Expression Trees in Linq To Entities - linq

Question

I'm trying to make an extension method that takes in an expression that points to a property on an entity, and a constant string to compare it to using String.Contains, that first checks whether the string is null or empty, and applies the filter only if the string has a value. This is my first dabble in Expression Trees though, so I'm not really sure what's going on, and I now have an exception which I'm not sure how to cure...
I'm at this so far:
<System.Runtime.CompilerServices.Extension()>
Public Function CheckAndFilter(Of T)(source As System.Linq.IQueryable(Of T), expressionField As System.Linq.Expressions.Expression(Of System.Func(Of T, String)), compareTo As String) As IQueryable(Of T)
If String.IsNullOrEmpty(compareTo) Then
Return source
Else
Dim memberExp As Expressions.MemberExpression = DirectCast(expressionField.Body, Expressions.MemberExpression)
Dim param = Expressions.Expression.Parameter(GetType(T))
Dim method As Reflection.MethodInfo = GetType(String).GetMethod("Contains")
Dim compareToExpression = Expressions.Expression.Constant(compareTo)
Dim finalExpr = Expressions.Expression.Call(memberExp, method, compareToExpression)
Dim lambda = Expressions.Expression.Lambda(Of Func(Of T, Boolean))(finalExpr, param)
Return source.Where(lambda)
End If
End Function
I'm calling it like this, against a DbContext where I have a Customer entity with a FirstName string property:
Dim results = repository.Customers.CheckAndFilter(Function(c) c.FirstName, searchText)
And the exception is:
{"The parameter 'c' was not bound in the specified LINQ to Entities query expression."}
Any thoughts?

Answer 1

A ha!
It's because I've declared a new parameter rather than re-using the one passed in via the expression...
<System.Runtime.CompilerServices.Extension()>
Public Function CheckAndFilter(Of T)(source As System.Linq.IQueryable(Of T), expressionField As System.Linq.Expressions.Expression(Of System.Func(Of T, String)), compareTo As String) As IQueryable(Of T)
If String.IsNullOrEmpty(compareTo) Then
Return source
Else
Dim memberExp As Expressions.MemberExpression = DirectCast(expressionField.Body, Expressions.MemberExpression)
Dim method As Reflection.MethodInfo = GetType(String).GetMethod("Contains")
Dim compareToExpression = Expressions.Expression.Constant(compareTo)
Dim finalExpr = Expressions.Expression.Call(memberExp, method, compareToExpression)
Dim lambda = Expressions.Expression.Lambda(Of Func(Of T, Boolean))(finalExpr, expressionField.Parameters)
Return source.Where(lambda)
End If
End Function