C# Expression Tree AndAlso for Entity Framework query

Question

I get an error when trying to combine two expressions with AndAlso.

Step 1: building expression for EF object with expression tree:

public override Expression<Func<T, bool>> ToExpression()
{
    var expressionParameter = Expression.Parameter(typeof(T), "p");
    var expressionField = Expression.PropertyOrField(expressionParameter, field);
    var expressionConstraint = Expression.Constant(value);

    BinaryExpression expression = Expression.Equal(expressionField, expressionConstraint);

    return Expression.Lambda<Func<T, bool>>(expression, expressionParameter);
}

If I'm calling .ToExpression(); and running this for one expression, this code works fine with EF, it produces expression:

{ p => (p.MyField1 == "X") }

But when I'm trying to do

Step 2: combine two expressions with AndAlso:

public override Expression<Func<T, bool>> ToExpression()
{
    Expression<Func<T, bool>> leftExpression = left.ToExpression();
    Expression<Func<T, bool>> rightExpression = right.ToExpression();

    BinaryExpression andExpression = Expression.AndAlso(leftExpression.Body, rightExpression.Body);

    return Expression.Lambda<Func<T, bool>>(andExpression, leftExpression.Parameters.Single());
}

This produces an expression:

{ p => ((p.MyField1 == "X") AndAlso (p.MyField2 == "Y")) }

It seems fine to me, but when trying to call it with .Where(expression) I get this error:

System.InvalidOperationException: The LINQ expression 'p' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'

Any idea why separate expressions work but combined with AndAlso don't?

Have you considered using PredicateBuilder / LinqKit instead of doing this manually by yourself? — Dai
– Dai, Commented Jul 17, 2023 at 13:49
No, I will take a look, but still curious why this doesn't work — Carlos28
– Carlos28, Commented Jul 17, 2023 at 13:55
I imagine it fails because AndAlso is the VB.NET short-circuiting version of VB.NET's And combinator (which famously is not short-circuiting) - whereas SQL (which is what I assume you're using this for?) does not define sequence-order for expressions so it makes no sense to support AndAlso instead of just And. — Dai
– Dai, Commented Jul 17, 2023 at 13:56
When you combine the expressions, I suspect ef core is not able to take "p" as same parameter reference , rather consider them different, perhaps you could use expressionVisitor to replace parameter stackoverflow.com/a/41652669/1398461 — kuldeep
– kuldeep, Commented Jul 17, 2023 at 14:13
Lambda expressions cannot be combined this way because they use different ParameterExpression instances (the name doesn't matter). Expression.Invoke is limited and not always supported, you need ParameterReplacer, there are a lot of examples here. — Ivan Stoev
– Ivan Stoev, Commented Jul 17, 2023 at 17:45

Bron Davies · Accepted Answer · 2023-07-17 14:57:33Z

1

Your Expression.Lambda calls probably need to start with a call to Expression.Invoke like this:

var expr = Expression.Invoke(expression, expressionParameter);
return Expression.Lambda<Func<T, bool>>(expr, expressionParameter);

answered Jul 17, 2023 at 14:57

Bron Davies

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Comments

Carlos28 · Accepted Answer · 2023-07-20 07:59:17Z

0

Implementing ParameterReplacer did the trick, implementation that I have found and used:

    internal class ReplaceExpressionVisitor : ExpressionVisitor
{
    private readonly Expression _oldValue;
    private readonly Expression _newValue;

    public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
    {
        _oldValue = oldValue;
        _newValue = newValue;
    }

    public override Expression Visit(Expression node) => node == _oldValue ? _newValue : base.Visit(node);
}

Thank's for your suggestions.

answered Jul 20, 2023 at 7:59

Carlos28

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2 Comments

Panagiotis Kanavos Over a year ago

What SQL did this emit? Was it identical to AND perhaps? SQL has no AndAlso and in fact, the order doesn't matter when it comes to generating an execution plan. The execution plan will pick indexes and filtering strategies that work on streams of data.

Svyatoslav Danyliv Over a year ago

@PanagiotisKanavos, EF Core translates Expression.AndAlso (and only this binary expression) into logical AND. Expression.And is bitwise operator and has different meaning.

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