C# Combine expressions from two others using a different property for each of them

Question

Context

• Three classes: MetaParticipant, MetaMovie and MetaPerson
• A MetaParticipant has one MetaMovie and one MetaPerson

To fix an issue, I created a IsEqual static method in all three.

For the independent ones MetaMovie and MetaPerson, I used (MetaPerson has the same except with its class instead):

public static System.Linq.Expressions.Expression<Func<MetaMovie, bool>> IsEqual(MetaMovie other)
{
    if (other.Id > 0) return m => other.Id == m.Id; // Using '> 0' so it skips the new ones in change tracker to the next identifier

    return m => other.MetaSource == m.MetaSource && other.ExternalId == m.ExternalId;
}

So, I would like to write the MetaParticipant.IsEqual method, but ain't able to figure out how.

This method will receive a MetaParticipant that can use its MetaMovie and MetaPerson to call the others.

Issue

Here is the MetaParticipant.Equals that IsEqual shall "replace":

public override bool Equals(object obj)
{
    if (obj == null) return false;
    if (base.Equals(obj)) return true;
    if (obj is not MetaParticipant other) return false;

    return Movie.Equals(other.Movie) && Person.Equals(other.Person) && JobTitle == other.JobTitle;
}

And where I am up to for IsEqual:

public static Expression<Func<MetaParticipant, bool>> IsEqual(MetaParticipant other)
{
    //var own = new Expression<Func<MetaParticipant, bool>() { return x => x.JobTitle == other.JobTitle; };

    var mm = MetaMovie.IsEqual(other.Movie);
    var mp = MetaPerson.IsEqual(other.Person);

    var body = Expression.AndAlso(
        Expression.Invoke(mm, Expression.Parameter(other.Movie.GetType(), "mm")),
        Expression.Invoke(mp, Expression.Parameter(other.Person.GetType(), "mp"))
        );
    //body = Expression.AndAlso(body, );

    var lambda = Expression.Lambda<Func<MetaParticipant, bool>>(body, Expression.Parameter(typeof(MetaParticipant)));
    return lambda;
    //return m => Expression.Invoke(mm, Expression.Variable(m.Movie.GetType())) && m.JobTitle == other.JobTitle;
}

Sorry, there is a bit of garbage I kept so you can see some tries I did.

Answer 1

With the help of @NetMage and someone else in another question (but unfortunately he deleted his answer), I figured out how to do it.

With .NET 7.0, I can use the class ReplacingExpressionVisitor to change both expressions to use the accordingly property.

public static Expression<Func<MetaParticipant, bool>> IsEqual(MetaParticipant other)
{
    var participantParam = Expression.Parameter(typeof(MetaParticipant), "m");

    var movieExpression = MetaMovie.IsEqual(other.Movie);
    var movieParamReplacer = new ReplacingExpressionVisitor(new[] { movieExpression.Parameters[0] }, new[] { Expression.Property(participantParam, nameof(Movie)) });

    var personExpression = MetaPerson.IsEqual(other.Person);
    var personParamReplacer = new ReplacingExpressionVisitor(new[] { personExpression.Parameters[0] }, new[] { Expression.Property(participantParam, nameof(Person)) });

    var jobTitleProperty = Expression.Property(participantParam, nameof(JobTitle));
    var otherJobTitle = Expression.Constant(other.JobTitle);
    var jobTitle = Expression.Equal(jobTitleProperty, otherJobTitle);

    var newBody = Expression.AndAlso(movieParamReplacer.Visit(movieExpression.Body), personParamReplacer.Visit(personExpression.Body));
    newBody = Expression.AndAlso(newBody, jobTitle);

    var lambda = Expression.Lambda<Func<MetaParticipant, bool>>(newBody, participantParam);
    return lambda;
}