How to use props of generic list which is part of generic method - c#

in my small app I would like to reuse method used for prices calculation for few different objects, that gives me an idea it could be generic method, sometimes it will receive main-products and return them, sometimes it will receive articles and return them, sometimes it will receive category-articles and returned them.

So to avoid writing 3 almost identical methods, I was thinking about making it generic. Something like this:

private async Task<List<T>> GetArticlesPolicyAsync<T>(List<T> articles, Guid externalCompanyId, CancellationToken cancellationToken)
{
	var externalArticleIds = articles.Select(x => x.ExternalArticleId).ToArray();

	var externalCategoryIds = articles.Select(x => x.ExternalCategoryId).ToArray();

	var externalSubCategoryIds = articles.Select(x => x.ExternalSubCategoryId).ToArray();

	var externalBrandIds = articles.Select(x => x.ExternalBrandId).ToArray();

	var pricePolicyItems = new List<ExternalPricePolicyItemGetDto>().... and so on....

Every sigle list that I receive as a prop 'articles' will definately contain this 4 props:

• ExternalArticleId

• ExternalCategoryId

• ExternalSubCategoryId

• ExternalBrandId

but unfortunately this is the error I am seeing:

> Error CS1061 'T' does not contain a definition for 'ExternalArticleId'
> and no accessible extension method 'ExternalArticleId' accepting a
> first argument of type 'T' could be found (are you missing a using
> directive or an assembly reference?)

I am not sure if this scenario can be generic 100% ?

Any kind of help would be awesome

Thanks in advance,

Cheers

To make this work with generics, the generic type must be constrained to a type that has all the properties you want to access (otherwise you only have access to members T as an object type).

One way to do this is to create an interface that defines these properties:

interface ExternalStuff
{
    int ExternalArticleId { get; set; }
    int ExternalCategoryId { get; set; }
    int ExternalSubCategoryId { get; set; }
    int ExternalBrandId { get; set; }
}

Then for all the types that implement these properties, you would need to specify that they fulfill the requirements of the new interface by adding : ExternalStuff after the class definition:

// You shouldn't have to do anything else inside the classes since you said 
// all the types already implemented the properties defined in the interface

class Article : ExternalStuff
{
    // ...
}

class MainProduct : ExternalStuff
{
    // ...
}

class CategoryArticle : ExternalStuff
{
    // ...
}

And finally you need to constrain the generic type T to this interface by adding where T : ExternalStuff at the end of the method declaration, which tells the compiler that only items that implement the interface can be used, and guarantees that the properties you're accessing will be members of those objects:

private async Task<List<T>> GetArticlesPolicyAsync<T>(List<T> articles, 
    Guid externalCompanyId, CancellationToken cancellationToken) 
    where T : ExternalStuff
{
    var externalArticleIds = articles.Select(x => x.ExternalArticleId).ToArray();
    var externalCategoryIds = articles.Select(x => x.ExternalCategoryId).ToArray();
    var externalSubCategoryIds = articles.Select(x => x.ExternalSubCategoryId).ToArray();
    var externalBrandIds = articles.Select(x => x.ExternalBrandId).ToArray();
    // etc...
}

You can read more about contraints here: Constraints on type parameters (C# Programming Guide)

What u need to do is to create an interface that holds the properties that are common for type T for example:

public interface IArticles
{
    int ExternalArticleId;

    int ExternalCategoryId;

    int ExternalSubCategoryId;

    int ExternalBrandId;
}

then define a constrain in the generic type T using the where clause.

Task<List<T>> GetArticlesPolicyAsync<T>(List<T> articles, Guid externalCompanyId, CancellationToken cancellationToken)
where T : IArticles
{
    var externalArticleIds = articles.Select(x => x.ExternalArticleId).ToArray();

    var externalCategoryIds = articles.Select(x => x.ExternalCategoryId).ToArray();

    var externalSubCategoryIds = articles.Select(x => x.ExternalSubCategoryId).ToArray();

    var externalBrandIds = articles.Select(x => x.ExternalBrandId).ToArray();

    var pricePolicyItems = new List<ExternalPricePolicyItemGetDto>().... and so on....

Hope this helps.
Regards.