Why Can't C# Constructors Inherit Type Inference from Generic Methods?

Why can’t C# constructors inherit type inference from generic methods?

In C#, constructors lack the type inference capabilities of generic methods. For example, in the following code:

<code class="language-csharp">public class MyType<T>
{
   private readonly T field;
   public MyType(T value) { field = value; }
}

var obj = new MyType(42); // 期望类型：MyType<int></code>

The compiler cannot infer that obj is of type MyType<int>. One solution is to use a factory class:

<code class="language-csharp">public class MyTypeFactory
{
   public static MyType<T> Create<T>(T value)
   {
      return new MyType<T>(value);
   }
}
var myObj = MyTypeFactory.Create(42);</code>

This raises questions about the reasons behind this restriction.

Is there a root cause?

There is no conclusive underlying reason preventing constructors from doing type inference. In contrast, identifying all relevant types and resolving overloads between constructors with different numbers of parameters requires a complex algorithm. Additionally, to ensure backward compatibility, non-generic constructors take precedence over inferred constructors.

Practical Obstacles: Weighing the Pros and Cons

Although type inference in constructors has a net benefit, this does not necessarily guarantee its implementation. A feature's viability depends on its relative value relative to other potential improvements, resource allocation, and compatibility issues.

Factory Pattern: A Clever Alternative

The factory method pattern provides an effective solution to the lack of constructor type inference. By introducing an additional layer of abstraction, type inference can be applied to factory methods to achieve similar results without modifying the constructor itself.

Changes that were never implemented

Although implementation was considered for C# 6, this proposed feature was ultimately removed from the final design. The fact that it was close to adoption highlighted its potential utility, but ultimately, other priorities prevailed.

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