I'm a little rusty with C# but something like this might work:

public List<Entity> GetEntities<FilterType>()
{
    // Assume all entities are valid
    List<Entity> entities = new List<Entity>(allEntities);
    var fields = FilterType.GetFields();
    foreach(var field in fields)
    {
        var componentType = field.FieldType;
        foreach(var entity in entities)
        {
            // Remove any invalid entities
            if(!entity.HasComponent(componentType))
                entities.Remove(entity);
        }
    }
    return entities;
}

This uses reflection to get the type of all (public!) fields in the struct/class FilterType and then checks if an entity does not have that component. In that case that entity is invalid and is removed from the final list result.

I'm a little rusty with C# but something like this might work:

public List<FilterType> GetEntities<FilterType>()
{
    // Assume all entities are valid
    List<Entity> entities = new List<Entity>(allEntities);
    var fields = FilterType.GetFields();
    foreach(var field in fields)
    {
        var componentType = field.FieldType;
        foreach(var entity in entities)
        {
            // Remove any invalid entities
            if(!entity.HasComponent(componentType))
                entities.Remove(entity);
        }
    }
    
    List<FilterType> filters = new List<FilterType>();
    foreach(var entity in entities)
    {
        FilterType filter = (FilterType) Activator.CreateInstance(FilterType);
        foreach(var field in fields)
            field.SetValue(filter, entity.GetComponent(field.FieldType));
        filters.Add(obj);
    }

    return filters;
}

This uses reflection to get the type of all (public!) fields in the struct/class FilterType and then checks if an entity does not have that component. In that case that entity is invalid and is removed from the final list result.