How to Pass Arguments to a C# Generic `new()` Constraint When Creating Objects?

Pass parameters to new() of C# generic template type

When trying to create a new object of generic type T using its constructor during list expansion, you may encounter a compilation error claiming "T: cannot supply arguments when creating variable instance".

This error occurs because you are trying to call a constructor with parameters for a generic type that is constrained using the "new" flag, which only allows instantiation of objects without constructor parameters.

Solution:

To create an object of a generic type with constructor parameters, you need to remove the "new" constraint and instead provide a delegate that creates the object based on the specified parameters. Here's an example:

<code class="language-csharp">public static string GetAllItems<T>(..., Func<ListItem, T> del) {
    ...
    List<T> tabListItems = new List<T>();
    foreach (ListItem listItem in listCollection) 
    {
        tabListItems.Add(del(listItem));
    }
    ...
}</code>

This function accepts a delegate del which takes a ListItem as parameter and returns an object of type T. You can then call this function and pass the lambda expression as a delegate to create an object of the required type with the necessary parameters:

<code class="language-csharp">GetAllItems<Foo>(..., l => new Foo(l));</code>

By using delegates, you can provide custom initialization logic for the objects you want to create while still maintaining the flexibility of generic programming.

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