Passing derived type to refactored method

Question

In my EF Fluent API configurations, I have a number of classes that derive from an abstract base class

public abstract class ImageBase
{
    public int ImageId { get; set; }
    public string ImageTitle { get; set; }
    public string ImageFileExtension { get; set; }
    public string ImageDescription { get; set; }
    [NotMapped]
    public string ImageFileName => BuildImageFilename();
    public string ImageUrl { get; set; }

    //set this property to true when part of a collection
    //if it is the main image of the collection
    public bool IsMainImage { get; set; }

    //for consistency, all image filenames are the image title, with spaces replaced with underscores
    //and the image file extension
    private string BuildImageFilename()
    {
        return $"{ImageTitle.Replace(" ", "_")}{ImageFileExtension}";
    }
}

I have a number of classes that derive from this, one example below

 public class ArticleImageUri : ImageBase
{
    [Required]
    public int ArticleId { get; set; }
}

In my fluent API for that class I originally had the following

public void Configure(EntityTypeBuilder<ArticleImageUri> builder)
    {
        //configure primary key
        builder.HasKey(u => new { u.ImageId, u.ArticleId })
            .HasName("PK_ArticleImageUri");

        //configure properties
        builder.Property(u => u.ArticleId)
           .ValueGeneratedNever()
           .IsRequired();

        builder.Property(u => u.ImageId)
          .ValueGeneratedOnAdd()
          .IsRequired();

        builder.Property(u => u.ImageDescription)
           .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Descriptions)
           .IsRequired();

        builder.Property(u => u.ImageFileExtension)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.ShortText)
            .IsRequired();

        builder.Property(u => u.ImageTitle)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
            .IsRequired();

        builder.Property(u => u.ImageFileName)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
            .IsRequired();

        builder.Property(u => u.ImageUrl)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Urls)
            .IsRequired();

        //ConfigurationHelpers.SetColumnSizesForImageClasses(builder);
    }

Note: ConfigurationHelpers.MaxStringLengths is a static class that returns an int for the size of the column. I refactored the API code to a method, SetColumnSizesForImageClasses.

 public static void SetColumnSizesForImageClasses(EntityTypeBuilder<ArticleImageUri> builder)
    {
        //configure column sizes
        builder.Property(u => u.ImageDescription)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Descriptions)
            .IsRequired();

        builder.Property(u => u.ImageFileExtension)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.ShortText)
            .IsRequired();

        builder.Property(u => u.ImageTitle)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
            .IsRequired();

        builder.Property(u => u.ImageFileName)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
            .IsRequired();

        builder.Property(u => u.ImageUrl)
            .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Urls)
            .IsRequired();
    }

and that works fine; for this class only.

I thought it would work just passing in the builder, as the builder is already of type but that doesn't work. I have a few other derived image types, for example.

 public class MachineImageUri : ImageBase
{
    [Required]

    public int MachineId { get; set; }

    public byte[] ImageThumbnail { get; set; }
}

I want to be able to use the refactored method to define the column sizes for the duplicate fields in every derived types Fluent API configuration but I cant figure out how to pass the proper parameters. I tried using T, which I think is the way to go, but could get no variant of that to work.

You can't reuse that method because `EntityTypeBuilder` generic class is invariant — Darjan Bogdan, Jan 10 '19 at 12:50

score 1 · Accepted Answer · answered Jan 10 '19 at 12:58

Since EntityTypeBuilder<T> is invariant, you won't be able to reuse Configure method out of the box. In other words you'll still need two (or more) distinct EF model configurations.

What you can do is to create "helper" method, which will take any T with certain type constraint.

static void ConfigureBase<T>(EntityTypeBuilder<T> builder) where T : ImageBase
{
   builder.Property(u => u.ImageDescription)
        .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Descriptions)
        .IsRequired();

    builder.Property(u => u.ImageFileExtension)
        .HasMaxLength(ConfigurationHelpers.MaxStringLengths.ShortText)
        .IsRequired();

    builder.Property(u => u.ImageTitle)
        .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
        .IsRequired();

    builder.Property(u => u.ImageFileName)
        .HasMaxLength(ConfigurationHelpers.MaxStringLengths.FileNames)
        .IsRequired();

    builder.Property(u => u.ImageUrl)
        .HasMaxLength(ConfigurationHelpers.MaxStringLengths.Urls)
        .IsRequired();
}

And then you'll be able to reuse it in a following way:

ArticleImageUri configuration:

public void Configure(EntityTypeBuilder<ArticleImageUri> builder)
{
   ConfigureBase<ArticleImageUri>(builder);
   //other specific configurations
}

MachineImageUri configuration:

public void Configure(EntityTypeBuilder<MachineImageUri> builder)
{
   ConfigureBase<MachineImageUri>(builder);
   //other specific configurations
}

Passing derived type to refactored method

1 Answers1