I want to implement a generic method to retrieve header/detail data from a database:

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Asked: May 23, 20262026-05-23T04:08:44+00:00 2026-05-23T04:08:44+00:00

I want to implement a generic method to retrieve header/detail data from a database:

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I want to implement a generic method to retrieve header/detail data from a database:

public static T RetrieveHeaderDetail<T>
    where T : Header<???>, new()
    // Where ??? means "what can I do here?"
{
    // ...
}

Here is the definition of the generic representing a document header:

public class Header<TDetail> where TDetail : class, new()
{
    public List<TDetail> Details;
}

And here are some instantiations:

public class RequestForQuotation : Header<RequestForQuotationDetail> { ... }
public class Order               : Header<OrderDetail>               { ... }
public class Invoice             : Header<InvoiceDetail>             { ... }
// ..

It is not hard to prove that, since .NET does not allow either multiple inheritance or “generic specialization” (which would allow a Header<U> to derive from some other Header<V>), for any specific T, there is at most one U such that T inherits (directly or indirectly) from Header<U>. Moreover, it is trivial to find the type U: iterate over T‘s base types until you find an instance of Header<U>, and then just take the generic’s argument! Still, C# wants me to specify the change my method’s definition to the following:

public static T RetrieveHeaderDetail<T,U>
    where T : Header<U>, new()
    where U : class,     new()
{
    // ...
}

Is there any way to get around this problem? I know it would be possible using Reflection, but I think it is a good practice to never do at runtime what could be done at compile time.

When I hit problems like this, I really, really miss C++.