C#•3y ago

Generic Dto Mapper

possibly oddball question, but I'm trying to make Dto mappers, and I came across List<T> and Array<T> items in my Dto's that I needed to map by using a foreach loop for each of these items. But I was wondering if there is a way to create a generic mapper of sorts for these things? I mean I tried starting, but I get stuck.. Currently each of these loops does the same, but with different properties in a different model class. Here's an example of the foreach loop they do:

foreach (TargetDto item in dto.ListProperty)
{
    sourceDto.ListProperty.Add(
      new()
      {
        // each property is set here like so:
        PropertyName = item.PropertyName
      });
}

foreach (TargetDto item in dto.ListProperty)
{
    sourceDto.ListProperty.Add(
      new()
      {
        // each property is set here like so:
        PropertyName = item.PropertyName
      });
}

however when I try to do this in a static helper class, I get stuck on the following:

public static ICollection<T> ComplexMapper<TS, T>(this ICollection<TS> source, ICollection<T> target)
{
    foreach (var item in target)
    {
      source.Add(
        // And now I'm stuck because I can't create 'new()' of T because it does not have a new() keyword
      );
    }
}

public static ICollection<T> ComplexMapper<TS, T>(this ICollection<TS> source, ICollection<T> target)
{
    foreach (var item in target)
    {
      source.Add(
        // And now I'm stuck because I can't create 'new()' of T because it does not have a new() keyword
      );
    }
}

49 Replies

Cracker•3y ago

Is there specific reason to not use reliable libraries for mapping such as AutoMapper ?

malkavOP•3y ago

because it's also educational for me 😅

Timtier•3y ago

Couldn't you add a condition for this mapper? Something along the lines of where T : new() behind the method definition?

malkavOP•3y ago

I've been using the project I'm working on to write a lot of these static generic helper methods to learn about them

Cracker•3y ago

Its hard to manage nested collections

malkavOP•3y ago

Apparently not, because T or TS does not have the new() constraint I know, which makes it a perfect educational topic 🤣

Timtier•3y ago

But you can enforce it using the snippet I described, right 🤔

malkavOP•3y ago

I tried that.. here's what I have:

public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) where T : new()
    {
        foreach (T t in target)
        {
            source.Add(
                new() // Here I get 'cannot create instance of type parameter 'T' because it does not have the new() constraint
                {
                    
                });
        }
    }

public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) where T : new()
    {
        foreach (T t in target)
        {
            source.Add(
                new() // Here I get 'cannot create instance of type parameter 'T' because it does not have the new() constraint
                {
                    
                });
        }
    }

Timtier•3y ago

And why specifically are you trying to add an instance of type T while the source is using type TS?

malkavOP•3y ago

Oh, my bad, it says TS.

malkavOP•3y ago

Timtier•3y ago

In that case the same where TS : new() would apply, wouldn't it? Pretty sure you can also do it for both

malkavOP•3y ago

okay, so how do I get all the keys now..

Timtier•3y ago

What's 'keys' in this context? Each property?

malkavOP•3y ago

yea each of the TS properties, as well as the T properties. So that I can do source.Add( new() { TS.PropertyOne = T.PropertyOne } ); sort of..

Timtier•3y ago

Something along the lines of, keep in mind I haven't validated it;

    public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) 
        where T : new()
        where TS : new()
    {
        // Get a collection of properties your target type has.
        var targetProps = typeof(T).GetProperties();

        foreach(var item in source)
        {
            // Construct a new object to fill.
            T obj = new T();

            // Loop through your properties, if it can be written to we try to get the value from TS and set it.
            foreach(var property in targetProps)
            {
                if (property is not null && property.CanWrite)
                {
                    var sourceValue = typeof(T).GetProperty(property.Name).GetValue(item);
                    property.SetValue(obj, sourceValue);
                }
            }

            target.Add(obj);
        }

        return target;
    }

    public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) 
        where T : new()
        where TS : new()
    {
        // Get a collection of properties your target type has.
        var targetProps = typeof(T).GetProperties();

        foreach(var item in source)
        {
            // Construct a new object to fill.
            T obj = new T();

            // Loop through your properties, if it can be written to we try to get the value from TS and set it.
            foreach(var property in targetProps)
            {
                if (property is not null && property.CanWrite)
                {
                    var sourceValue = typeof(T).GetProperty(property.Name).GetValue(item);
                    property.SetValue(obj, sourceValue);
                }
            }

            target.Add(obj);
        }

        return target;
    }

This is ofc very naive and does not consider if it does not exist on the source type etc etc.

cap5lut•3y ago

u could pass a delegate for the actual mapping, but then u more or less have the same thing as linq:

public static ICollection<TTo> ComplexObjectMapper<TFrom, TTo>(this IEnumerable<TFrom> source, ICollection<TTo> target, Func<TFrom, TTo> mapper)
{
  foreach (var element: source)
  {
    target.Add(mapper(element));
  }
}

public static ICollection<TTo> ComplexObjectMapper<TFrom, TTo>(this IEnumerable<TFrom> source, ICollection<TTo> target, Func<TFrom, TTo> mapper)
{
  foreach (var element: source)
  {
    target.Add(mapper(element));
  }
}

which u could use like:

var source = new int[] { 1, 2, 3 };
var target = new List<string>();
source.ComplexObjectMapper(target, element => element.ToString());

var source = new int[] { 1, 2, 3 };
var target = new List<string>();
source.ComplexObjectMapper(target, element => element.ToString());

(if u want filtering u could add Func<TFrom, bool> parameter.) but u could also just do something like var target = source.Where(element = element >= 2).Select(element => element.ToString()); (this is linq btw) by using a delegate as parameter u can avoid writing dozens of similar methods

malkavOP•3y ago

okay, so I used this, but then I must be doing something wrong 😅 help me understand this You're mapping from source into target, by getting all the properties from source, and telling the target object to use those property names and give the source value of that propertyName to the target propertyName. but now, when I want to do SomeListOfComplexItems.ComplexObjectMapper<ComplexItem, ComplexDto>(SomeListOfComplexDtoItems); I get the following error: Cannot convert from System.Collections.Generic.List<ComplexModel> to System (and that's where the error message cuts off, but I recon it says System.Collections.Generic.List<ComplexDto>

cap5lut•3y ago

hard to tell without knowing the real code

malkavOP•3y ago

Wow... now my mind is melting 🤣 My objects are not as simple as ints and strings though.. I basically have:

public class SomeModel
{
  public string SomeProperty {get;set;}
  public int SomeIdentifier {get;set;}
}  

public class SomeDto
{
  public string SomeProperty {get;set;}
  public int SomeIdentifier {get;set;}
}

public class SomeModel
{
  public string SomeProperty {get;set;}
  public int SomeIdentifier {get;set;}
}  

public class SomeDto
{
  public string SomeProperty {get;set;}
  public int SomeIdentifier {get;set;}
}

so I would then use Linq to do what now?

var source = new List<SomeModel>();
var target = new List<SomeDto>();
source.Where(element => element..... what now???

var source = new List<SomeModel>();
var target = new List<SomeDto>();
source.Where(element => element..... what now???

okay, lemme give you what I have now..

public static model MapHoursToModel(dto dto)
    {
        model result = new()
        {
            ModelId = dto.ModelId,
            CreatedAt = dto.CreatedAt,
            UpdatedAt = dto.UpdatedAt,
            Month = dto.Month,
            Year = dto.Year,
            UserMonth = dto.UserMonth,
            WorkedDays = new()
        };
        result.WorkedDays = dto.WorkedDays.ComplexObjectMapper<HostDaysWorked, DomainDaysWorked>(result.WorkedDays);
// foreach (DomainDaysWorked item in dto.WorkedDays)
        // {
        //     result.WorkedDays.Add(
        //         new()
        //         {
        //             DayWorked = item.DayWorked,
        //             Hours = item.Hours,
        //             Assignment = item.Assignment,
        //             Remarks = item.Remarks,
        //             EntryState = item.EntryState,
        //             TypeOfHours = item.TypeOfHours
        //         });
        // }
  return result;
}

public static model MapHoursToModel(dto dto)
    {
        model result = new()
        {
            ModelId = dto.ModelId,
            CreatedAt = dto.CreatedAt,
            UpdatedAt = dto.UpdatedAt,
            Month = dto.Month,
            Year = dto.Year,
            UserMonth = dto.UserMonth,
            WorkedDays = new()
        };
        result.WorkedDays = dto.WorkedDays.ComplexObjectMapper<HostDaysWorked, DomainDaysWorked>(result.WorkedDays);
// foreach (DomainDaysWorked item in dto.WorkedDays)
        // {
        //     result.WorkedDays.Add(
        //         new()
        //         {
        //             DayWorked = item.DayWorked,
        //             Hours = item.Hours,
        //             Assignment = item.Assignment,
        //             Remarks = item.Remarks,
        //             EntryState = item.EntryState,
        //             TypeOfHours = item.TypeOfHours
        //         });
        // }
  return result;
}

I commented out the foreach, because that's how I did it when I went literal. I'm doing the same for a much more complex object with four to six of those foreach loops that do basically the same, but with different properties the ComplexObjectMapper() is what was sent earlier:

public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) 
        where TS : new()
        where T : new()
    {
        PropertyInfo[] targetProperties = typeof(T).GetProperties();

        foreach (TS item in source)
        {
            T obj = new();

            foreach (PropertyInfo? prop in targetProperties)
            {
                if (prop is null) continue;
                object? sourceValue = typeof(T).GetProperty(prop.Name).GetValue(item);
                prop.SetValue(obj, sourceValue);
            }
            target.Add(obj);
        }

        return target;
    }

public static ICollection<T> ComplexObjectMapper<TS, T>(this ICollection<TS> source, ICollection<T> target) 
        where TS : new()
        where T : new()
    {
        PropertyInfo[] targetProperties = typeof(T).GetProperties();

        foreach (TS item in source)
        {
            T obj = new();

            foreach (PropertyInfo? prop in targetProperties)
            {
                if (prop is null) continue;
                object? sourceValue = typeof(T).GetProperty(prop.Name).GetValue(item);
                prop.SetValue(obj, sourceValue);
            }
            target.Add(obj);
        }

        return target;
    }

cap5lut•3y ago

u would use a Select() call there, which is used to transform/map elements. Where() is to filter elements

var target = source.Select(element => new SomeDto {
  SomeProperty: element.SomeProperty,
  SomeIdentifier: element.SomeIdentifier
});

var target = source.Select(element => new SomeDto {
  SomeProperty: element.SomeProperty,
  SomeIdentifier: element.SomeIdentifier
});

malkavOP•3y ago

Ah, so a Linq way of doing my commented out foreach loops 🤣 that's already much closer to what I wanted

cap5lut•3y ago

note that target is now only a IEnumerable<SomeDto>, if u would for example would want a List<SomeDto>, u would do an ToList() call at the end

malkavOP•3y ago

fair enough. I'm just trying to figure it out though.. eh... So List<SomeDto> target = source.Select(element => new SomeDto { SomeProperty: element.SomeProperty } ).ToList() right?

cap5lut•3y ago

yep

malkavOP•3y ago

Dude...

malkavOP•3y ago

https://tenor.com/view/the-universe-tim-and-eric-mind-blown-mind-blown-meme-mind-explosion-mind-explosion-meme-gif-18002878

Tenor

malkavOP•3y ago

you're a hero and a legend 🤣

cap5lut•3y ago

actually the linq authors r ;p

malkavOP•3y ago

though still I'd love to learn how to generic that though... but I'll figure that out eventually yea they are too.. Linq has saved my ass so many times now

cap5lut•3y ago

how to generic that though

basically without passing a delegate?

mrphil2105•3y ago

Uhhhh reflection is quite slow I'd suggest using Mapster or AutoMapper

cap5lut•3y ago

yeah, im not sure what they r meaning, so thats why i was asking linq isnt that fast either ;p

mrphil2105•3y ago

Faster than reflection though AFAIK

malkavOP•3y ago

o.O

cap5lut•3y ago

ofc, reflection is really slow and complex, an alternative would be source generators but thats even more complex. imo either way it would be adding too much magic to it which can be quite error prone and thus not worth it

malkavOP•3y ago

Fair enough..

mrphil2105•3y ago

Some LINQ functions are getting a speed boost though in .NET 7 Min and Max for example are getting vectorization finally

cap5lut•3y ago

well, talking about optimization at this stage isnt really helpful, this might not even be the bottleneck of the program. write it working -> refactor it clean -> check bottlenecks -> optimize them

mrphil2105•3y ago

Still, using reflection for mapping is not a good idea like this Just use Mapster :)

malkavOP•3y ago

The thing is that the senior at my work gave me this "skeleton" format I should follow, and he's making me write my own mappers 😅

mrphil2105•3y ago

Mapster has really good performance And is easy to use

malkavOP•3y ago

fair enough

mrphil2105•3y ago

It can also generate mapping code

cap5lut•3y ago

oh mapster uses code generation

mrphil2105•3y ago

You can do that too You can also just use the global mapper Or use the interface it provides

malkavOP•3y ago

The skeleton I'm using is basically: Host.MyApp Host.UnitTests Domain.PortsAndAdapterInterfaces Adapter.Persistence and a Mapper between each of those boundaries

mrphil2105•3y ago

So it has a few options

malkavOP•3y ago

But you guys have been legends, I'll close the post now as I'm helped! thanks!!!

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