LINQ Expression Problem

Question

I have a Linq Expression that is giving me a real headache and hope someone can help me find out why my provider does not like it.

If I do the following it works fine;

Expression<Func<Employee, bool>> expr = e => e.Participant == "Y";
_context.Employees.Where(expr).ToList<IEmployee>();

However, if I do the following my provider doesn't like it.

Expression<Func<IEmployee, bool>> expr = e => e.Participant == "Y";
Expression<Func<Employee, bool>> convertedExpr = ParameterReplacer.Replace
 <Func<IEmployee, bool>, Func<Employee, bool>> 
 (expr,
 expr.Parameters.Single(), 
 Expression.Parameter(typeof(Employee)));
_context.Employees.Where(convertedExpr).ToList<IEmployee>();

The reason I am doing the conversion is that the upper layer of my application only knows about the interface type, so I convert it to a Expression that uses the concrete type in the lower layer using the ParameterReplacer (kindly supplied by another SO member).

I have compared the linq expressions between the version that works and the one that doesn't and the only difference I can see is in what I see in the DebugView property in that the parameter names are different.

Here is the code for the ParameterReplacer;

public static class ParameterReplacer
{
 // Produces an expression identical to 'expression'
 // except with 'source' parameter replaced with 'target' parameter.
 public static Expression<TOutput> Replace<TInput, TOutput>(Expression<TInput> expression, ParameterExpression source, ParameterExpression target)
 { 
 return new ParameterReplacerVisitor<TOutput>(source, target).VisitAndConvert(expression);
 } 
 private class ParameterReplacerVisitor<TOutput> : ExpressionVisitor
 { 
 private ParameterExpression _source;
 private ParameterExpression _target;
 public ParameterReplacerVisitor(ParameterExpression source, ParameterExpression target)
 { 
 _source = source; 
 _target = target;
 } 
 internal Expression<TOutput> VisitAndConvert<T>(Expression<T> root)
 { 
 return (Expression<TOutput>)VisitLambda(root);
 } 
 protected override Expression VisitLambda<T>(Expression<T> node)
 { 
 // Leave all parameters alone except the one we want to replace.
 var parameters = node.Parameters.Select(p => p == _source ? _target : p);
 return Expression.Lambda<TOutput>(Visit(node.Body), parameters);
 } 
 protected override Expression VisitParameter(ParameterExpression node)
 { 
 // Replace the source with the target, visit other params as usual.
 return node == _source ? _target : base.VisitParameter(node);
 }
 } 
}

Is this an issue with the ParameterReplacer or my provider? Can any one help me out?

The provider I am using is Telerik OpenAccess and the exception its throwing is;

InnerException: System.NullReferenceException Message=Object reference not set to an instance of an object. Source=Telerik.OpenAccess.35.Extensions StackTrace: at Telerik.OpenAccess.Query.ExpressionCompiler.PerformDatabaseQueryImpl(Type type, Int32 elementAt, Object[] groupResolutionParamValues, Boolean single, Boolean checkOid) at Telerik.OpenAccess.Query.ExpressionCompiler.PerformDatabaseQuery(Type type, Int32 elementAt, Object[] groupResolutionParamValues, Boolean single, Boolean checkOid) InnerException:

• David: The provider I am using is Telerik OpenAccess and its throwing a null reference exception. Added exception to question.

Answer 1

I have never used your provider, so I can't be sure, but considering the circumstances, I think this is good bet:

Since, the only difference is that the parameter in the second expression has null Name and the exception you are getting is a null reference exception, I think that's exactly the problem. Try changing the Name to something non-null.

That means changing the creation of the new parameter to something like:

Expression.Parameter(typeof(Employee), "e")

• David: Excellent! That was exactly the problem. At last, I can move on.