How to use MethodCallExpression.Update - linq

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);