How Can I Instantiate Generic Types with Parameterized Constructors in C#?

C# generic types and instantiation of parameterized constructors

Suppose you have a generic method that accepts a generic type, and you want to instantiate an instance of that type with specific constructor parameters. However, the type's constructor requires a parameter, and you don't know if you can do that.

The following code snippet demonstrates an attempt to create a fruit object in a generic method:

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

This approach may fail because BaseFruit does not have a parameterless constructor. It takes an integer parameter to specify the weight of the fruit.

To solve this problem, C# provides the Activator class. You can create an instance of a type using the Activator.CreateInstance method and pass an array of objects as a constructor argument:

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

Note that using the T constraint on new() only ensures that the compiler checks for public parameterless constructors at compile time. The actual code used to create the type is handled by the Activator class.

While this approach allows you to instantiate a type with a parameterized constructor, you must ensure that the specific constructor is present in the type definition. Relying on this approach may indicate potential code smells and suggest that you should explore other design options.

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