Why Does C# Generic Type Inference Fail with Interface Constraints?

Limitations of C# generic type inference

C#’s type inference mechanism can usually intelligently infer the generic parameters of generic methods. However, type inference may fail in some cases, as shown in the following example:

<code class="language-csharp">interface IQuery<TResult> { }

interface IQueryProcessor
{
    TResult Process<TQuery, TResult>(TQuery query)
        where TQuery : IQuery<TResult>;
}

class SomeQuery : IQuery<string>
{ }</code>

In the above code, when calling SomeQuery's IQueryProcessor method using a Process instance, the compiler is unable to infer the correct generic parameters. Explicitly specifying the parameters solves this problem:

<code class="language-csharp">p.Process<SomeQuery, string>(query);</code>

Reason for inference failure

Contrary to initial assumptions, constraints alone are not sufficient for type inference. C#'s type inference is entirely based on parameters and their corresponding formal parameter types. Since the provided parameter type is SomeQuery (which implements IQuery<string>), the compiler cannot automatically infer that the generic parameters should be TQuery = SomeQuery and TResult = string.

Eric Lippert in the article "Constraints are not part of the signature" (https://www.php.cn/link/4cf06252cc21d496e754ad7185d0617d.

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