Creating a GroupJoin expression tree

void Main()
{
    var fn = GroupJoinExp<IndexString, IndexNumber, JoinResult>();

    Console.WriteLine(fn.ToString());
}

// Define other methods and classes here
public Expression<Func<IQueryable<TOuter>, IEnumerable<TInner>, IQueryable<TResult>>> GroupJoinExp<TOuter, TInner, TResult>()
{
    var tOuter = typeof(TOuter);
    var miOuterKey = tOuter.GetMember("index")[0];
    var miOuterValue = tOuter.GetMember("value")[0];
    var tInner = typeof(TInner);
    var miInnerKey = tInner.GetMember("index")[0];

    var pOuter = Expression.Parameter(tOuter, "o");
    var pInner = Expression.Parameter(tInner, "i");

    var miResStr = typeof(TResult).GetMember("str")[0];
    var miResGrp = typeof(TResult).GetMember("grp")[0];


    // selector expression:
    var pInnerEnum = Expression.Parameter(typeof(IEnumerable<TInner>), "g");

    var meminit =
            Expression.MemberInit(
                Expression.New(typeof(TResult)),
                Expression.Bind(miResStr, Expression.MakeMemberAccess(pOuter, miOuterValue)),
                Expression.Bind(miResGrp, pInnerEnum)
            );
    var selector = Expression.Lambda(meminit, pOuter, pInnerEnum);

    // parameters for GroupJoin:
    var gjpOuter = Expression.Parameter(typeof(IQueryable<TOuter>), "outer");
    var gjpInner = Expression.Parameter(typeof(IEnumerable<TInner>), "inner");
    var gjpOuterKeySelector = Expression.Lambda(Expression.MakeMemberAccess(pOuter, miOuterKey), pOuter);
    var gjpInnerKeySelector = Expression.Lambda(Expression.MakeMemberAccess(pInner, miInnerKey), pInner);
    var gjpSelector = selector;

    // GroupJoin expression
    var fnGrpJn = Expression.Call(typeof(Queryable), "GroupJoin", new Type [] { tOuter, tInner, typeof(Int32), typeof(TResult) },
        gjpOuter,
        gjpInner,
        gjpOuterKeySelector,
        gjpInnerKeySelector,
        gjpSelector
    );

    var res = (Expression<Func<IQueryable<TOuter>, IEnumerable<TInner>, IQueryable<TResult>>>) Expression.Lambda(fnGrpJn, gjpOuter, gjpInner);
    return res;
}

public class IndexString
{
    public int index;
    public string value;
}

public class IndexNumber
{
    public int index;
    public double dbl;
}

public class JoinResult
{
    public string str;
    public IEnumerable<IndexNumber> grp;
}