Question

There was a library of dynamic LINQ extensions methods released as a sample with Visual Studio 2008. I’d like to extend it with a join method. The code below fails with a parameter miss match exception at run time. Where is the problem?

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,                 innerSelectorLambda.Body.Type,                 resultsSelectorLambda.Body.Type             },             outer.Expression, inner.AsQueryable().Expression,             Expression.Quote(outerSelectorLambda),             Expression.Quote(innerSelectorLambda),             Expression.Quote(resultsSelectorLambda))         ); } 

Answer 1

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);     }