Why Can't C# Infer Generic Types with Interface Constraints?

Limitations of C# generic type inference

Why can’t C# infer the generic type in this case?

In the use of generic methods, C# type inference will usually identify the correct generic parameters. However, in some cases it may fail, causing the programmer to have to specify the type explicitly. This article discusses a situation in which a compiler is said to be deficient in making inferences.

Consider the following code snippet:

interface IQuery<TResult> { }

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

class SomeQuery : IQuery<string>
{
}

In a Test method, when calling SomeQuery with a Process instance as argument, the compiler cannot infer the generic argument:

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

        // 无法编译 :-(
        p.Process(query);

        // 必须显式编写所有参数
        p.Process<SomeQuery, string>(query);
    }
}

The question is, why doesn't the compiler successfully infer the generic parameters in this case?

Constraints and inferences

The answer lies in how C# handles generic parameter constraints. Constraints (such as TQuery : IQuery<TResult>) limit the types that can be used as arguments. However, constraints are not considered part of the method signature and therefore cannot be used for type inference.

In this particular case, the compiler can determine that the query parameter implements IQuery<TResult>, but it cannot use the constraints to infer the concrete type of TResult. Therefore, it requires explicit type parameters.

Conclusion

When using generics, it is crucial to understand the limitations of C# type inference. While the compiler usually does a pretty good job of inferring types, in some cases constraints prevent it from doing so. In this case, specific generic parameters need to be provided to guide the compiler.

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