Using a CSharp Syntax Rewriter

One interesting feature of the roslyn engine is the CSharpSyntaxRewriter. This is a base class which you can extend that implements the visitor pattern. You simply override the Visit method that is appropriate for your use case and you can rewrite any portion of a syntax tree.

Consider the following code:

public class Foo
{
    public string _bar = "baz";
}

Now, let’s say we want to change "baz" to something else. To do this, we simply implement a new CSharpSyntaxRewriter that overrides the VisitLiteralExpression so that our code is only executed on LiteralExpression syntax nodes. We then check to see if it is a StringLiteralExpression and if it is, then we create a new node.

class LiteralRewriter : CSharpSyntaxRewriter
{
    public override SyntaxNode VisitLiteralExpression(LiteralExpressionSyntax node)
    {
        if (!node.IsKind(SyntaxKind.StringLiteralExpression))
        {
            return base.VisitLiteralExpression(node);
        }

        var retVal = SyntaxFactory.LiteralExpression(SyntaxKind.StringLiteralExpression, 
                                                      SyntaxFactory.Literal("NotBaz"));
        return retVal;
    }
}

The rewriter can be passed any SyntaxNode and it will run on that node and its descendants. So, to use this, I can get a SyntaxTree from a SemanticModel that I get from a CSharpCompilation. Here is a full working sample:

var tree = CSharpSyntaxTree.ParseText(@"
  public class Foo
  {
      public string _bar = ""baz"";
  }");
var Mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
var compilation = CSharpCompilation.Create("MyCompilation",
    syntaxTrees: new[] { tree }, references: new[] { Mscorlib });
var model = compilation.GetSemanticModel(tree);
var root = model.SyntaxTree.GetRoot();

var rw = new LiteralRewriter();
var newRoot = rw.Visit(root);

Console.WriteLine(newRoot.GetText());
Console.ReadLine();

As you can see, a rewriter is a quick and easy way to manipulate a syntax node that you have access to. It’s a great tool to use when you have some simple changes you need to make to a syntax tree.

If you have come up with a good use for syntax rewriters, leave a comment below.