Visualizing an AST created with ANTLR (in a .Net environment)

Correct, the interpreter only shows what rules are used in the parsing process, and ignores any AST rewrite rules.

What you can do is use StringTemplate to create a Graphviz DOT-file. After creating such a DOT-file, you use some 3rd party viewer to display this tree (graph).

Here's a quick demo in Java (I know little C#, sorry).

Take the following (overly simplistic) expression grammar that produces an AST:

grammar ASTDemo;

options { 
  output=AST; 
}

tokens {
  ROOT;
  EXPRESSION;
}

parse
  :  (expression ';')+ -> ^(ROOT expression+) // omit the semi-colon
  ;

expression
  :  addExp -> ^(EXPRESSION addExp)
  ;

addExp
  :  multExp
     ( '+'^ multExp
     | '-'^ multExp
     )*
  ;

multExp
  :  powerExp
     ( '*'^ powerExp
     | '/'^ powerExp
     )*
  ;

powerExp
  :  atom ('^'^ atom)*
  ;

atom
  :  Number
  |  '(' expression ')' -> expression // omit the parenthesis
  ;

Number
  :  Digit+ ('.' Digit+)?
  ;

fragment
Digit
  :  '0'..'9'
  ;

Space
  :  (' ' | '\t' | '\r' | '\n') {skip();}
  ;

First let ANTLR generate lexer and parser files from it:

java -cp antlr-3.2.jar org.antlr.Tool ASTDemo.g 

then create a little test harness that parses the expressions "12 * (5 - 6); 2^3^(4 + 1);" and will output a DOT-file:

import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
import org.antlr.stringtemplate.*;

public class MainASTDemo {
    public static void main(String[] args) throws Exception {
        ANTLRStringStream in = new ANTLRStringStream("12 * (5 - 6); 2^3^(4 + 1);");
        ASTDemoLexer lexer = new ASTDemoLexer(in);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        ASTDemoParser parser = new ASTDemoParser(tokens);
        ASTDemoParser.parse_return returnValue = parser.parse();
        CommonTree tree = (CommonTree)returnValue.getTree();
        DOTTreeGenerator gen = new DOTTreeGenerator();
        StringTemplate st = gen.toDOT(tree);
        System.out.println(st);
    }
}

Compile all .java files:

// *nix & MacOS
javac -cp .:antlr-3.2.jar *.java

// Windows
javac -cp .;antlr-3.2.jar *.java

and then run the main class and pipe its output to a file named ast-tree.dot:

// *nix & MacOS
java -cp .:antlr-3.2.jar MainASTDemo > ast-tree.dot

// Windows
java -cp .;antlr-3.2.jar MainASTDemo > ast-tree.dot

The file ast-tree.dot now contains:

digraph {

    ordering=out;
    ranksep=.4;
    bgcolor="lightgrey"; node [shape=box, fixedsize=false, fontsize=12, fontname="Helvetica-bold", fontcolor="blue"
        width=.25, height=.25, color="black", fillcolor="white", style="filled, solid, bold"];
    edge [arrowsize=.5, color="black", style="bold"]

  n0 [label="ROOT"];
  n1 [label="EXPRESSION"];
  n1 [label="EXPRESSION"];
  n2 [label="*"];
  n2 [label="*"];
  n3 [label="12"];
  n4 [label="EXPRESSION"];
  n4 [label="EXPRESSION"];
  n5 [label="-"];
  n5 [label="-"];
  n6 [label="5"];
  n7 [label="6"];
  n8 [label="EXPRESSION"];
  n8 [label="EXPRESSION"];
  n9 [label="^"];
  n9 [label="^"];
  n10 [label="^"];
  n10 [label="^"];
  n11 [label="2"];
  n12 [label="3"];
  n13 [label="EXPRESSION"];
  n13 [label="EXPRESSION"];
  n14 [label="+"];
  n14 [label="+"];
  n15 [label="4"];
  n16 [label="1"];

  n0 -> n1 // "ROOT" -> "EXPRESSION"
  n1 -> n2 // "EXPRESSION" -> "*"
  n2 -> n3 // "*" -> "12"
  n2 -> n4 // "*" -> "EXPRESSION"
  n4 -> n5 // "EXPRESSION" -> "-"
  n5 -> n6 // "-" -> "5"
  n5 -> n7 // "-" -> "6"
  n0 -> n8 // "ROOT" -> "EXPRESSION"
  n8 -> n9 // "EXPRESSION" -> "^"
  n9 -> n10 // "^" -> "^"
  n10 -> n11 // "^" -> "2"
  n10 -> n12 // "^" -> "3"
  n9 -> n13 // "^" -> "EXPRESSION"
  n13 -> n14 // "EXPRESSION" -> "+"
  n14 -> n15 // "+" -> "4"
  n14 -> n16 // "+" -> "1"

}

which can be viewed with one of the viewers here. There are even online viewers. Take this one for example: https://dreampuf.github.io/GraphvizOnline/

When feeding it the contents of ast-tree.dot, the following image is produced: