Delete rows in two tables with relations in both directions using Entity Framework Core

You need to call SaveChanges in between, with a transaction over the whole thing.

public void DeleteImageSeries(int imageSeriesToRemoveId)
{
    using var tran = dbContext.Database.BeginTransaction();
    var imageSeries = dbContext.ImageSeries.First(is => is.Id == imageSeriesToRemoveId);
    imageSeries.MainImageId = null;
    dbContext.SaveChanges();

    var images = dbContext.Images.Where(i => i.ImageSeriesId == imageSeriesToRemoveId);
    foreach (var image in images)
    {
        dbContext.Remove(image);
    }
    dbContext.Remove(imagesSeries);
    dbContext.SaveChanges();

    tran.Commit();
}

Alternatively, a better idea is to just use ExecuteUpdate and ExecuteDelete without loading the entities at all.

public void DeleteImageSeries(int imageSeriesToRemoveId)
{
    using var tran = dbContext.Database.BeginTransaction();
    var imageSeriesQuery = dbContext.ImageSeries
        .Where(is => is.Id == imageSeriesToRemoveId);
    imageSeriesQuery.ExecuteUpdate(setter => setter
        .SetProperty(is => is.MainImageId, null));
    dbContext.Images
        .Where(i => i.ImageSeriesId == imageSeriesToRemoveId)
        .ExecuteDelete();
    imageSeriesQuery.ExecuteDelete();
    tran.Commit();
}

I agree with the others that you should probably just make an IsMain property on the Image (and add a filtered unique index to prevent duplicates). Then you can remove the circular reference.

An alternative setup for this common pattern of collections with one "favored" item is to have all references in the owned entity. EF-core has various ways to implement 1:1 relationships and here one of them is used:

public class ImageSeries
{
    public int Id { get; set; }
    public ICollection<Image> Images { get; set; } = [];
    public required Image MainImage { get; set; }
}

public class Image
{
    public int Id { get; set; }
    public int? MainOfImageSeriesId { get; set; }
    public int ImageSeriesId { get; set; }
}

The mapping:

modelBuilder.Entity<ImageSeries>()
    .HasMany(e => e.Images)
    .WithOne()
    .HasForeignKey(i => i.ImageSeriesId)
    .OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<ImageSeries>()
    .HasOne(e => e.MainImage)
    .WithOne()
    .HasForeignKey<Image>(i => i.MainOfImageSeriesId);

In this model it's a piece of cake to add an ImageSeries with Images and a MainImage in one transaction, and to delete them:

using (var db = getContext())
{
    var images = Enumerable.Range(1,5).Select(e => new Image()).ToList();
    db.ImageSeries.Add(new ImageSeries
    {
        Images = images,
        MainImage = images[0]
    });
    db.SaveChanges();
}
using (var db = getContext())
{
    db.Remove(db.ImageSeries.FirstOrDefault()); 
    db.SaveChanges();
}

If you have an existing database, note that the Image table differs from what you probably have now and you may have to change the schema, if possible/permitted:

CREATE TABLE [Image] (
  [Id] int NOT NULL IDENTITY,
  [MainOfImageSeriesId] int NULL,
  [ImageSeriesId] int NOT NULL,
  CONSTRAINT [PK_Image] PRIMARY KEY ([Id]),
  CONSTRAINT [FK_Image_ImageSeries_ImageSeriesId] FOREIGN KEY ([ImageSeriesId])
      REFERENCES [ImageSeries] ([Id]) ON DELETE CASCADE,
  CONSTRAINT [FK_Image_ImageSeries_MainOfImageSeriesId] FOREIGN KEY ([MainOfImageSeriesId])
      REFERENCES [ImageSeries] ([Id])
);

I.e. both the 1:n and the 1:1 foreign keys are in the Image table, which is the reason that both relationships can be removed in one go.

Try to call this, after changing the mainImageId to null, which should tell EF to do an update before trying to delete:

    dbContext.Entry(imageSeries).Property(x => x.MainImageId).IsModified = true;

Also, use RemoveRange instead of the foreach:

    dbContext.Images.RemoveRange(images);