How Can I Pass Constructor Arguments to a Generic `new()` Constraint in C#?

Passing generics for template types in C# new() constructor parameters

Question:

A compilation error occurs when trying to create a new object of type T with a constructor argument in a generic method using the new() constraint. The error message states that the parameter cannot be supplied during object initialization.

<code class="language-csharp">public static string GetAllItems<T>(...) where T : new()
{
   List<T> tabListItems = new List<T>();
   foreach (ListItem listItem in listCollection) 
   {
       tabListItems.Add(new T(listItem)); // 此处出错。
   } 
   ...
}</code>

Solution:

To create an object of a generic type using constructor arguments, it is not enough to just use the new constraint. A custom function must be introduced to handle initialization with necessary parameters. Here’s how:

<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)); // 使用函数创建对象。
  }
  ...
}</code>

By using a custom function, you can call a generic method, specify the type and provide an initialization function as follows:

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

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