.NET: How Can I Combine LINQ Expressions for EF4 Without Using Expression.Invoke?

Below is the explanation and a possible solution using a parameter replacement technique.

The Problem

Entity Framework 4’s LINQ provider does not support the Expression.Invoke method. Your code uses Expression.Invoke in the loop to “call” the next lambda expression against the initial lambda’s parameters. Removing Invoke leads to trying to merge two Expression(Of Func(Of T, Boolean)) lambdas with Expression.OrElse, which isn’t allowed because Expression.OrElse expects Boolean expressions rather than whole lambda expressions.

In EF Core (and in compiled in-memory queries) these extra layers are handled, but in EF4 the query translator cannot process the Invoke call.

The Solution

To combine the expressions without invoking, you need to “inline” the body of each lambda by replacing its parameter with the unified parameter used for the final lambda. One way to do this is to create a custom ExpressionVisitor (often called a ParameterReplacer) that substitutes one parameter for another.

Here’s how you can do that in VB:

VBCopied
Imports System.Linq.Expressions

' Helper class to replace parameters in an expression.
Public Class ParameterReplacer
    Inherits ExpressionVisitor

    Private ReadOnly _oldParam As ParameterExpression
    Private ReadOnly _newParam As ParameterExpression

    Public Sub New(oldParam As ParameterExpression, newParam As ParameterExpression)
        _oldParam = oldParam
        _newParam = newParam
    End Sub

    Protected Overrides Function VisitParameter(node As ParameterExpression) As Expression
        ' Replace the old parameter with the new one.
        If node = _oldParam Then
            Return _newParam
        End If
        Return MyBase.VisitParameter(node)
    End Function
End Class

Public Function CombineExpressions(Of T)(ParamArray filters() As Expression(Of Func(Of T, Boolean))) As Expression(Of Func(Of T, Boolean))
    Dim firstFilter = filters.FirstOrDefault()

    ' If no filters, return an expression that is always true.
    If firstFilter Is Nothing Then
        Dim alwaysTrue As Expression(Of Func(Of T, Boolean)) = Function(x) True
        Return alwaysTrue
    End If

    ' Use a single parameter for the final lambda.
    Dim param As ParameterExpression = Expression.Parameter(GetType(T), "x")

    ' Replace the parameter in the first expression.
    Dim replacer = New ParameterReplacer(firstFilter.Parameters(0), param)
    Dim body As Expression = replacer.Visit(firstFilter.Body)

    ' For each additional filter, replace its parameter and combine the bodies.
    For Each filter In filters.Skip(1)
        Dim visitor = New ParameterReplacer(filter.Parameters(0), param)
        Dim filterBody As Expression = visitor.Visit(filter.Body)
        body = Expression.OrElse(body, filterBody)
    Next

    ' Create the final lambda expression.
    Return Expression.Lambda(Of Func(Of T, Boolean))(body, param)
End Function

How It Works

1. Parameter Unification:
   A new parameter (param) is declared. For each filter expression, the original parameter is replaced by this unified parameter using the ParameterReplacer.

2. Inlining Expressions:
   By replacing the parameters, each filter’s body becomes a Boolean expression that can be combined directly using Expression.OrElse.

3. Building the Final Expression:
   The combined body is wrapped in a lambda with the unified parameter, resulting in an expression tree without any Invoke calls—something EF4 can translate.

Conclusion

EF4 doesn’t support Expression.Invoke in LINQ-to-Entities queries. By replacing the parameters and inlining the lambda bodies, you create a combined expression that EF4 can process. Another alternative is to use a library like LinqKit that provides similar functionality, but the above code demonstrates the approach inline.

This solution should work with Entity Framework 4.