Generate dapper type handlers outside the domain project

[marcwittke]

Describe the feature

Referring to the Note in the docs I'd like to request dapper type handlers to be generated outside the project. It was mentioned in #541 but it seems as it wasn't implemented for Dapper.

[marcwittke]

I found a workaround and I wonder if this could be a first class citizen in Vogen (or in a separate extension project Vogen.Dapper?) Using a little bit of reflection I have a generic type handler:

using System.Data;
using System.Reflection;
using Dapper;


public class StrongIdValueTypeHandler<TValueObject> : SqlMapper.TypeHandler<TValueObject> where TValueObject : struct, IEquatable<int>
{
    // ReSharper disable once StaticMemberInGenericType
    private static readonly MethodInfo FromMethod;

    // ReSharper disable once StaticMemberInGenericType
    private static readonly PropertyInfo ValueProperty;

    static StrongIdValueTypeHandler()
    {
        FromMethod = typeof(TValueObject).GetMethod("From", BindingFlags.Public | BindingFlags.Static)
            ?? throw new InvalidOperationException($"The type {typeof(TValueObject)} must have a static 'From' method.");

        ValueProperty = typeof(TValueObject).GetProperty("Value", BindingFlags.Public | BindingFlags.Instance)
            ?? throw new InvalidOperationException($"The type {typeof(TValueObject)} must have a 'Value' property.");
    }

    public override void SetValue(IDbDataParameter parameter, TValueObject valueObject)
    {
        parameter.DbType = DbType.Int32;
        parameter.Value = ValueProperty.GetValue(valueObject) ?? DBNull.Value;
    }

    public override TValueObject Parse(object? value)
    {
        if (value is null or DBNull)
        {
            return default;
        }

        if (value is int intValue)
        {
            return (TValueObject)(FromMethod.Invoke(null, [intValue])
                ?? throw new InvalidOperationException($"Method {typeof(TValueObject)}.From(int value) returned null."));
        }

        throw new InvalidCastException($"Unable to cast value of type {value.GetType()} to {typeof(TValueObject)}");
    }
}

then, it is applied for all value objects wrapping an int and having the GeneratedCodeAttribute with Tool=="Vogen" using reflection:

var strongIdTypes = assembly.GetTypes()
            .Where(
                type => type is { IsValueType: true, IsPrimitive: false }
                    && typeof(IEquatable<int>).IsAssignableFrom(type)
                    && type.GetCustomAttributes<GeneratedCodeAttribute>().Any(attr => attr.Tool == "Vogen"))
            .ToArray();

        foreach (var strongIdType in strongIdTypes)
        {
            var handlerType = typeof(StrongIdValueTypeHandler<>).MakeGenericType(strongIdType);
            object handler = Activator.CreateInstance(handlerType)
                ?? throw new InvalidOperationException($"Could not create an instance of {handlerType}");

            SqlMapper.AddTypeHandler(strongIdType, (SqlMapper.ITypeHandler) handler);
        }

This way, no code generation is required, and the overhead is neglectable since the reflection results are cached. What do you think? ChatGPT liked it 😄

[SteveDunn]

Thanks for the feedback @marcwittke - I intend to add the ability to generate most of the converters/serialisers into the project where they're declared rather than the project where the value object is declared.

Thanks for suggesting your approach. However, I wouldn't want to add any Reflection code as I think it'd break a fair few users who use Vogen in trimmed/AOT apps (I know I'd break my own projects if I did this).

It doesn't look to difficult to implement, so I'll attempt it at some point in the new couple of weeks.

Thanks again,

Steve