Efcore: Value Conversion from Enum to string different from enum value

[OculiViridi]

In a read only database, I've an entity object with an Enum property that is mapped to a VARCHAR DB column. The problem is that the string value IS NOT equivalent to the Enum .ToString() value, so how am I supposed to write the necessary custom conversion code?

Entity

public class PhaseArticle
{
    public int Id { get; set; }
    public string Code { get; set; }
    public string Description { get; set; }
    public string UnitOfMeasure { get; set; }
    public string Category { get; set; }
    public string Group { get; set; }
    public ArticleFamily Family { get; set; } // --> Enum value
    public double UnitCost { get; set; }
    public string AdditionalDescription { get; set; }
    public string ExternalCode { get; set; }
    public string ColorCode { get; set; }
    public string Note { get; set; }
}

Now I'm using a static helper class with this simple method:

public static class PhaseHelper
{
    public static ArticleFamily GetArticleFamilyFromString(string family)
    {
        switch (family)
        {
            case "CEL":
                return ArticleFamily.Cell;
            case "STR":
                return ArticleFamily.String;
            case "RAW":
                return ArticleFamily.OtherRawMaterial;
            case "SFP":
                return ArticleFamily.SemiFinishedPanel;
            case "FP":
                return ArticleFamily.FinishedPanel;
            default:
                return ArticleFamily.Other;
        }
    }
}

Custom conversion code

public static void ApplyPhaseConversions<T>(this ModelBuilder modelBuilder)
{
    modelBuilder
        .Entity<PhaseArticle>()
        .Property(e => e.Family)
        .HasConversion<?>(
            v => 
            { ? },
            v => 
            { ? });
}

Technical details

EF Core version: Microsoft.EntityFrameworkCore v2.1.4
Database Provider: Microsoft.EntityFrameworkCore.SqlServer v2.1.4
Operating system: Windows 10
IDE: Visual Studio 2017 v15.9.6

[smitpatel]

Inverse of the GetArticleFamilyFromString can easily do it. Your input parameter would be enum value and put switch statement on it and return appropriate string value for each enum value.

[OculiViridi]

I'm trying but I'm not able to make it compile.

It's complaining about:

Cannot convert Lambda expression into a tree of expressions.

[ajcvickers]

@OculiViridi This is because of limitations in the C# compiler such that it can't create expression trees for this code. The solution is to factory the conversion code into methods and then call those in HasConversion. For example:
```C#
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity()
.Property(e => e.Family)
.HasConversion(
v => FromEnum(v),
v => FromString(v));
}

private static string FromEnum(ArticleFamily family)
{
// Convert code here...
}

private static ArticleFamily FromString(string family)
{
// Convert code here...
}
```
This allows the compiler to create expression trees for the method calls, which in this case is what you want.