio, choose Debug -> Toggle Breakpoint.

Run the program.

In Visual Studio, choose Debug -> Start Debugging.

Inside the Main method, create a simple IdentifierNameSyntax representing the name of the System namespace and assign it to a variable. As you build up a QualifiedNameSyntax from this node you will reuse this variable so declare this variable to be of type NameSyntax to allow it to store both types of SyntaxNode - DO NOT use type inference:

VB.NET

        Dim name As NameSyntax = IdentifierName("System")

Set this statement as the next statement to be executed and execute it.

Right-click this line and choose Set Next Statement.
In Visual Studio, choose Debug -> Step Over, to execute this statement and initialize the new variable.
You will need to repeat this process for each of the following steps as we introduce new variables and inspect them with the debugger.

Open the Immediate Window.

In Visual Studio, choose Debug -> Windows -> Immediate.

Using the Immediate window type the expression ? name.ToString() and press Enter to evaluate it. You should see the string "System" as the result.

Note that you can also type statements in the Immediate Window to be executed. You must begin an expression evaluation in the Visual Basic Immediate Window with the question mark ?' to differentiate expression evaluations from statement executions.

Next, construct a QualifiedNameSyntax using this name node as the left of the name and a new IdentifierNameSyntax for the Collections namespace as the right side of the QualifiedNameSyntax:

VB.NET

        name = QualifiedName(name, IdentifierName("Collections"))

Execute this statement to set the name variable to the new QualifiedNameSyntax node.
Using the Immediate Window evaluate the expression ? name.ToString(). It should evaluate to "System.Collections".
Continue this pattern by building another QualifiedNameSyntax node for the Generic namespace:

VB.NET

        name = QualifiedName(name, IdentifierName("Generic"))

Execute this statement and again use the Immediate Window to observe that ? name.ToString() now evaluates to the fully qualified name "System.Collections.Generic".

Modifying Nodes with With* and ReplaceNode Methods

Because the syntax trees are immutable, the Syntax API provides no direct mechanism for modifying an existing syntax tree after construction. However, the Syntax API does provide methods for producing new trees based on specified changes to existing ones. Each concrete class that derives from SyntaxNode defines With* methods which you can use to specify changes to its child properties. Additionally, the ReplaceNode extension method can be used to replace a descendent node in a subtree. Without this method updating a node would also require manually updating its parent to point to the newly created child and repeating this process up the entire tree - a process known as re-spining the tree.

Example - Transformations using the With* and ReplaceNode methods.

This example uses the WithName method to replace the name in an ImportsStatementSyntax node with the one constructed above.

Continuing from the previous example above, add this code to parse a sample code file:

VB.NET

        Dim tree = VisualBasicSyntaxTree.ParseText(
"Imports System
Imports System.Collections
Imports System.Linq
Imports System.Text
 
Namespace HelloWorld
    Module Module1
        Sub Main(args As String())
            Console.WriteLine(""Hello, World!"")
        End Sub
    End Module
End Namespace")
 
        Dim root As CompilationUnitSyntax = tree.GetRoot()

Note that the file uses the System.Collections namespace and not the System.Collections.Generic namespace.

Execute these statements.
Create a new SimpleImportsClauseNode node using the SimpleImportsClauseSyntax.WithName method to update the "System.Collections" import with the name we created above:

VB.NET

        Dim oldImportClause As SimpleImportsClauseSyntax = 
                root.Imports(1).ImportsClauses(0)
 
        Dim newImportClause = oldImportClause.WithName(name)

Execute these statements.
Using the Immediate Window evaluate the expression ? root.ToString() and observe that the original tree has not been changed to contain this new updated node.
Add the following line using the ReplaceNode extension method to create a new tree, replacing the existing import with the updated newImportClause node, and store the new tree in the existing root variable:

VB.NET

        root = root.ReplaceNode(oldImportClause, newImportClause)

Execute this statement.
Using the Immediate Window evaluate the expression ? root.ToString() this time observing that the tree now correctly imports the System.Collections.Generic namespace.
Stop the program.

In Visual Studio, choose Debug -> Stop Debugging.

Your Module1.vb file should now look like this:

VB.NET

Option Strict Off

Imports Microsoft.CodeAnalysis.VisualBasic.SyntaxFactory


Module Module1

    Sub Main()
        Dim name As NameSyntax = IdentifierName("System")

        name = QualifiedName(name, IdentifierName("Collections"))

        name = QualifiedName(name, IdentifierName("Generic"))

        Dim tree = VisualBasicSyntaxTree.ParseText(
"Imports System
Imports System.Collections
Imports System.Linq
Imports System.Text
 
Namespace HelloWorld
    Module Module1
        Sub Main(args As String())
            Console.WriteLine(""Hello, World!"")
        End Sub
    End Module
End Namespace")

        Dim root As CompilationUnitSyntax = tree.GetRoot()

        Dim oldImportClause As SimpleImportsClauseSyntax =
                root.Imports(1).ImportsClauses(0)

        Dim newImportClause = oldImportClause.WithName(name)

        root = root.ReplaceNode(oldImportClause, newImportClause)
    End Sub

End Module

Transforming Trees using SyntaxRewriters

The With* and ReplaceNode methods provide convenient means to transform individual branches of a syntax tree. However, often it may be necessary to perform multiple transformations on a syntax tree in concert. The SyntaxRewriter class is a subclass of SyntaxVisitor which can be used to apply a transformation to a specific type of SyntaxNode. It is also possible to apply a set of transformations to multiple types of SyntaxNode wherever they appear in a syntax tree. The following example demonstrates this in a naive implementation of a command-line refactoring which removes explicit types in local variable declarations anywhere where type inference could be used. This example makes use of techniques discussed in this walkthrough as well as the Getting Started: Syntactic Analysis and Getting Started: Semantic Analysis walkthroughs.

Example - Creating a SyntaxRewriter to transform syntax trees.

Create a new Visual Basic Stand-Alone Code Analysis Tool project.

In Visual Studio, choose File -> New -> Project... to display the New Project dialog.
Under Visual Basic -> Extensibility, choose Stand-Alone Code Analysis Tool.
Name your project "TransformationVB" and click OK.

Enter the following lines at the top of your Module1.vb file:

VB.NET

Option Strict Off

Imports System.IO

Add a new class file to the project.

In Visual Studio, choose Project -> Add Class...
In the "Add New Item" dialog type TypeInferenceRewriter.vb as the filename.

Enter the following line at the top of your TypeInferenceRewriter.vb file:

VB.NET

Option Strict Off

Make the TypeInferenceRewriter class extend the VisualBasicSyntaxRewriter class:

VB.NET

Public Class TypeInferenceRewriter
    Inherits VisualBasicSyntaxRewriter

Add the following code to declare a private read-only field to hold a SemanticModel and initialize it from the constructor. You will need this field later on to determine where type inference can be used:

VB.NET

    Private ReadOnly SemanticModel As SemanticModel
 
    Public Sub New(semanticModel As SemanticModel)
        Me.SemanticModel = semanticModel
    End Sub

Override the VisitLocalDeclarationStatement method:

VB.NET

    Public Overrides Function VisitLocalDeclarationStatement (
                                  node As LocalDeclarationStatementSyntax
                              ) As SyntaxNode

    End Function

Note that the VisitLocalDeclarationStatement method returns a SyntaxNode, not LocalDeclarationStatementSyntax. In this example you'll return another LocalDeclarationStatementSyntax node based on the existing one. In other scenarios one kind of node may be replaced by another kind of node entirely - or even removed.

For the purpose of this example you'll only handle local variable declarations, though type inference may be used in For Each loops, For loops, LINQ expressions, and Lambda expressions. Furthermore this rewriter will only transform declarations of the simplest form:

VB.NET

Dim variable As Type = expression

The following forms of variable declarations in VB are either incompatible with type inference or left as an exercise to the reader.

VB.NET

' Multiple types in a single declaration.
Dim variable1 As Type1 = expression1, 
    variable2 As Type2 = expression2
' Multiple variables in a single declaration.
Dim variable1, variable2 As Type 
' No initializer.
Dim variable1 As Type 
Dim variable As New Type
' Already inferred.
Dim variable = expression

Add the following code to the body of the VisitLocalDeclarationStatement method to skip rewriting these forms of declarations:

VB.NET

        If node.Declarators.Count > 1 Then Return node
        If node.Declarators(0).Names.Count > 1 Then Return node
        If node.Declarators(0).AsClause Is Nothing Then Return node
        If node.Declarators(0).AsClause.Kind = SyntaxKind.AsNewClause _
            Then Return node
        If node.Declarators(0).Initializer Is Nothing Then Return node

Note that returning the node parameter unmodified results in no rewriting taking place for that node.

Add these statements to extract the type name specified in the declaration and bind it using the SemanticModel field to obtain a type symbol.

VB.NET

        Dim declarator As VariableDeclaratorSyntax = node.Declarators(0)
        Dim asClause As SimpleAsClauseSyntax = declarator.AsClause
        Dim variableTypeName As TypeSyntax = asClause.Type
 
        Dim variableType As ITypeSymbol = 
                SemanticModel.GetSymbolInfo(variableTypeName).Symbol

Now, add this statement to bind the initializer expression:

VB.NET

        Dim initializerInfo As TypeInfo = 
                SemanticModel.GetTypeInfo(declarator.Initializer.Value)

Finally, add the following If statement to remove the As clause if the type of the initializer expression matches the type specified in the As clause:

VB.NET

        If variableType Is initializerInfo.Type Then
 
            Dim newDeclarator As VariableDeclaratorSyntax =
                    declarator.WithAsClause(Nothing)
 
            Return node.ReplaceNode(declarator, newDeclarator)
        Else
            Return node
        End If

Note that this conditional is required because if the types don't match, the declaration may be casting the initializer expression to a base class or interface or performing an implicit conversion. Removing the explicit type in these cases would change the semantics of a program.
Note that passing Nothing to the **WithAsClause **method causes the existing As clause to be removed.
Note also that it's simpler to use ReplaceNode rather than With* methods to transform the LocalDeclarationStatementSyntax because LocalDeclarationStatementSyntax nodes hold a SeparatedSyntaxList(Of VariableDeclaratorSyntax). Using ReplaceNode avoids the requirement to construct and populate a new list as an intermediate step.

Your TypeInferenceRewriter.vb file should now look like this:

VB.NET

Option Strict Off

Public Class TypeInferenceRewriter
    Inherits VisualBasicSyntaxRewriter

    Private ReadOnly SemanticModel As SemanticModel

    Public Sub New(semanticModel As SemanticModel)
        Me.SemanticModel = semanticModel
    End Sub

    Public Overrides Function VisitLocalDeclarationStatement(
                                  node As LocalDeclarationStatementSyntax
                              ) As SyntaxNode
        If node.Declarators.Count > 1 Then Return node
        If node.Declarators(0).Names.Count > 1 Then Return node
        If node.Declarators(0).AsClause Is Nothing Then Return node
        If node.Declarators(0).AsClause.Kind = SyntaxKind.AsNewClause _
            Then Return node
        If node.Declarators(0).Initializer Is Nothing Then Return node

        Dim declarator As VariableDeclaratorSyntax = node.Declarators(0)
        Dim asClause As SimpleAsClauseSyntax = declarator.AsClause
        Dim variableTypeName As TypeSyntax = asClause.Type

        Dim variableType As ITypeSymbol =
                SemanticModel.GetSymbolInfo(variableTypeName).Symbol

        Dim initializerInfo As TypeInfo =
                SemanticModel.GetTypeInfo(declarator.Initializer.Value)

        If variableType Is initializerInfo.Type Then

            Dim newDeclarator As VariableDeclaratorSyntax =
                    declarator.WithAsClause(Nothing)

            Return node.ReplaceNode(declarator, newDeclarator)
        Else
            Return node
        End If
    End Function
End Class

Return to your Module1.vb file.
To test your TypeInferenceRewriter you'll need to create a test Compilation to obtain the SemanticModel required for the type inference analysis. You'll do this step last. In the meantime declare a placeholder variable representing your test Compilation:

VB.NET

        Dim test As Compilation = CreateTestCompilation()

After a short delay you should see an error squiggle appear reporting that no CreateTestCompilation method exists. Press Ctrl+Period to open the smart tag and then select the Generate method Module1.CreateTestCompilation' option. This will generate a method stub for the CreateTestCompilation method in Module1. You'll come back to fill this in later:
Next, write the following code to iterate over each SyntaxTree in the test Compilation. For each one initialize a new TypeInferenceRewriter with the SemanticModel for that tree:

VB.NET

        For Each sourceTree As SyntaxTree In test.SyntaxTrees
 
            Dim model As SemanticModel = test.GetSemanticModel(sourceTree)
 
            Dim rewriter As New TypeInferenceRewriter(model)

        Next

Lastly, inside the above For Each statement that you just created, add the following code to perform the transformation on each source tree and conditionally write out the new transformed tree if any edits were made. Remember, your rewriter should only modify a tree if it encountered one or more local variable declarations that could be simplified using type inference:

VB.NET

            Dim newSource As SyntaxNode = rewriter.Visit(sourceTree.GetRoot())
 
            If newSource IsNot sourceTree.GetRoot() Then
                File.WriteAllText(sourceTree.FilePath, newSource.ToFullString())
            End If

You're almost done! There's just one step left: Creating a test Compilation. Since you haven't been using type inference at all during this walkthrough it would have made a perfect test case. Unfortunately, creating a Compilation from a VB project file is beyond the scope of this walkthrough. But fortunately, if you've been following instructions very carefully there's hope. Replace the contents of the CreateTestCompilation method with the following code. It creates a test compilation that coincidentally matches the project described in this walkthrough:

VB.NET


        Dim globalImports As String() =
            {"Microsoft.CodeAnalysis",
             "Microsoft.CodeAnalysis.VisualBasic",
             "Microsoft.CodeAnalysis.VisualBasic.Syntax"}

        Dim options = New VisualBasicCompilationOptions(OutputKind.ConsoleApplication).
                          WithGlobalImports(From s In globalImports
                                            Select GlobalImport.Parse(s))

        Dim module1Tree As SyntaxTree =
                VisualBasicSyntaxTree.ParseText(File.ReadAllText("..\..\Module1.vb"),, "..\..\Module1.vb",)
        Dim rewriterTree As SyntaxTree =
               VisualBasicSyntaxTree.ParseText(File.ReadAllText(
"..\..\TypeInferenceRewriter.vb"),, "..\..\TypeInferenceRewriter.vb",)

        Dim sourceTrees As SyntaxTree() = {module1Tree, rewriterTree}

        Dim mscorlib As MetadataReference =
            MetadataReference.CreateFromFile(GetType(Object).Assembly.Location)
        Dim codeAnalysis As MetadataReference =
            MetadataReference.CreateFromFile(GetType(SyntaxTree).Assembly.Location)
        Dim vbCodeAnalysis As MetadataReference =
            MetadataReference.CreateFromFile(GetType(VisualBasicSyntaxTree).Assembly.Location)

        Dim references As MetadataReference() = {mscorlib, codeAnalysis, vbCodeAnalysis}

        Return VisualBasicCompilation.Create("TransformationVB",
                                             sourceTrees,
                                             references,
                                             options)

Your Module1.vb file should look like this now:

VB.NET

Option Strict Off

Module Module1

    Sub Main()

        Dim test As Compilation = CreateTestCompilation()

        For Each sourceTree As SyntaxTree In test.SyntaxTrees

            Dim model As SemanticModel = test.GetSemanticModel(sourceTree)

            Dim rewriter As New TypeInferenceRewriter(model)

            Dim newSource As SyntaxNode = rewriter.Visit(sourceTree.GetRoot())

            If newSource IsNot sourceTree.GetRoot() Then
                File.WriteAllText(sourceTree.FilePath, newSource.ToFullString())
            End If
        Next

    End Sub

    Private Function CreateTestCompilation() As Compilation

        Dim globalImports As String() =
            {"Microsoft.CodeAnalysis",
             "Microsoft.CodeAnalysis.VisualBasic",
             "Microsoft.CodeAnalysis.VisualBasic.Syntax"}

        Dim options = New VisualBasicCompilationOptions(OutputKind.ConsoleApplication).
                          WithGlobalImports(From s In globalImports
                                            Select GlobalImport.Parse(s))

        Dim module1Tree As SyntaxTree =
                VisualBasicSyntaxTree.ParseText(File.ReadAllText("..\..\Module1.vb"),, "..\..\Module1.vb",)
        Dim rewriterTree As SyntaxTree =
               VisualBasicSyntaxTree.ParseText(File.ReadAllText(
"..\..\TypeInferenceRewriter.vb"),, "..\..\TypeInferenceRewriter.vb",)

        Dim sourceTrees As SyntaxTree() = {module1Tree, rewriterTree}

        Dim mscorlib As MetadataReference =
            MetadataReference.CreateFromFile(GetType(Object).Assembly.Location)
        Dim codeAnalysis As MetadataReference =
            MetadataReference.CreateFromFile(GetType(SyntaxTree).Assembly.Location)
        Dim vbCodeAnalysis As MetadataReference =
            MetadataReference.CreateFromFile(GetType(VisualBasicSyntaxTree).Assembly.Location)

        Dim references As MetadataReference() = {mscorlib, codeAnalysis, vbCodeAnalysis}

        Return VisualBasicCompilation.Create("TransformationVB",
                                             sourceTrees,
                                             references,
                                             options)

    End Function

End Module

Cross your fingers and run the project.

In Visual Studio, choose Debug -> Start Debugging.

You should be prompted by Visual Studio that the files in your project have changed. Click "Yes to All" to reload the modified files. Examine them to observe your awesomeness :)

Note how much cleaner the code looks without all those explicit and redundant type specifiers.

Congratulations! You've just used the Compiler APIs to write your own refactoring that searches all files in a project for certain syntactic patterns, analyzes the semantics of source code that matches those patterns, and transforms it. You're now officially a Refactoring guru!