Why Can't C# Infer Generic Type Arguments in Methods with Interface Constraints?

C# Generic Type Inference Limitations: The Case of Interface Constraints

C#'s generic methods provide adaptable data handling across diverse types. However, the compiler's automatic type inference isn't always successful. This limitation becomes apparent when dealing with interface constraints in generic methods.

Consider this example:

interface IQuery<TResult> { }

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

class SomeQuery : IQuery<string> { }

class Test
{
    void Test(IQueryProcessor p)
    {
        var query = new SomeQuery();

        // Compilation Error: Type inference failed
        p.Process(query);

        // Explicit type arguments required for successful compilation
        p.Process<SomeQuery, string>(query);
    }
}

The compiler fails to infer TQuery and TResult in the first p.Process(query) call. The reason is that C#'s type inference mechanism relies on the types of the arguments provided. While query is of type SomeQuery, this alone doesn't fully define TQuery and TResult.

The constraint where TQuery : IQuery<TResult> allows for multiple IQuery implementations, but the compiler cannot deduce the precise implementation from the argument type. Therefore, explicit type arguments (<SomeQuery, string>) are needed to resolve the generic types.

As Eric Lippert explains (https://www.php.cn/link/4a3cffe005397d4cffdee044f1c8d30e), constraints are not part of a method's signature and hence are not used for type inference. Inference is based solely on the formal parameter types, which, crucially, exclude constraints.

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