Custom ValueConverter from class to string ignores column type and unicode property settings

I have created a custom value converter to convert from a class to a string and vice versa. When using this property in a Contains lambda the SQL generation for the WHERE IN clause does not take into consideration the property builder extension functions IsUnicode or HasColumnType. This results in a SQL statement where a varchar field is being cast to nvarchar(max) and the string values in the IN clause are unicode (N'my string').

This is a huge performance hit and I would expect the IsUnicode and HasColumnType would be good enough hints for the SQL generation engine to treat the strings as ASCII.

I have also tried adding CustomMappingHints to the ValueConverter which did not change the SQL generation at all either.

Steps to reproduce

public class ValueString
{
    private readonly string _value;

    public ValueString(string value)
    {
        _value = value;
    }

    public static implicit operator string(ValueString valueString)
    {
        return valueString._value;
    }

    public override bool Equals(object obj)
    {
        if (!(obj is ValueString valueString))
           return false;

       return _value == valueString._value;
    }

    public override int GetHashCode()
    {
        return _value.GetHashCode();
    }

    public override string ToString()
    {
        return _value;
    }
}

public static class PropertyBuilderExtensions
{

    public static PropertyBuilder<ValueString> HasValueStringConversion<ValueString>(this PropertyBuilder<ValueString> propertyBuilder)
    {
        var converter = new ValueConverter<ValueString, string>(
            t => t.ToString(),
            s => new ValueString(s));

        return propertyBuilder.HasConversion(converter);
    }
}

public class MyClass
{
    public ValueString Hello { get; set; }
}

public class MyClassEntityConfiguration : IEntityTypeConfiguration<MyClass>
{
    public void Configure(EntityTypeBuilder<MyClass> builder)
    {
        ...
        builder.Property(e => e.Hello)
            .IsUnicode(false)
            .HasValueStringConversion();
    }
}

Sample call:

context.MyClass.Where(a => list.Contains(a.Hello));

Results in:

SELECT Hello FROM MyClass
WHERE CAST(Hello AS nvarchar(max)) IN (N'sample', N'test')

Further technical details

EF Core version: 3.1.2 Database provider: Microsoft.EntityFrameworkCore.SqlServer v3.1.2 Target framework: .NET Core 3.1 Operating system: Windows 10 Enterprise IDE: Rider

[lukepfeiffer10]

Thanks for the assistance, it is working now that the type is changed to IEnumerable<ValueString>. I think the implicit conversion between ValueString and string was causing unnecessary confusion for me on how the compiler would know what to do.

[lukepfeiffer10]

I see what you mean. I’ll try that.