Roslyn IfStatement

Question

I'm using the Roslyn syntax tree to update if/else statements. Here's my code:

foreach (StatementSyntax statement in blockNode.Statements)
{
    if (statement.IsKind(SyntaxKind.IfStatement))
    {
        BlockSyntax ifBlock = statement.ChildNodes().OfType<BlockSyntax>().FirstOrDefault();
        if (ifBlock != null)
        {
            ReturnStatementSyntax newRSS = ifBlock.ChildNodes().OfType<ReturnStatementSyntax>().FirstOrDefault();
            blockNode = blockNode.InsertNodesBefore(newRSS, newExitCode);
        }
        ElseClauseSyntax elseBlock = statement.ChildNodes().OfType<ElseClauseSyntax>().FirstOrDefault();
        if (elseBlock != null)
        {
            BlockSyntax block = elseBlock.ChildNodes().OfType<BlockSyntax>().FirstOrDefault();
            if (block != null)
            {
                ReturnStatementSyntax newRSS = block.ChildNodes().OfType<ReturnStatementSyntax>().FirstOrDefault();
                blockNode = blockNode.InsertNodesBefore(newRSS, newExitCode);
            }
        }
        newBlock = newBlock.AddRange(blockNode.Statements);
    }
}

Can anyone explain why the first blockNode insert nodes works, but the second one does not? I see the code I want inserted both times, but only the first one updates the syntax tree. The second one does nothing.

Update: I've made the changes suggested by JoshVarty. I used the DocumentEditor to load of the changes. I'm now getting an exception when I call GetChangedDocument. Here's my code:

DocumentEditor editor = DocumentEditor.CreateAsync(doc).Result;
editor.InsertBefore(blockNode, newEntryCode);
editor.InsertAfter(blockNode, newExitCode);
Document newDoc = editor.GetChangedDocument();

The exception is: An exception of type 'System.InvalidOperationException' occurred in Microsoft.CodeAnalysis.CSharp.dll but was not handled in user code

Additional information: The item specified is not the element of a list.

Do I have to use the Generator? What did I miss?

Thanks

Answer 1

Here's how I solved the issue. I used the SyntaxGenerator to rewrite the if statement and then used DocumentEditor to hold all the changes to the syntax tree as I rewrite certain methods. Here's the relevant code:

SyntaxGenerator synGen = editor.Generator;
foreach (StatementSyntax statement in blockNode.Statements)
{
    if (statement.IsKind(SyntaxKind.IfStatement))
    {
        IfStatementSyntax ifs = statement as IfStatementSyntax;
        SyntaxList<StatementSyntax> trueStatements = new SyntaxList<StatementSyntax>();
        SyntaxList<StatementSyntax> falseStatements = new SyntaxList<StatementSyntax>();

        BlockSyntax ifBlock = ifs.ChildNodes().OfType<BlockSyntax>().FirstOrDefault();
        if (ifBlock != null)
        {
            ReturnStatementSyntax newRSS = ifBlock.ChildNodes().OfType<ReturnStatementSyntax>().FirstOrDefault();
            SyntaxList<StatementSyntax> ifStatements = ifBlock.Statements;
            foreach (StatementSyntax ss in ifStatements)
            {
                if (ss.Kind() != SyntaxKind.ReturnStatement)
                {
                    trueStatements = trueStatements.Add(ss);
                }
             }
             foreach (StatementSyntax ss in newExitCode)
             {
                 trueStatements = trueStatements.Add(ss);
             }
             trueStatements = trueStatements.Add(newRSS);
             ElseClauseSyntax elseBlock = ifs.ChildNodes().OfType<ElseClauseSyntax>().FirstOrDefault();
             if (elseBlock != null)
             {
                 BlockSyntax block = elseBlock.ChildNodes().OfType<BlockSyntax>().FirstOrDefault();
                 if (block != null)
                 {
                     ReturnStatementSyntax newRSS = block.ChildNodes().OfType<ReturnStatementSyntax>().FirstOrDefault();
                     SyntaxList<StatementSyntax> elseStatements = block.Statements;
                     foreach (StatementSyntax ss in elseStatements)
                     {
                         if (ss.Kind() != SyntaxKind.ReturnStatement)
                         {
                             falseStatements = falseStatements.Add(ss);
                         }
                     }
                     foreach (StatementSyntax ss in newExitCode)
                     {
                         falseStatements = falseStatements.Add(ss);
                     }
                     falseStatements = falseStatements.Add(newRSS);
                 }
             }
             IfStatementSyntax newIfStatement = (IfStatementSyntax)synGen.IfStatement(ifs.Condition, trueStatements, falseStatements);
             newBlock = newBlock.Add(newIfStatement);
        }
        else
        {
            if (!statement.IsKind(SyntaxKind.ReturnStatement))
            {
                newBlock = newBlock.Add(statement);
            }
            else
            {
                newBlock = newBlock.AddRange(newExitCode);
                newBlock = newBlock.Add(statement);
            }
        }
    }
}
var newBody = SyntaxFactory.Block(SyntaxFactory.Token(SyntaxKind.OpenBraceToken), newBlock, SyntaxFactory.Token(SyntaxKind.CloseBraceToken));
var newMethod = SyntaxFactory.MethodDeclaration(mds.AttributeLists, mds.Modifiers, mds.ReturnType, mds.ExplicitInterfaceSpecifier, mds.Identifier, mds.TypeParameterList, mds.ParameterList, mds.ConstraintClauses, newBody, mds.ExpressionBody);
editor.ReplaceNode(mds, newMethod);
SyntaxNode newRoot = editor.GetChangedRoot();
var newFormattedRoot = Formatter.Format(newRoot, Workspace);
Document newDoc = editor.GetChangedDocument();
doc = doc.WithSyntaxRoot(newFormattedRoot);
methodsChanged++;

Thanks to Josh Varty and Jeroen Vannevel for their assistance in figuring this one out.

Answer 2

I believe the problem here is that you create a new tree from statement and then try to use parts of that new tree to compare to statement afterward.

Basically this line doesn't do anything the second time around:

blockNode = blockNode.InsertNodesBefore(newRSS, newExitCode);

blockNode is an entirely new tree you've created and does not contain newRSS. So it cannot find newRss and insert your newExitCode.

• newRss is from block
• block is from elseBlock
• elseBlock is from the original statement

There are three options you have when trying to apply multiple changes to a syntax tree at once:

1. Use the DocumentEditor - See: https://stackoverflow.com/a/30563669/300908
2. Use Annotations (Lines 235 and 239)
3. Use .TrackNodes()

My understanding is the DocumentEditor is the easiest option and takes care of tracking/annotating the nodes for you under the cover.