Can Generic Types with Parameterized Constructors Be Instantiated in C#?

C# generic types and instantiation of parameterized constructors

This article explores how to instantiate a generic type with a parameterized constructor in C#.

Suppose there is a generic method for adding fruits to the fruit manager:

<code class="language-csharp">public void AddFruit<T>() where T : BaseFruit
{
    BaseFruit fruit = new T(weight); // 例如：new Apple(150)
    fruit.Enlist(fruitManager);
}</code>

where BaseFruit class has a constructor that takes an integer weight as parameter.

Question: Can we instantiate a fruit object with a specific weight in this generic method?

Answer: Yes, but not directly like in the example. There are two methods:

1. Use Activator class:

Using the Activator class, we can dynamically instantiate an object of type T and pass the required parameters as an array of objects:

<code class="language-csharp">return (T)Activator.CreateInstance(typeof(T), new object[] { weight });</code>

Note that this requires the BaseFruit class to have a public parameterless constructor for the compiler to check, but in reality it uses the Activator class to create the instance.

2. Use generic parameters with constructor:

C# restricts the use of constructors that require parameters to instantiate generic types. As a workaround, you can create a generic parameter with the same name as the type and specify the constructor parameter in its definition:

<code class="language-csharp">public void AddFruit<T>(T fruit) where T : new(int weight)
{
    fruit.Enlist(fruitManager);
}

// 使用示例：
AddFruit(new Apple(150));</code>

However, this approach is generally not recommended as it can lead to code smells related to constructor requirements in generic types.

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