Question

I’m trying and failing to use an ExpressionVisitor to modify an expression that calls a method. I have a SearchService that encapsulates the search logic and want to be able to amend the search arguments passed.

The class in which the SearchFunc should be modified and run:

  public class SearchService
  {
    public Expression<Func<string, string, List<int>>> SearchFunc { get; set; }

    public void Run()
    {
      SearchModifier modifier = new SearchModifier();
      Expression<Func<string, string, List<int>>> newFunc = (Expression<Func<string, string, List<int>>>)modifier.Modify(SearchFunc);
    }
  }

SearchModifier is defined as:

      public class SearchModifier : ExpressionVisitor
  {
    public Expression Modify(Expression expression)
    {
      return Visit(expression);
    }

    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
      Debug.Print(string.Format("VisitMethodCall: {0}", node.ToString()));
      //VisitMethodCall: value(ExpressionTree_test.MainWindow)._adminRepository.SearchUsers("orig val", "orig val2")

      //trying to use the Update method to create an amended MethodCallExpression
      List<ConstantExpression> newargs = new List<ConstantExpression>();      
      newargs.Add(Expression.Constant("my new arg 1", typeof(string)));
      newargs.Add(Expression.Constant("my new arg 2", typeof(string)));

      MethodCallExpression methodCallExpression = node.Update(node, newargs);
      //causes exception
      //Method 'System.Collections.Generic.List`1[System.Int32] SearchUsers(System.String, System.String)' declared 
      //on type 'ExpressionTree_test.AdminRepository' cannot be called 
      //with instance of type 'System.Collections.Generic.List`1[System.Int32]'

      Debug.Print(string.Format("Amended VisitMethodCall: {0}", methodCallExpression.ToString()));

      return base.VisitMethodCall(node);
    }

The Run method is called like this:

_searchService = new SearchService();
      _searchService.SearchFunc = (t, s) => _adminRepository.SearchUsers("orig val", "orig val2");

I can’t find much information on using the MethodCallExpression.Update method so am not sure I’m doing this correctly. How to I change the values of the arguments in the method?
Of course there may be a better way of doing this and any suggestions gratefully received…

Answer 1

You’re not using the result of the Update method. You should pass it to base.VisitMethodCall instead of node:

return base.VisitMethodCall(methodCallExpression);

---

EDIT

Sorry, I misread the question… The first argument to Update is not the expression node being visited, it’s the instance on which the method is called. So the code should be:

node.Update(node.Object, newargs);