EF Core Value Converter — Complete Guide

In this tutorial, you'll learn about EF Core Value Converter. We cover key concepts, practical examples, and best practices.

Your entity has a string[] tags property, but the database column is nvarchar. Or an enum is stored as int but you want string. EF Core value converters transform values between the CLR type and the database type automatically.

Wrong

public class Product
{
    public int Id { get; set; }
    public string Tags { get; set; } // Manual serialization
}

// Every access requires:
var tags = product.Tags.Split(','); // Boilerplate everywhere

Output: Works. But every read/write requires manual conversion. No type safety.

Right

public class Product
{
    public int Id { get; set; }
    public string[] Tags { get; set; } // Natural CLR type
}

// Value converter
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>()
        .Property(p => p.Tags)
        .HasConversion(
            v => string.Join(',', v),      // To database
            v => v.Split(',', StringSplitOptions.RemoveEmptyEntries) // From database
        );
}

Output: EF Core converts string[] to comma-separated string for storage and back. The entity works with the natural CLR type.

Reusable converter:

public class StringArrayConverter : ValueConverter<string[], string>
{
    public StringArrayConverter()
        : base(
            v => string.Join(',', v),
            v => v.Split(',', StringSplitOptions.RemoveEmptyEntries))
    { }
}

Prevention

• Use value converters for enum-to-string, JSON serialization, and custom type mapping.
• Use built-in converters for bool to int, char to string, and DateTime to long.
• Create reusable converter classes for types used across multiple entities.
• Use HasConversion<TProvider>() for simple conversions.
• Use ValueConverter class for complex conversions with null handling.
• Use ConverterMappingHints for database column type hints (size, unicode).
• Test converter round-trips with edge cases (null, empty, special characters).

Common Mistakes with core value converter

1. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
2. Using head and tail instead of pattern matching, causing runtime errors on empty lists
3. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks

These mistakes appear frequently in real-world EF code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Value converters are used in DodaTech for enum-to-string mapping in audit logs. For more EF Core, visit DodaTech.