I’ve fixed it myself now. It was a schoolboy error passing too many parameters to the CreateQuery(… ) call. Paste the following code into the Dynamic.cs file within the DynamicQueryable class for a dynamic Join extension method. You can find the source for the DynamicQuery sample project at http://code.msdn.microsoft.com/csharpsamples.
Enjoy.
public static IQueryable Join(this IQueryable outer, IEnumerable inner, string outerSelector, string innerSelector, string resultsSelector, params object[] values)
{
if (inner == null) throw new ArgumentNullException("inner");
if (outerSelector == null) throw new ArgumentNullException("outerSelector");
if (innerSelector == null) throw new ArgumentNullException("innerSelector");
if (resultsSelector == null) throw new ArgumentNullException("resultsSelctor");
LambdaExpression outerSelectorLambda = DynamicExpression.ParseLambda(outer.ElementType, null, outerSelector, values);
LambdaExpression innerSelectorLambda = DynamicExpression.ParseLambda(inner.AsQueryable().ElementType, null, innerSelector, values);
ParameterExpression[] parameters = new ParameterExpression[] {
Expression.Parameter(outer.ElementType, "outer"), Expression.Parameter(inner.AsQueryable().ElementType, "inner") };
LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, resultsSelector, values);
return outer.Provider.CreateQuery(
Expression.Call(
typeof(Queryable), "Join",
new Type[] {outer.ElementType, inner.AsQueryable().ElementType, outerSelectorLambda.Body.Type, resultsSelectorLambda.Body.Type },
outer.Expression, inner.AsQueryable().Expression, Expression.Quote(outerSelectorLambda), Expression.Quote(innerSelectorLambda), Expression.Quote(resultsSelectorLambda)));
}
//The generic overload.
public static IQueryable<T> Join<T>(this IQueryable<T> outer, IEnumerable<T> inner, string outerSelector, string innerSelector, string resultsSelector, params object[] values)
{
return (IQueryable<T>)Join((IQueryable)outer, (IEnumerable)inner, outerSelector, innerSelector, resultsSelector, values);
}