Question

How should I construct Expression tree for string.IndexOf("substring", StringComparison.OrdinalIgnoreCase)?

I can get it working without the second argument: StringComparison.OrdinalIgnoreCase. These are my attempts so far:

var methodCall = typeof (string).GetMethod("IndexOf", new[] {typeof (string)});
Expression[] parms = new Expression[]{right, Expression.Constant("StringComparison.OrdinalIgnoreCase", typeof (Enum))};
var exp =  Expression.Call(left, methodCall, parms);
return exp;

Also tried this:

var methodCall = typeof (string).GetMethod(method, new[] {typeof (string)});
Expression[] parms = new Expression[]{right, Expression.Parameter(typeof(Enum) , "StringComparison.OrdinalIgnoreCase")};
var exp =  Expression.Call(left, methodCall, parms);
return exp;

Please remember that I can get it working if I ignore the OrdinalIgnoreCase parameter.

Thanks

Answer 1

I suspect there are two problems.

The first is the way you’re getting the method – you’re asking for a method with only a single string parameter, instead of one with two parameters:

var methodCall = typeof (string).GetMethod("IndexOf",
                            new[] { typeof (string), typeof(StringComparison) });

The second is the value you’re giving – it should be the actual value of the constant, not a string:

Expression[] parms = new Expression[] { right, 
    Expression.Constant(StringComparison.OrdinalIgnoreCase) };

EDIT: Here’s a complete working sample:

using System;
using System.Linq.Expressions;

class Test
{
    static void Main()
    {
        var method = typeof (string).GetMethod("IndexOf",
                new[] { typeof (string), typeof(StringComparison) });

        var left = Expression.Parameter(typeof(string), "left");
        var right = Expression.Parameter(typeof(string), "right");

        Expression[] parms = new Expression[] { right, 
                Expression.Constant(StringComparison.OrdinalIgnoreCase) };

        var call = Expression.Call(left, method, parms);
        var lambda = Expression.Lambda<Func<string, string, int>>
            (call, left, right);

        var compiled = lambda.Compile();
        Console.WriteLine(compiled.Invoke("hello THERE", "lo t"));
    }
}